Hydrochloric acid 30% vs 33%: strength, fuming and tank material

The answer, up front

Three answers, because the question has three parts.

1. Legally, there is no difference. We went looking for a threshold anywhere in Indian law that 30% and 33% hydrochloric acid fall on opposite sides of, and there is not one. The only concentration line we could find anywhere is 5% by weight, in the state poisons schedules, and both strengths clear it by a factor of six. Note that even the transport classification does not draw one: packing group for a class 8 corrosive is assigned on corrosivity test criteria, not on a stated concentration. Nothing in the poisons rules, the hazardous chemical rules, the GST rate, the transport classification, the Factories Act exposure schedule or the waste rules distinguishes them.
2. Physically, there is a difference, it is real, and it is smaller than the difference the ambient temperature makes. The partial pressure of hydrogen chloride over the liquid is what people mean by fuming, and it rises steeply with both strength and temperature. Going from roughly 30% to roughly 32% at the same temperature roughly doubles it. Warming the same 30% acid from 20 °C to 50 °C multiplies it by nearly seven. In an Indian tank farm in May, the tank-top temperature is the variable that matters, not the second decimal of the assay.
3. Commercially, the difference should be nothing at all, and if it is something you have written the enquiry wrongly. Hydrochloric acid is bought for its hydrogen chloride. A tonne of 33% acid contains about a tenth more HCl than a tonne of 30% acid, so a per-tonne price for “acid” is not a price for the product. Quote and settle on contained HCl and the question dissolves.

What follows is the detail behind each of those, plus the part of the question that actually deserves the engineering attention: what the tank, the lining, the gaskets and the vent are made of, because that is where hydrochloric acid punishes a mistake.

This is a buyer’s and plant engineer’s guide, not a design document and not legal or safety advice. It quotes no price and no market level. Where it gives a figure it names the document the figure comes from, so that you can check whether it still reads that way, and where the published sources disagree or stop, it says so rather than filling the gap. Nothing here replaces the supplier’s own safety data sheet for the material you are buying, the equipment manufacturer’s instructions for the equipment you are using, or a competent engineer.

What the Indian standard actually specifies — and what it does not

The current edition is IS 265 : 2021, and most people are reading the old one

IS 265 : 2021, Hydrochloric Acid — Specification, Fifth Revision is the current edition. It supersedes IS 265 : 1993, which had been reaffirmed in 2015 and carried five printed amendments. This matters more than a version number usually does, because the freely circulating full text of “IS 265” on the internet is the 1993 edition. Anyone who read the standard without buying it almost certainly read the superseded one, and Indian buyers were still writing “conforming to IS: 265/1993” into live tender documents as recently as the end of 2024.

The fifth revision made four changes a buyer would notice: it added a Food grade; it brought road tankers into the packing clause; it deleted screw-capped stoneware bottles; and it added a total organic carbon limit, which is the characteristic that in practice separates acid made from pure chlorine and hydrogen from acid recovered as a by-product of chlorination.

The grades are purity grades. They are not concentration grades.

This is the single most commonly misunderstood thing about buying hydrochloric acid in India. IS 265 : 2021 defines five grades — Technical, Chemical pure, Food, Analytical reagent and Boiler water. There is no 30% grade and no 33% grade. What the grades carry is two assay tiers, and the assay is a minimum: total acidity as HCl of 30.0% by mass minimum for Technical, Boiler water and Food, and 32.0% by mass minimum for Chemical pure and Analytical reagent. Every other characteristic in the specification table — residue on ignition, sulphates, iron, free chlorine and bromine, sulphites, heavy metals, arsenic, mercury — is a maximum.

So there is no band and no upper limit on strength anywhere in the standard. Three consequences follow, and all three are commercial.

• “Our acid is IS 265 grade” is close to meaningless in a contract. It commits the seller to at least 30% (or at least 32% for the two higher-purity grades) and to the impurity ceilings for the grade named. It does not commit the seller to a target strength at all. If you want 33%, specify 33% separately, with a tolerance.
• Nothing stops a 30.0% minimum being met at 30.1%. If your process was designed around acid that has historically arrived at 32% and your purchase order says “Technical grade to IS 265”, a supplier who switches you to a 30.2% delivery has not breached anything.
• The impurity that usually matters is iron, and the grade sets it, not the strength. The Technical grade iron ceiling is tens of times looser than the Chemical pure one. If iron matters to your application — and in several it matters a great deal, including anywhere zirconium equipment is in contact with the acid, as the materials section below explains — then the grade is the lever, not the percentage.

One further point that only appears in the 2021 edition: it adds a total organic carbon limit, and that limit is the practical reason the production route matters. Most Indian commercial hydrochloric acid is recovered as a by-product of chlorination rather than made as a primary product from chlorine and hydrogen, and by-product acid from chloro-organic processes is where organic carbon comes from. The limit is a proxy for the route rather than a determination of it — a clean by-product acid will pass it — but for food, pharmaceutical or electronics duty it is the characteristic to specify, and before 2021 the standard had nothing to say about it at all.

We have read the specification table and Annex A of IS 265 : 2021 only through a scanned copy with an optical character layer, and two successive readings of the density annex returned slightly different numbers in its upper rows. We are therefore quoting the structure of the standard — five grades, two assay minima, every other characteristic a maximum — which is unambiguous, and we are not reproducing its impurity limits at all. Be careful with the two assay figures as well. The 30.0% and 32.0% minima quoted above are also the figures in the superseded 1993 edition, which we could read cleanly, and we could not independently confirm from a reliable copy that the 2021 revision left them unchanged. Treat them as indicative of the standard’s shape, and buy the standard from the Bureau of Indian Standards before you write any of its numbers into a purchase order. The same caution applies, in principle, to everything else we have taken from that scan — the grade names, the two-tier structure, the test method, the conformity criterion and the annex status. The five grade names we were able to confirm against a separate Bureau document; the rest we could not, and we have said so rather than letting one disclaimer stand for the lot.

Stating the strength, and the three ways it goes wrong

Litres or kilograms is a lawful choice, which is exactly why it is a trap

Start here, because it is the error that costs money quietly. Under the Legal Metrology (Packaged Commodities) Rules, 2011 the general rule is mass for solids and volume for liquids — but that rule is expressly subject to the Fourth Schedule, and the Fourth Schedule contains entries for “Acids in liquid form” and “Liquid chemicals”, each of which may be declared by weight or volume.

So both are lawful, and two suppliers can quite properly quote you on different bases. Thirty-three per cent acid is denser than thirty per cent acid, so a per-litre price and a per-kilogram price are not interchangeable between them, and neither is a price for hydrogen chloride. Three numbers, three different answers, all of them describing the same delivery. Say in the enquiry which one you want.

For a bulk buyer there is a second point in those Rules worth knowing: Chapter II does not apply to packages containing more than 25 kg or 25 litres, nor to packaged commodities meant for industrial or institutional consumers. A carboy over 25 kg, a drum, an intermediate bulk container and a road tanker are therefore outside that chapter on two independent grounds. The general article in this series on how the landed price of a bulk chemical is built up covers the industrial-consumer limb and its conditions in detail; we are not restating it here.

Baumé, and the half-point that quietly goes to the seller

Hydrochloric acid is widely talked about in degrees Baumé rather than per cent, and the conversion in circulation is wrong in the seller’s favour. Twenty degrees Baumé is not 32% HCl.

The American standard Baumé scale for liquids heavier than water uses a modulus of 145 on a 60/60 °F basis — the National Bureau of Standards said in 1918 that it had “always held” that this modulus should govern such liquids. On that scale 20 °Bé is a specific gravity of exactly 1.1600, which the United States Coast Guard chemical hazard database gives as 31.5% HCl and a peer-reviewed monograph compiled from producers’ own data sheets gives as 31.45%. Likewise 22 °Bé is about 35.2%, not 36%, and 23 °Bé about 37.1%.

On a twenty-tonne tanker, half a percentage point of HCl is a hundred kilograms of product. “Twenty Baumé, call it 32%” is a systematic half-point giveaway, repeated on every load.

And there is not one Baumé scale. The same 1918 Bureau of Standards paper documents at least three moduli in simultaneous use for liquids heavier than water — 145 (the American standard, at 60/60 °F), 144, and 146.78 (at 17.5 °C) — and records that “many different Baume scales have been proposed and used in the past”. A Baumé figure with no modulus and no reference temperature attached is not a specification. Separately, we found no published source establishing that the Baumé convention is an Indian trade practice rather than an imported American one; IS 265 uses relative density throughout and does not mention Baumé at all. If your supplier quotes in Baumé, ask which scale.

A hydrometer is a receiving check. It is not a settlement basis.

This follows from the standard itself, in three places.

• The reference method for total acidity is titration against standard alkali with methyl orange indicator, using a Lunge-Rey pipette. That is what IS 265 : 2021 specifies as the test.
• The density correlation table is in an annex marked “for information”, tagged to the foreword rather than to the requirements clause. It is a convenience, not a method of determination.
• Conformity for total acidity is a statistical criterion over five samples, not a single reading. The lot conforms if the mean of five individual results, less six-tenths of their range, is at least the tabulated value. That is the clause to cite in a dispute, and almost nobody does.

A hydrometer dip satisfies none of that, and on top of it a Baumé reading carries an undeclared scale and an uncorrected temperature. The correct position: hydrometry is a legitimate sixty-second check at the gate for gross error — a load that is nowhere near what was ordered — and the commercial weight should settle on a titrated assay against the standard’s own method and its own conformity criterion. Write both into the purchase order: the receiving check, and what settles the invoice.

We were not able to find a published figure for how much a hydrometer reading on hydrochloric acid shifts per degree of temperature, and we are not going to invent one. That gap is itself part of the argument: if you cannot state the temperature correction, you cannot settle a weight on a density reading taken on a tanker that has been standing in the sun.

Fuming: what the numbers actually say

There is no threshold

The language of the trade suggests a switch — that acid above some strength “fumes” and below it does not, and that the Indian standard’s own description of the two higher-purity grades as “fuming” marks the line. There is no switch. The partial pressure of hydrogen chloride above the liquid is a smooth and steeply rising function of both strength and temperature, with no discontinuity anywhere in the commercial range.

The authoritative measurements are old and good. A United States Bureau of Mines report of 1980 tabulates partial pressures of gaseous HCl over aqueous hydrochloric acid, taking its 0 to 50 °C values from a 1956 paper in Industrial and Engineering Chemistry which remains the canonical primary measurement. Converted from the molality basis it is tabulated on to weight per cent, it says this:

Read the columns, then read the rows. Along a row, warming the acid by thirty degrees multiplies the partial pressure by about six — nearly seven at the bottom of the band, nearer five and a half at the top. Down a column, adding not quite two percentage points of strength roughly doubles it. Temperature beats strength, and it is not close. A 30% acid at 50 °C fumes nearly three times harder than a 33% acid at 20 °C — 80 mm Hg against something nearer 30, interpolating between the tabulated rows, since 33% is not itself a tabulated strength. For a tank standing in a Gujarat or Tamil Nadu summer, where tank-top temperatures in the forties are ordinary, shading, tank colour, vent design and where the tank sits are worth more than the choice between 30% and 33%.

And both strengths are an immediate-danger atmosphere in the headspace

This is the part that should settle the question for anyone treating 30% as the safer purchase. At 20 °C, a partial pressure of 11.75 mm Hg corresponds to a headspace of roughly 15,000 parts per million of hydrogen chloride. The Second Schedule to the Factories Act, 1948 sets the Indian workplace limit for hydrogen chloride at 5 parts per million as a ceiling — a value not to be exceeded at any time — and the United States National Institute for Occupational Safety and Health puts the concentration immediately dangerous to life or health at 50 parts per million.

So the headspace above a 30% tank is already some three thousand times the Indian workplace ceiling, and some hundreds of times the immediately-dangerous level. Thirty-three per cent roughly doubles that. Doubling a number that is already three hundred times the immediately-dangerous concentration does not change what the hatch is. Opening the hatch on either strength without breathing apparatus is the same decision, and it is the wrong one. The strength difference is a rate-of-loss and corrosion-of-surroundings question, not a safe-versus-unsafe switch.

Two honest limits on those figures. The partial-pressure table was read through a scanned 1980 document whose own optical character layer the reader warned was error-prone, and the conversion from its molality basis to weight per cent is our arithmetic, as is the conversion from millimetres of mercury to parts per million at one atmosphere. The qualitative findings — no threshold, temperature dominating strength, both strengths giving a dangerous headspace — are robust to any plausible reading error. Use the figures as orders of magnitude, not as design data.

The vapour pressure on the safety data sheet is probably not your acid

A specific and expensive trap. Safety data sheets for hydrochloric acid are routinely written to cover a range of concentrations — one producer’s sheet covers 31% to 36.7% — and the physical properties in section 9 of such a sheet are quoted at the worst case in the range. That sheet gives a vapour pressure of 84 mm Hg at 20 °C, which corresponds to roughly the 36% end of its own range. The total vapour pressure over 32% acid at the same temperature is about a third of that.

So section 9 of a range-covering safety data sheet is not a physical-property source for the concentration you actually bought. If a number matters, get it from a concentration-specific source or from the producer in writing for the grade on the delivery note. The same caution applies to anything you find on an aggregator page, and to the one other widely copied vapour-pressure table for hydrochloric acid, which disagrees with the Bureau of Mines measurements by about an order of magnitude and additionally quotes a Reid vapour pressure — a petroleum volatility test that has no meaning for an aqueous acid.

A related and more serious habit: a great deal of published material about “hydrochloric acid” is actually about hydrogen chloride, the anhydrous liquefied gas, which is a different article of commerce in a different package under a different transport entry. The international chemical safety card numbered 0163, published under the International Programme on Chemical Safety, is the card for hydrogen chloride: its physical data are the anhydrous gas’s, and it notes the aqueous solution separately. To its credit that card keeps the two apart, carrying UN 1050 and class 2.3 for the gas and a separate entry of UN 1789, class 8, packing group II or III for the solution. The confusion lives in the secondary material that copies the gas figures across and drops the solution entry — which is why you will see a boiling point of minus 85 °C and a vapour pressure of forty atmospheres attached to liquid acid. A tanker or carboy of 30–33% acid is UN 1789, class 8, with the packing group assigned on the corrosivity test criteria rather than on concentration — producer safety data sheets for this strength give packing group II, while the card itself says II or III without resolving it. Check which substance a figure is about before you use it.

Why the band sits where it does

Worth understanding because it explains what you can and cannot ask a supplier for.

You cannot distil acid up to strength. The azeotrope runs the other way.

Hydrochloric acid and water form a maximum-boiling azeotrope at about 20.2% HCl by mass, boiling at about 108.6 °C at one atmosphere on the figure most widely published, though a peer-reviewed compilation we read gives 110 °C. The degree and a half does not matter to the point. Because the azeotrope is at a maximum rather than a minimum, distillation drives any commercial-strength acid down towards 20%, not up. A National Academy of Sciences laboratory safety summary puts it plainly: concentrated acid evolves hydrogen chloride from about 60 °C, leaving the constant-boiling composition behind.

So every strength above about 20% exists only because hydrogen chloride gas was absorbed into water. Anyone who tells you spent acid can be boiled back up to 32% in a simple still is wrong; that needs cooled or isothermal absorption, or an extractive route, not evaporation.

The upper end is set by three things, and none of them is winter

• The partial pressure, which is the table above. Above about 32% the fuming rate starts to rise sharply, and with it product loss, corrosion of everything near the tank, and the scrubber duty.
• How the acid is made. An adiabatic absorber heats up as it absorbs, and that temperature rise is what limits the concentration it can reach; an equipment maker’s own account of the process notes that strong hydrochloric acid has a rather low boiling point and gives 33% by weight as the worked example of what an adiabatic column with an external cooler produces. Thirty-three per cent is, in effect, what the common process makes.
• Solubility. Hydrogen chloride saturates water at roughly 38 to 40% by mass at Indian ambient temperatures — two independent sources bracket it there. Commercial 36% acid is already close to the ceiling, which is why nothing above about 37% is traded in bulk.

What does not explain the band is cold. You will find Indian copy explaining the 30–33% range by winter crystallisation, and we could find no source putting the freezing point of acid in the high twenties to high thirties anywhere above about minus 25 °C. Cold crystallisation is not an operative constraint anywhere in industrial India. It is a northern-hemisphere logistics concern that has migrated into material where it does not belong.

Two gaps we will not paper over. First, we found no reliable strength-by-strength freezing-point table for commercial hydrochloric acid — the safety data sheets we could read disagree with each other by more than ten degrees and one of them prints a freezing point that is also a melting point and spans eighty-four degrees across both signs, which is a corrupted field rather than data. So we state only that it is well below any Indian ambient, and publish no table. Second, the lower end of the band has no published explanation that we could find; the commercial logic is that below about 30% you are paying freight on an increasing proportion of water, and we offer that as reasoning rather than as a sourced fact.

And is 33% even a grade?

An awkward question for an article with this title, so we will answer it. We could not find a single Indian standard, Indian producer specification or Indian tender that names 33% as a grade. IS 265 has no such tier. One large Indian chlor-alkali producer advertises a 30–35% band without numbers. The 33% figure appears in equipment literature as what an adiabatic absorber makes.

The honest conclusion is that “30 versus 33” is a market convention rather than a pair of specified grades, which is the deepest reason the question is the wrong one. Ask for a minimum strength with a tolerance, a grade from the standard for the impurity ceilings, and a price on contained HCl. Three specified things in place of one unspecified one.

Tank, lining and gasket material: where the real risk is

If the strength question is largely a distraction, this is not. Hydrochloric acid is among the most aggressive commodity chemicals towards ordinary engineering materials, and the consequences of getting it wrong run from a leaking flange to a fatality.

One structural caveat that governs this whole section, and that most writing on the subject conceals. Almost all published alloy corrosion data for hydrochloric acid stops at 20% by weight, because the standard laboratory flask-and-condenser test is not valid above the azeotrope — one major alloy producer states that limitation explicitly in its own literature. So for 30–33% acid, hard published metal data is genuinely thin, and any confident alloy limit you are quoted for this concentration band is an extrapolation. Everything below is attributed, and where we have no attribution we say so instead of estimating.

Steel is out, and stainless steel is out, including the grades people reach for

Carbon steel is not a candidate at any commercial strength. One alloy producer’s published corrosion table rates carbon steel in 30–37% hydrochloric acid at 20 °C in its worst category, which its own key defines as a corrosion rate above 1 millimetre per year — “material not usable”. An acid producer’s handbook puts it in a sentence: hydrochloric acid is extremely corrosive to carbon steel, stainless steel, nickel, Monel, bronze, brass, copper, Inconel and aluminium.

And the hazard is not only the hole. Where acid reaches metal it generates hydrogen, which is flammable. Both major acid producers’ literature frames this as the consequence of a lining failure: if the lining of a tank car or a storage tank fails, the acid attacks the shell and evolves hydrogen, and one of them notes that prolonged metal contact can generate enough to support combustion. It is why smoking is prohibited in unloading areas for a non-combustible liquid. A lining failure is a fire risk as well as a leak risk, and that is the single best argument for taking lining inspection seriously.

On stainless steel the published data is unusually emphatic, and it defeats the usual escalation. In the same producer table at 30–37% acid and 20 °C, every grade listed sits in the unusable category — including 317L and EN 1.4439, both of which carry more molybdenum than 316L, and 904L, a six-per-cent-molybdenum superaustenitic. The table gives no rating at all for the higher superaustenitics and duplexes at that concentration. Separately, a glass producer’s own published immersion comparison put 304 and 316L in the “severe damage after four hours” group; and in an accelerated chloride-cracking screen in boiling magnesium chloride, one alloy producer reports 316L cracking in two hours — a different mechanism from general attack in strong acid, cited here as evidence about chlorides and stainless steel rather than as a second reading of the same test.

The correct position is an absence of support rather than a demonstrated universal, and it points the same way: no stainless grade in that published table is rated usable at this concentration even at 20 °C, and the higher superaustenitics and the duplexes are not rated at all — so there is no published basis on which to select any of them for this duty. Reaching for a higher stainless grade is not an escalation path here; it is an unrated material at a higher price. If a fabricator proposes one, ask for the producer’s own data at your concentration and temperature and see what arrives.

The ladder that does exist

In ascending order of capability and cost, the materials that are actually used are non-metallic or lined construction first, then the nickel-molybdenum alloys, then zirconium, then tantalum. Two notes on the metals, both of which are warnings rather than recommendations.

• The nickel-molybdenum alloys are the top wrought choice for pure acid, and the producer’s published data stops at 20%. Its own statement is that its best-performing alloy resists pure hydrochloric acid at all temperatures to boiling in the 0 to 20% range, and its rate table stops there. It also warns that oxidising impurities such as ferric and cupric ions negatively affect these alloys, and that field tests are encouraged before industrial use. Any temperature limit quoted to you for these alloys at 30–33% is extrapolated beyond the published data.
• Zirconium is described by its producer as resistant to hydrochloric acid at all concentrations — and that sentence is a trap in a real plant. The same producer’s guide records that ferric and cupric ions break down zirconium’s protective oxide, with effects appearing at as little as 50 parts per million of ferric iron; that below 20% acid the attack is pitting while above 20% it is intergranular; and that it observed stress corrosion cracking in 32% acid with 500 ppm ferric iron at both 30 °C and 77 °C. Thirty-two per cent is exactly this article’s band. Iron arrives from a corroding upstream component or from particles embedded by a carbon-steel tool during maintenance. Zirconium’s resistance is conditional on the acid staying clean, and this is where the grade’s iron ceiling stops being a paper specification.

We found no producer corrosion chart for tantalum in hydrochloric acid and are therefore quoting no limit for it. The widely repeated figure for tantalum resisting the acid to about 190 °C at all concentrations is one we could not verify. What is published, from the zirconium producer’s comparative remarks, is that tantalum is the more expensive option, about three times the density, and more susceptible to hydrogen embrittlement than zirconium.

Rubber-lined steel, and the temperature number people get wrong

The Indian standard itself says bulk supply of hydrochloric acid is made in rubber-lined mild steel road tankers, and rubber-lined steel is the mainstream answer for large storage. An acid producer’s handbook specifies it concretely: welded steel tank to the relevant storage-tank standard, with an acid-resistant soft rubber lining of at least three-sixteenths of an inch — about 4.8 mm.

Now the trap, and it is a twenty-degree trap. The same handbook says the lining should withstand a maximum of 160 °F, which is 71 °C. A lining producer’s technical bulletin for a chlorobutyl-and-PVC soft lining sold explicitly for hydrochloric acid tankers and tanks states excellent resistance to concentrated hydrochloric acid up to 38% at up to 60 °C — even though the product’s general temperature range is quoted as minus 30 to plus 80 °C. A graphite-filled hard rubber lining from the same producer quotes a product range of minus 15 to plus 100 °C and gives no acid-specific figure at all.

A lining’s product temperature range and its hydrochloric acid temperature limit are different numbers, and the acid number is lower. Ask for the acid-specific figure, at your concentration, in writing. All of these bulletins add that the figures depend on the actual duty and can vary.

On inspection, the relevant practice is the standard for discontinuity or holiday testing of non-conductive coatings on metallic substrates. It divides methods by thickness: low-voltage wet sponge up to half a millimetre, high-voltage spark above it. A 4.8 mm rubber lining is firmly in high-voltage territory. But the test voltage is a calculated number, derived from total film thickness, dielectric strength and atmospheric conditions, and the standard warns that high-voltage testing damages coatings thinner than half a millimetre and requires the coating manufacturer’s approval in the margin below it. We are deliberately publishing no voltage figure: a guessed voltage punctures the lining you are inspecting. Take it from the standard and the lining manufacturer.

Two things nobody has published, and which you should treat with suspicion when quoted. A realistic service life in years for a rubber lining in hydrochloric acid — we found no published figure; the twenty-year figure in circulation is one producer’s statement about fibre-reinforced plastic tanks, not about rubber lining. And what fails first in such a lining — a blister at a weld seam, delamination at the adhesive, or permeation through to the steel — which no primary source we found states. Any confident number or mechanism you are offered for either is somebody’s experience presented as data, which may still be worth having, but is not the same thing.

Fibre-reinforced plastic: the resin matters, and so does the glass

Reinforced plastic is the other mainstream tank, and it has a specific and well-documented failure mode: hydrochloric acid attacks the glass. The same glass producer immersion study that damaged 304 and 316L in four hours put ordinary E-glass in the same severe-damage group, while a boron-free corrosion-resistant E-glass showed no damage.

That is why the construction requirements, not the resin name, are what to specify. A major vinyl-ester resin producer’s own corrosion-resistance guide, in its rows for hydrochloric acid up to 33% and for a 31% acid cleaner, carries footnotes requiring a double surfacing veil and a corrosion barrier of at least 5 mm, a double C-veil in that barrier (corrosion-resistant E-glass veil also permitted), and acid-resistant glass in the corrosion liner. Its guide notes in terms that strongly diffusing media such as hydrochloric acid need special precautions, and that thicker, better-cured liners resist diffusion for longer. Some grades in those same rows are marked not recommended at any temperature.

The governing construction standards come at the same risk from a different direction, and with different numbers — they specify the laminate rather than conditioning a resin rating, so do not read the two sets of figures as one. The American specification for contact-moulded corrosion-resistant reinforced tanks requires the inner surface to be a resin-rich layer a quarter to half a millimetre thick with usually less than 20% reinforcement by weight, caps the combined inner liner and interior layer at 27 plus or minus 5% glass by weight, and requires an organic-fibre surface mat in environments that attack glass.

What both sources agree on is the direction: for the corrosion barrier, the instinct that more glass makes a better tank is exactly wrong. Glass is the thing the acid attacks; the barrier’s entire job is to keep acid away from it. A glass-rich inner surface is a defect.

And do not read the structural temperature limit as a chemical one. Those construction standards say that service above 180 °F, about 82 °C, needs special design consideration — that is a design-review threshold for the laminate. The chemical limits in the resin producer’s hydrochloric acid rows are considerably lower, in the region of 65 to 80 °C depending on grade, and the guide states that even those are the highest temperatures at which equipment has given good service or passed a particular test method, and are “not necessarily maximum service temperatures”.

An Indian buyer specifying a reinforced-plastic acid tank has to call up a foreign standard, and should know it. We could not find an Indian Standard covering fibre-reinforced plastic tanks for chemical service. The nearest Indian standard by name, IS 14399, is for hot press-moulded sectional water storage tanks in E-glass, with no corrosion barrier or chemical-resistance requirements at all. Do not let a fabricator cite it for an acid tank. Call up the American contact-moulded or filament-wound specification, or the European standard for above-ground reinforced-plastic tanks and vessels, and specify the barrier thickness, the veil and the glass type explicitly. We could not prove no Indian standard exists — only that we could not find one.

Thermoplastics, and the rating that is true but misleading

Published producer tables give: high-density polyethylene rated resistant to 30% aqueous acid at 20, 40 and 60 °C; unplasticised PVC rated generally resistant at 23 and at 60 °C, 60 being the highest column its guide carries; chlorinated PVC rated recommended for 36% concentrated acid across columns well above that.

The trap is the duty, not the chemistry. The acid producer’s own handbook says that thermoplastic piping — PVC, CPVC, PVDF, PFA — is used mainly for tank vents and other low-stress applications, and that lined steel pipe is often preferred for duty because it is structurally rigid. The other producer notes that thermoplastic tanks are usually limited in size, are prone to over-pressure and vacuum damage, puncture and ultraviolet degradation, and that their lifetime is highly variable and generally shorter than reinforced plastic. Specifying PVC for a pumped acid header on the strength of a compatibility rating is the mistake: the rating describes the polymer against the chemical, not the pipe against the pressure.

We are publishing no figure for polypropylene, PVDF or PTFE in 30–33% acid, because we could not obtain a producer’s own chart for any of them at this concentration — one is gated, one rates only by chemical class, and one we could not reach. All three appear in acid producers’ qualitative lists of suitable liner and valve materials, which is not the same as a rating. We also found no primary source at all on whether hydrochloric acid permeates high-density polyethylene in a way that matters to a steel outer shell or external hardware; the claim is widely repeated and we could not verify it. Ask the tank manufacturer for its own chart, at your concentration and your temperature.

Gaskets: the premium elastomer is not the answer

A clean and slightly counter-intuitive finding. In a major gasket producer’s own published chemical-resistance chart, the hydrochloric acid row splits by product family: its PTFE-based sheet styles are rated suitable, with little or no adverse effect, while its elastomer-bound compressed fibre sheets are rated not suitable, with moderate to severe adverse effect.

On the elastomers themselves, a pipe producer’s seal table for 30% aqueous acid at 20 °C rates EPDM best — little or no effect — with fluoroelastomer (the Viton family) and nitrile both a category worse, at a volume change of 10 to 20% and possibly suitable for static service only. The acid producer’s own gasket list is “Teflon envelope, Hycar rubber, ethylene-propylene rubber, or vinyl materials”. Fluoroelastomer is not on it.

So the default instinct — use the premium fluoroelastomer, it must be best for acid — is not what either producer’s data points to. Note the limit of that, though: absence from a recommendation list is not a rating, and the two sources do not agree on the elastomers — Hycar is a trade-name family best known for nitrile rubber, so on that reading the acid producer recommends for flange service an elastomer the pipe producer rates as marginal and static-only. What both support is PTFE-based sheet for a flange. On the elastomer ranking behind it, EPDM is the one that comes out best in the only table we could trace, and the disagreement is a reason to ask your gasket supplier for its own data at your concentration and temperature rather than to settle it from here. One important caveat on duty: fluoroelastomer, natural rubber, neoprene, butyl and the chlorosulphonated polyethylenes do appear in the acid producer’s list of materials with good resistance for hoses, which is a dynamic duty with a different failure mode. Do not move a hose rating onto a static seal, or the reverse.

And one thing you may not need at all: on lined steel pipework, the acid producer’s handbook says a separate gasket may not be wanted at all at flanged joints, because the liner itself acts as the gasket mating material. Adding a sheet gasket where two liner flares are meant to seal against each other, and then torquing to the sheet gasket’s figure, is how joints get over-compressed.

Bolt torque, which has killed someone

This deserves its own heading because the mechanism is so easy to reproduce. In November 2020 at a polysilicon plant in Tennessee, contractor pipefitters torqued the flange bolts on a hydrogen chloride heat-exchanger outlet to roughly two and two-thirds times the manufacturer’s figure for the graphite connection they were working on — applying instead the manual’s value for a PTFE-to-PTFE joint, a pairing the manual covered and the graphite joint it did not. The graphite nozzle fractured in brittle overload and released gaseous hydrogen chloride. One worker died escaping the release, two were seriously injured, an apprentice suffered chemical burns.

The investigating board’s findings are about procedure, not about graphite: there were no detailed, job-specific torquing procedures; the instructions came from an equipment manual covering only one material pairing and were relayed verbally through three people; the term “hot torque” was undefined and used inconsistently; and torquing on live hazardous equipment was not treated as a line break with a risk assessment before the permit was issued. Its recommendation was to develop detailed torquing procedures showing differing specifications visually and defining terms such as hot torque.

We are not printing a torque figure anywhere in this article, including the two in that incident, and you should distrust any article that does. On acid service the figure is material- and joint-specific, it comes from the component manufacturer, it belongs in a written job-specific procedure, and using a real documented figure for the wrong material is itself the release mechanism. That incident was in gaseous hydrogen chloride service rather than 30–33% liquid, which we say for accuracy — and it is why we have named it as hydrogen chloride service above. The transferable lesson is exact.

Valves and pumps: the published failure modes

From an acid producer’s own equipment guidance, because these are specific and actionable. Lined ball valves — PFA- or PTFE-lined steel, or PVC and CPVC bodies — for shut-off. Butterfly and wafer-sphere valves are not recommended where tight shut-off is required, because acid leaks through worn disc-to-liner interfaces; the guidance names tank shut-off, loading and unloading shut-off, and tank dike valves specifically as places to avoid them, leaving butterfly valves for flow throttling. On pumps, seal-less magnetically driven pumps are preferred, with the warnings that they fail if run dry and that dead-heading ruins them; where a mechanical seal is used, the guidance is double seals, and shrouding the seal and shaft area because it is a product-exposure point.

Venting: the quiet way an acid tank is destroyed

If one section of this article is worth passing to whoever receives your tankers, it is this one. An acid producer’s technical bulletin on venting hydrochloric acid tanks states the requirement plainly: all such tanks must be properly vented so as not to exceed the pressure or vacuum rating of the tank, and a pressure and vacuum relief device should be fitted in addition to the vent. Both directions are failure modes. Filling raises the pressure; withdrawal without adequate venting pulls a partial vacuum that can crack or collapse the tank. The other producer lists the absence of a pressure and vacuum relief device, “especially for HCl”, among its causes of tank failure.

The number that makes this concrete: unloading a road tanker by air pressure produces an air surge of about 1,350 standard cubic feet per minute at the end of the transfer. That figure comes with a stated basis — a two-inch hose twenty-five feet long, a maximum of 30 psig in the trailer or tank car, an eight-and-a-half-foot tank, and a vertical vent line no more than three feet long. Hold that number next to the pressure rating of your own tank. The bulletin does go on to give vent diameters, and the important thing about them is how they are derived: the diameter is set by the tank’s own design pressure, so that the surge does not exceed it, and the design pressures it works through span a factor of twenty-five, from one inch of water column to twenty-five. We are deliberately not reproducing that table. Its diameters are valid only on its own assumptions and only against the tank rating each one is paired with, and a diameter lifted out of it without both is exactly how a tank ends up under-vented.

A published vent diameter is useless without the assumptions behind it, which is why we have not reproduced one. A larger unloading line, a longer vent line, or a tie-in to a scrubber all require a larger vent, and a vent connected to a scrubber additionally requires a separate pressure and vacuum relief device on the tank — a scrubber-connected vent is a restriction, not a free improvement. Anyone who takes a diameter out of a table as a universal answer, and has a longer vent run or a bigger unloading line, will be under-vented. Have the vent and the relief device sized against your own arrangement, by someone competent to do it, and against your tank’s actual rating.

Two further design points from the second producer’s guidance, both of which are commonly got wrong. A vent must not double as an overflow, and a non-metallic tank’s vent should be at least twice the inlet pipe diameter. An overflow should be at least one and a half times the inlet diameter, side-wall mounted, typically set to relieve at 90% of tank volume, discharging into containment without splashing, and with a vapour trap in the overflow line. Its list of improper practices includes over-pressurising the tank and using excess pad gas during unloading. For a reinforced-plastic or thermoplastic tank, which often cannot withstand either pressure or vacuum, under-venting during an air-pressure transfer is a tank-rupture scenario, not a nuisance.

Scrubbers

Water is the commonest scrubbing medium for hydrochloric acid — cheap, reusable, and the weak acid it makes has a use. A recirculating scrubber solution builds strength to a maximum of about 15% HCl, at which point the weak acid is drawn off for use or for waste treatment and the tank recharged with fresh water. Alkaline scrubbing, usually caustic soda, needs lined construction and dilute solution, generates heat and solids, needs monitoring for sparger blockage and produces neutralised material requiring disposal. On efficiency, one producer’s handbook gives recirculating scrubbers at roughly 50 to 60% and packed columns at 99% or better.

The practical advice in both producers’ guidance is the same: have the scrubber designed by a qualified engineering service, add capacity for the tanker or rail-car blowdown if you offload with air, test the solution frequently, inspect it before operation, check for obstructions that cause back-pressure, and keep a written inspection and maintenance plan.

Receiving a tanker, and the two incidents everyone should know

Who verifies the connection

There is a folk rule that the driver connects the hose because it is the driver’s job. The published position after a mass-casualty incident is the opposite of that.

In October 2016 at a processing plant in Kansas, a driver connected a 30% sulphuric acid delivery hose to a sodium hypochlorite fill line. About 4,000 gallons of acid went into a tank that was 90% full of 12.5% hypochlorite. The reaction ran for about forty-five minutes, venting a chlorine-bearing cloud through a three-inch vent and an eighteen-inch manway. Over 140 people sought medical attention, some were hospitalised, and shelter-in-place and evacuation orders were issued. The two fill lines were eighteen inches apart, looked similar, were not clearly marked, and the hypochlorite line was unlocked.

The investigating board’s published guidance is specific, and it is the citable answer to the question:

• Both the facility’s own personnel and the driver should verify a correct connection before any transfer begins, and facility personnel should be physically present during deliveries, with both parties monitoring the unloading.
• Pipe markings should be accurate and legible, placed as close to the fill line as possible.
• Fill lines should be locked after every delivery, and unloading procedures should include measures to ensure it.
• Use uniquely shaped and colour-coded fittings for each chemical or class of chemicals, and design fill lines so that a given hose can only connect to a fitting mated to receive it.
• Physically separate or space the fill lines, and consider interlocks and other mitigation during unloading.
• Work with your chemical distributor to run a risk assessment and agree the unloading procedure, with responsibilities clearly defined, taking account of how operators and drivers actually interact with the equipment.

That last point is the one a buyer can act on this week, and it is a reasonable thing to ask of any supplier.

The rest of the unloading discipline

From an acid producer’s handbook, the points that recur. Pump unloading produces less vapour than air pressure, avoids depressurising the vehicle, reduces hose drainage and is generally faster. Where air is used, the pressure must not exceed the lower of the vehicle’s own rated working pressure and the figure in the unloading procedure agreed with your supplier — one producer’s handbook gives 30 psig as its own ceiling, for its own fleet, as an assumption inside its own venting calculation. We found no published pressure rating for Indian rubber-lined mild steel road tankers, so take that figure from the vehicle’s own nameplate rather than from any article. Never remove the blind flange on a discharge pipe until the vehicle is fully vented and depressurised. Before transfer, verify that the storage tank is vented to a properly sized scrubber and that it can hold the entire load — the other producer’s rule of thumb is a tank capacity of at least one and a half times the largest incoming delivery. Attend the transfer continuously and, if it is interrupted, stop and close all valves. On pump unloading of a trailer, keep the vapour return open so as not to pull a vacuum on the vehicle. Afterwards, drain and contain residual acid in the hose, wash down spills, and secure all openings. Connections should be rubber-lined or plastic flanges throughout, and inadequate support for the delivery hose and the receiving connector is a listed cause of failure.

One instruction we are not giving you, because we could not source it. Earthing or bonding a hydrochloric acid road tanker before unloading is widely repeated as routine practice, and we found no published requirement for it in normal acid transfer. Hydrochloric acid is not combustible. “Ground all equipment” appears in the emergency-response guidance for a spill or leak. The real and published ignition concern is narrower: if a lining fails, acid attacks the metal and generates hydrogen, which is why smoking is prohibited in unloading areas. If your procedure requires bonding, keep it — we are not telling anyone to remove a control — but understand the hazard it is and is not addressing, and do not let it displace lining inspection.

What must not be stored with it

Hydrochloric acid is a chlor-alkali product, and so is sodium hypochlorite. They are frequently delivered by the same supplier, sometimes on the same day, and they must not share containment, drainage, a transfer pump or a stock of spill absorbent. Hypochlorite on contact with acids produces highly toxic fumes of chlorine gas. The Kansas incident above is what that looks like in practice.

The regulator’s framing of segregation is the useful one, because it goes further than most plants do. Britain’s Health and Safety Executive, in its technical guidance on segregating hazardous materials, asks not only whether incompatible substances are stored apart but whether they could mix within the drainage system, or wherever leaks and spills collect; whether there is an inventory system sorting materials into compatible groups; whether bunding is sufficient for the maximum credible spill volume; and whether hazardous areas sit on impervious ground with an adequate drainage slope. It also names strong acids and cyanide compounds as a pair to keep apart. A shared bund or a shared drain is itself the segregation failure, not merely a contributing factor.

The reaction list from the hazard databases, for the avoidance of doubt. Hydrochloric acid reacts exothermically with organic and inorganic bases; exothermically with carbonates and hydrogen carbonates, generating carbon dioxide; with sulphides, carbides, borides and phosphides to generate toxic or flammable gases; with many metals to generate flammable hydrogen; and violently with oxidants, producing chlorine. Its published violent-reaction list includes acetic anhydride, ammonium hydroxide, calcium phosphide, chlorosulphonic acid, ethylenediamine, oleum, perchloric acid, propylene oxide, sodium hydroxide, vinyl acetate and calcium carbide. And mixtures with concentrated sulphuric acid can evolve hydrogen chloride gas at a dangerous rate — so the two acids a plant is most likely to hold together are themselves a pair to separate.

Where we are relying on a generic source we say so. The hypochlorite hazard is taken from the hypochlorite’s own hazard datasheet, not from the acid’s; the cyanide pair rests on the regulator’s generic segregation statement rather than on any acid-specific document we could find; and neither of the two producer safety data sheets we read mentions hypochlorite, sulphides or cyanides at all. That is itself worth knowing: a safety data sheet’s incompatibility list can be narrower than the real chemistry. Do not treat section 10 of a sheet as a complete segregation matrix.

Spills: the sequence matters more than the reagent

This is the most dangerous paragraph in any article about acid, so read the order carefully. The acid producer’s handbook sets out the sequence, and the reason for it:

1. Containment first. Containment of a spill or leak is, in its words, of primary importance.
2. Then dilute with a water spray. Not because dilution is the objective, but because the reaction between concentrated hydrochloric acid and an alkali “gives off heat and can be quite violent”. The spray also suppresses fuming.
3. Then neutralise. The handbook tabulates quantities of quicklime, hydrated lime, soda ash and caustic soda needed per hundred gallons of acid at a given strength. It does not rank them and does not recommend one.

So “throw soda ash on it” is dangerously incomplete advice, and the reason is a second hazard that rarely gets mentioned: hydrochloric acid reacts exothermically with carbonates and hydrogen carbonates to generate carbon dioxide. Soda ash or bicarbonate on a concentrated pool therefore means heat, plus vigorous gas evolution, plus aerosolised acid, in a confined bund, while the fume is already an inhalation hazard. Note too that sodium bicarbonate is not among the agents that handbook tabulates at all. Anyone entering the spill area needs self-contained breathing apparatus; responders near it need full protective equipment including rubber boots, and should stay upwind.

And the instruction that must never be merged with the one above. The emergency response guide assigned to UN 1789 — which is the guidance for a transport incident, not for a facility spill — says, twice, “do not get water inside containers”, and warns that containers may explode when heated or contaminated with water. Its method for a small spill is dry: cover with dry earth or dry sand and collect with clean non-sparking tools. The water spray in the sequence above is for a pool already contained in a dike at a fixed facility, applied by people in breathing apparatus. These are two different situations and the instructions are not interchangeable. Nobody should ever read either of them as “hose water into a leaking drum or tanker”.

On the bund itself, both producers specify a coating, and the mechanism is ordinary acid attack on cement paste, with carbonate aggregate and carbonated paste as further routes — hydrochloric acid reacting exothermically with carbonates, including building materials containing limestone, is published as a hazard in its own right. Bare concrete bunding is therefore not adequate for acid. The guidance is a concrete dike with an acid-resistant coating, cracks and open expansion joints repaired, residue cleaned up promptly, frequent inspection, a rinse hose available, a safety shower and eyewash, and — a good discipline this — standing liquid in the bund tested before it is discharged anywhere.

Where Indian law actually bites

The regulatory picture for hydrochloric acid in India has four features worth a buyer’s attention, and three widely believed things in it are wrong.

It is a scheduled poison, and the duty is on the seller and separately on you

Hydrochloric acid above 5% by weight is a scheduled poison under the state poisons rules made under the Poisons Act, 1919 in each of the states whose rules we were able to read. In Gujarat it appears in the schedule to the 2014 acid rules, where the relevant entry is item 4, and again at item 10 in different words: item 4 catches the acid “beyond 5% concentration by weight”, while item 10 catches it except where the substance contains less than 5 per cent, so a solution at exactly 5% falls outside one limb and inside the other. The same pair appears in another state’s identically-drafted schedule, so it is in the drafting rather than a transcription error — and it is academic for anyone buying 30% or 33%, which both limbs catch several times over.

The rule-making power under the Act sits with the State Government, and central rules exist only for importation. So there is no pan-India poisons licence: a licence issued in one state authorises nothing in another, and the schedules and thresholds differ from state to state. If you buy into plants in several states, that is a question to put to each supplier separately.

The part buyers most often miss is that the Gujarat rules put duties on the user, not only on the seller. A non-individual user must display a standard operating procedure for security, storage and incident management; must make a named person accountable for possession and safe keeping; must keep the acid under a double lock system; and must keep a usage register and file it with the Sub-Divisional Magistrate every quarter. Those are your obligations as the occupier of the receiving site. No supplier discharges them for you, and we have not found many buyers who know about the quarterly filing.

Two honest limits here. We could not reach an official Gujarat gazette copy of the 2014 rules — what we read was a commercial database reproduction, corroborated against another state’s official-sourced equivalent — and we could not establish whether those rules have themselves been amended since 2014. We verified two states and could not read a third. Nothing in this article should be taken as the position in a state we have not named, and anyone relying on the detail should have the gazette text checked. Separately, the figure of fifty thousand rupees that circulates as the penalty under these rules is misattributed: it comes from a Supreme Court direction relayed by a 2013 Ministry of Home Affairs advisory, it is exercisable by the Sub-Divisional Magistrate against a seller who fails to declare stock within fifteen days, alongside confiscation — and, under a separate paragraph of the same advisory, against any person in breach of any of its own directions, which on the user side cover the usage register filed with the Sub-Divisional Magistrate, a named accountable person and supervised storage. So the exposure is not only the seller’s. The remaining Gujarat duties are rule-made, and breach of a rule carries the Poisons Act penalty rather than this one. The statutory penalty under the Poisons Act for breach of a rule is a much smaller sum that has not been changed since 1919.

The hazardous chemical rules are about the gas, not the solution

This is the most confidently-wrong thing said about hydrochloric acid storage in India, and getting it right changes a plant’s compliance picture.

Under the Manufacture, Storage and Import of Hazardous Chemical Rules, 1989, the entry in the first schedule’s list of hazardous chemicals is printed “Hydrochloric acid (Gas)”, and the threshold-quantity entries in the isolated-storage and major-accident-hazard schedules are for “Hydrogen chloride (liquefied gas)” — anhydrous hydrogen chloride, at 25 tonnes and 250 tonnes. Those figures are not tonnages of 30% acid in a tank farm.

Nor is acid solution swept in by the general criteria, because the first schedule’s own criteria are acute toxicity, flammability and explosives — corrosivity is not among them, and the definition of a hazardous chemical in those Rules is a closed three-limb test against the schedules. So a 30–33% aqueous solution is not pulled in merely for being corrosive.

And where the thresholds do apply, to the liquefied gas, the ladder is commonly misstated too. Read the threshold columns’ own headings and the split is this: 25 tonnes brings everything except the safety report — the general occupier duty, notification of a major accident, site notification and approval, the on-site and off-site emergency plans and the duty to inform persons liable to be affected. It is 250 tonnes that brings the safety report and the safety audit.

The principal notification dates from November 1989 and we traced four amendments, the last in January 2000, with no later amendment appearing in either official state mirror we read. We found no notification repealing or replacing those Rules; the proposed consolidated chemical management rules remain a draft. The two mirrors disagree on which part of the major-accident-hazard schedule the hydrogen chloride entry sits in, so we have cited the entry and the tonnages and not the part.

GST: hydrochloric acid did not move, and a lot of masters say it did

Commercially the most useful fact in this article. In the rate changes effective 22 September 2025, sulphuric acid, nitric acid and ammonia moved from 18% to 5%. Hydrochloric acid did not. Heading 2806, which is where hydrogen chloride and hydrochloric acid sit, does not appear in the new rate notification at all, so the acid falls in the residual entry for all inorganic chemicals at 18%.

And the reason is on the record, which is what makes it safe to rely on. The GST Council’s own release described that measure as a correction of the inverted duty structure in the fertiliser sector, and named those three. Hydrochloric acid is not a fertiliser-sector input and was never in scope. It does not appear anywhere in the release.

So from that date a buyer of hydrochloric acid pays 18% while a buyer of sulphuric acid pays 5%. For a registered buyer with taxable output this is a cash-flow and working-capital difference rather than a cost, because the tax is creditable — the companion article on landed cost sets out the four situations in which it is not. But any price list, ERP tax master or supplier template that moved hydrochloric acid to 5% on 22 September 2025 is wrong, and the error will surface as a credit problem rather than as a pricing one. Ask which HSN code and rate your supplier will invoice under, and check it against the notification rather than against a summary.

Two limits on that paragraph, in the interests of letting you check it. The absence of heading 2806 is a search across the readable text of the rate notification in both its central and integrated versions, which were truncated before their commencement paragraphs; it is strongly corroborated by the Council release and by the residual entry, but it is a negative finding over an incomplete read. And the 22 September date itself comes from the Council release rather than from the notification’s own words — we could not read its commencement paragraph. And note what did not move: fertiliser-grade phosphoric acid is sometimes listed with the three that did, but the Council release does not mention it, and we have not established the rate it moved from. Before you price against a rate, get the notification.

Three things it is not

• It is not under a Quality Control Order. The Department of Chemicals and Petrochemicals’ own list of implemented and extended chemical orders does not include hydrochloric acid. Note that it is not a general exemption for mineral acids — ortho-phosphoric and polyphosphoric acid are both on it. So IS 265 is a voluntary standard for hydrochloric acid, binding only because your contract says so — which is a reason to name the edition and the grade in the purchase order rather than relying on a mark.
• It is not a controlled substance in India. Hydrochloric acid is in Table II of the 1988 United Nations convention against illicit traffic, and appears as such on the international narcotics board’s own list — but India has never notified it under the narcotics controlled substances order, through the 2013 order or any of its amendments up to January 2025. Six of that table’s eight chemicals are in the same position. This is a useful correction to have to hand, because an overseas counterparty’s compliance team may well treat the acid as a controlled precursor on the strength of the convention and be wrong about India.
• It does not need a petroleum or explosives licence as a solution. The gas cylinder rules name hydrogen chloride among obnoxious or poisonous gases and bring licensing and a no objection certificate into play for cylinders of gas. None of that reaches 30–33% acid in a tank or a drum.

Two further Indian points, stated briefly because they are about your site rather than about the purchase. The Second Schedule to the Factories Act, 1948 carries hydrogen chloride at a 5 parts per million ceiling — a value not to be exceeded at any time, with deliberately no short-term average; there is no entry for “hydrochloric acid” as such, and note that the international occupational-hygiene consensus has since moved to a lower figure, so the Indian statutory number is not the most protective one available. And spent acid is a listed hazardous waste in the hands of the user, by reference to the user’s process rather than to the chemical — spent pickling liquor, spent acid from metal surface treatment, from dyes, from pesticides and from ore processing all appear as entries, and there is a further entry for corrosive wastes arising from the use of strong acids and bases, though that one sits under the pulp and paper industry rather than standing as a general catch-all. Buying in packed form creates one too, and it is the entry buyers most often miss: the same schedule lists empty barrels, containers and liners contaminated with hazardous chemicals as a hazardous waste in their own right. So the drum has a disposal cost as well as the acid, and both belong in the cost of the process rather than in a surprise.

One genuinely open question we are not going to pretend to answer. Whether a trader storing and drumming hydrochloric acid in Gujarat needs a consent to establish and operate, and in which category, we could not establish. The central classification list offers both “isolated storage of hazardous chemicals”, which is in the Red category, and “refilling, repacking and blending of chemicals”, which is in the Green — three categories apart, and the list does not resolve which applies to a drum-filling trader. Manufacture of acid is squarely Red. If this matters to your own site, it is a question for the State Pollution Control Board and not for an article.

What a trader should be able to tell you

Velora Chemicals is a bulk supplier and trader sourcing from qualified manufacturers, not a manufacturer. On hydrochloric acid that shapes what we can and cannot usefully offer, and it is better said plainly.

What a trade should be worth on this product: matching the grade to the application rather than to the grade name — which on this product mostly means the iron and free chlorine ceilings, not the strength; telling you the production route, because by-product acid and acid made from pure chlorine and hydrogen are not interchangeable for every duty — and the 2021 standard’s total organic carbon limit is a proxy for the route rather than a determination of it, so the route is something a supplier has to state and the limit something you specify separately; quoting on contained hydrogen chloride so that two offers are comparable; and knowing which of your receiving arrangements the material can actually be delivered into.

What it is not worth: an opinion about your tank lining. We do not design storage, we do not operate a laboratory, and the materials section of this article is assembled from producers’ and standards bodies’ published documents rather than from our own testing. Where the published sources stop — and on this chemical they stop at 20% by weight for most metals — the right answer is the equipment manufacturer’s own data for your concentration and temperature, in writing, and a competent engineer.

We state no licence or registration position for Velora in this article, and we are not going to ask of other suppliers what we have not put in writing ourselves. Hydrochloric acid above 5% by weight is a scheduled poison in the states whose rules we could read, and the licensing duty on a seller is the seller’s. So ask us in the enquiry which states we can supply into and on what basis, and we will answer in writing, along with whether we can supply against your grade, strength and tolerance, delivery point and packing. Ask any supplier the same question, and do not accept a grade name as the answer to it. One further disclosure, since we have told you to settle the weight on a titration: we do not operate a laboratory, so where a contract settles on a titrated assay we will tell you whose laboratory produces it and on what method.

The short version

Stop asking for 30% or 33% and start asking for three things: a minimum strength with a tolerance, a grade from IS 265 : 2021 for the impurity ceilings, and a price on contained HCl. The standard has no 30% or 33% grade, in the current edition or in the 1993 one; its assays are minima and its grades are purity grades. Settle the weight on a titrated assay, not a hydrometer, and remember that a liquid acid may lawfully be declared by weight or by volume, so say which you want. On fuming, temperature beats strength by a wide margin, both strengths give a headspace that is hundreds of times the immediately-dangerous concentration, and the money is in shading, vent design and where the tank sits. On materials, no stainless grade has any published basis for this duty, the glass in a reinforced-plastic tank is what the acid attacks so the corrosion barrier is the specification, a lining’s product temperature range is not its acid temperature limit, PTFE-based gaskets beat the premium elastomers, and the vent and the pressure-vacuum relief device are what stand between an air-pressure delivery and a ruptured tank. In Indian law the only concentration line we could find is 5%, in the poisons schedules, and both strengths clear it six times over; the hazardous chemical thresholds are about the gas and not the solution; and hydrochloric acid stayed at 18% GST when the other acids went to 5%. Everything else is arithmetic on numbers you can check.