Available chlorine in bleaching powder — how it is measured, and why it falls in storage

The answer, up front

Available chlorine is a measure of oxidising power, not of how much chlorine is in the bag. It is expressed as the mass of elemental chlorine (Cl₂) that would do the same amount of oxidising, as a percentage of the sample mass. A bleaching powder at 34% available chlorine oxidises as much as 340 g of chlorine gas per kilogram of powder — it does not contain 340 g of chlorine atoms.

That distinction is why the number can exceed what a naive reading of the formula suggests, and why it is the only basis on which two offers can be compared. A tonne of powder at 34% delivers 340 kg of chlorine equivalent; a tonne at 30% delivers 300 kg. On the same rupee-per-tonne price the second is 13% more expensive for the job it does, and no amount of negotiating on the tonne rate recovers that.

Where the number comes from

The convention is electron-for-electron. Chlorine gas takes two electrons when it is reduced to chloride:

Cl₂ + 2e⁻ → 2Cl⁻

and the hypochlorite ion takes two electrons for the same journey:

OCl⁻ + 2H⁺ + 2e⁻ → Cl⁻ + H₂O

So one mole of hypochlorite is worth one mole of Cl₂ — 70.90 g — in oxidising terms. Pure calcium hypochlorite, Ca(OCl)₂, has a molar mass of 142.98 g/mol and carries two hypochlorite ions, giving a theoretical available chlorine of 2 × 70.90 ÷ 142.98 = 99.2%. Commercial high-test hypochlorite is sold at 65–70% because it is not the pure salt: it carries calcium chloride, calcium hydroxide, calcium carbonate and water alongside it.

Bleaching powder is not CaOCl₂

The formula CaOCl₂ that appears in school chemistry is a simplification, and it is worth being clear about because it drives a lot of confused enquiries. Bleaching powder — chlorinated lime — is made by passing chlorine over slaked lime, and the product is a mixture, not a single compound. The phases identified in it include calcium hypochlorite Ca(OCl)₂, basic calcium hypochlorite, calcium chloride hydroxide (CaCl(OH)), calcium hydroxide and water of hydration, in proportions that depend on how the material was chlorinated.

That mixture is the reason the commercial grades sit where they do:

“Stable” in stable bleaching powder refers to formulation and bound water content that slow the rate at which the assay falls in ordinary storage. It does not mean inert, and it does not mean stabilised against the self-accelerating decomposition described further down this page. The carriage guidance issued jointly by the International Group of P&I Clubs and CINS is blunt about it: the lower-strength UN 2208 material has a higher likelihood of self-accelerating decomposition than the stronger grades, even though the consequences are smaller. Do not relax warehouse controls because a grade is called stable.

The Indian Standard, and the part number that matters

The Indian Standard is IS 1065, and since 2019 it has been published in two parts: IS 1065 (Part 1) : 2019, Stable Bleaching Powder — Specification, Part 1 Household and Industrial Use, and IS 1065 (Part 2) : 2019, … Part 2 Treatment of Water Intended for Drinking.

If the material is going into a potable supply, specify Part 2 and check the certificate says so. Part 2 is not a relabelled Part 1: it caps lead and arsenic, and the industrial part does not. An unqualified “conforms to IS 1065” on a certificate tells a waterworks buyer nothing about heavy metals. Name the part and the grade or type as well — the 1989 edition set Grade 1 at 34.0% available chlorine minimum and Grade 2 at 32.0% minimum, and a purchase order that says only “IS 1065” has bought you no assay floor at all.

Both parts were confirmed by title from the BIS catalogue, but the full requirement tables of the 2019 editions could not be retrieved when this article was written. The 34.0% / 32.0% grade minima are from IS 1065:1989 and a producer specification sheet. Work from the current standard itself before writing a figure into a contract. BIS certification to IS 1065 is voluntary — there is no Quality Control Order making it mandatory for this product.

How it is actually measured

Available chlorine is determined by iodometric titration, and the method is worth understanding because it tells you what can go wrong. A weighed sample is suspended and diluted, acetic acid and an excess of potassium iodide are added, and the hypochlorite liberates iodine in proportion to its oxidising power:

OCl⁻ + 2I⁻ + 2H⁺ → I₂ + Cl⁻ + H₂O

The liberated iodine is then titrated against standardised sodium thiosulphate, with starch added near the end point so the blue colour discharges sharply:

I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻

Two moles of thiosulphate therefore correspond to one mole of Cl₂ equivalent, so 1 mL of exactly 0.1 N sodium thiosulphate ≡ 3.545 mg of available chlorine. The determination is set out in IS 1065; a result is only comparable against the same method, so read which method the certificate cites.

Where the error usually is

In practice the titration is not the weak link — the sampling is. Bleaching powder is a heterogeneous, caking solid whose strength varies between bags in the same consignment and between the top and bottom of one bag. A single grab sample from an opened drum can read several points away from the lot. Draw a composite from a number of bags across the consignment, mix and quarter it down, and test that. If you and your supplier disagree on a result, the sampling plan is the first thing to compare, not the titration.

Two other practical points: the sample must be weighed and analysed promptly rather than left open on the bench, since it is losing strength while it sits; and the thiosulphate must be freshly standardised, because thiosulphate solutions drift on storage themselves.

Why it drops in storage

Available chlorine is not a fixed property of the material. Three separate reactions spend it, and all three run faster when it is hot, damp or dirty.

1. Carbon dioxide from the air

Atmospheric CO₂ displaces the weaker hypochlorous acid from its calcium salt:

Ca(OCl)₂ + CO₂ + H₂O → CaCO₃ + 2HOCl

The hypochlorous acid then decomposes and escapes. This is why an old bag smells strongly of chlorine when you open it — that smell is product leaving, not a sign of strength. It is also why bags and drums should be kept closed and why bleaching powder stored loose in a humid godown loses faster than the same material in sealed HDPE.

2. Disproportionation to chlorate

Hypochlorite is thermodynamically unstable with respect to chlorate and chloride, and on long standing it converts:

3 OCl⁻ → ClO₃⁻ + 2Cl⁻

The products are calcium chlorate and calcium chloride. Chlorate is not reduced by iodide under the conditions of the titration, so it carries no available chlorine and the assay falls. The reaction is strongly accelerated by heat.

For potable-water duty there is a second reason to care. Chlorate formed as hypochlorite ages is carried into the treated water, and the WHO sets a provisional guideline value of 0.7 mg/L for it, noting that there is no low-cost way to remove chlorate once it has formed. IS 10500:2012 sets no chlorate limit — its only chlorine parameter is a minimum free residual chlorine of 0.2 mg/L at the consumer end, or 0.5 mg/L where protection against viral infection is required — so in India this is a design consideration rather than a compliance limit.

3. Catalytic decomposition to oxygen

Hypochlorite also decomposes straight to chloride and oxygen:

2 OCl⁻ → 2Cl⁻ + O₂

This route is catalysed by transition-metal ions — iron, copper, cobalt, nickel and manganese in particular. Rust from a drum, scale, or dust picked up off a dirty floor is enough to start it, which is a practical argument for lined or plastic containers, clean handling equipment and not decanting into whatever vessel is free.

Taken together these mean the commercial reality: bleaching powder has a shelf life, the certificate has a date on it, and a drum that has been through an Indian summer in an uninsulated shed is not the material the certificate describes. Store it cool, dry, dark and closed, use oldest stock first, and re-test before dosing if it has been held a long time.

The safety problem this creates

The same instability that costs you assay is a serious fire and gas hazard, and it is routinely underestimated because the material looks inert. Calcium hypochlorite is classified corrosive — H314, causes severe skin burns and serious eye damage — together with Ox. Sol. 2 (H272, may intensify fire), Acute Tox. 4 oral (H302), acute inhalation toxicity (H331, toxic if inhaled), and Aquatic Acute 1 (H400, very toxic to aquatic life), with EUH031 (contact with acids liberates toxic gas). It is not merely an irritant dust.

Decomposition begins at around 100 °C and becomes rapid above roughly 175 °C, releasing oxygen and chlorine. Because it is exothermic, a large mass that starts to decompose heats itself and accelerates. There is no single published self-accelerating decomposition temperature for a 33% stable bleaching powder — the carriage guidance works from a critical ambient temperature instead, and sets a control temperature of 10 °C for refrigerated ocean carriage.

Fire — and the two extinguishers you must not reach for

Flood a calcium hypochlorite fire with water. The ERG guide assigned to UN 2208 directs water on small fires and flooding from a distance on large ones, and states explicitly that dry chemicals and foams are not to be used. IS 15738:2007, Stable Bleaching Powder — Code of Safety, says the same: extinguish with copious water, and do not use foam, alcohol foam, carbon tetrachloride, dry ammonium compound extinguishers or carbon dioxide. This matters because the default extinguisher standing in an Indian godown is DCP or CO₂ — which is to say, the two things that are prohibited. Check what is mounted next to your bleaching powder store before you need it.

IS 15738 also sets the storage itself: a non-combustible floor (wooden floors are prohibited), at least one metre clearance from electrical fittings, heat detection, an approved fire hydrant with copious water, and enforced no-smoking.

What it must not touch

• Water and damp are a fire risk, not only an assay risk. A trace of water reaching contaminated material can initiate reaction. In one well-documented warehouse incident, rainwater reaching several hundred drums of solid chlorinating chemical produced three days of explosions and fire and a mass evacuation. Keep it dry, off the floor, and under cover.
• Organic material of any kind — oil, grease, rags, sawdust, paper, brake fluid — and the organic chemicals on the same site: acetone, methanol, isopropyl alcohol, glycerine, phenol, stearic acid. Contamination can start a self-sustaining decomposition that ends in a fire.
• Any acid. Acid liberates chlorine gas immediately. That means all of it — hydrochloric, sulphuric, nitric, phosphoric, acetic, oxalic and citric, and acidic surfactants such as LABSA. Separate bund, not the far end of the same one.
• Other oxidisers and chlorine donors. Hydrogen peroxide reacts violently with hypochlorite, and chlorinated isocyanurates — trichloroisocyanuric acid and the like — can react violently with calcium hypochlorite. Never store the two chlorine chemistries together.
• Ammonia and ammonium salts, including ammonium sulphate fertiliser and some cleaning products: the reaction forms nitrogen trichloride, which is explosive.
• Transition-metal compounds, including copper sulphate — copper is one of the metals that catalyses its decomposition.
• Drains and watercourses. It is very toxic to aquatic life and is a marine pollutant for transport. Spillage goes to a bunded collection point, never to a storm drain.

PPE, exposure and first aid

Handle the dry powder in chemical-resistant gloves, goggles or a face shield, and respiratory protection against the dust — this is a corrosive, not an irritant. There is no OSHA permissible exposure limit, NIOSH recommended limit or ACGIH threshold limit value assigned to calcium hypochlorite itself; the AIHA workplace environmental exposure level sets a 15-minute short-term value of 2 mg/m³. For the decomposition product, IS 15738 cites a chlorine time-weighted average of 1 ppm.

• Eyes — irrigate with running water for 15 to 20 minutes holding the lids apart. Immediate medical attention.
• Skin — remove contaminated clothing and wash for 15 to 20 minutes.
• Inhalation — move to fresh air, keep at rest, give oxygen if trained to. Seek medical assessment even if the casualty seems well: chlorine-induced pulmonary oedema can be delayed by several hours.
• Ingestion — do not induce vomiting. Rinse the mouth, give water or milk to drink if the casualty is conscious, and give no acidic drinks. Medical attention immediately.
• Work to the safety data sheet for the lot you are actually handling, and keep it where the material is stored rather than in a file.

Transport — the entry is read off the lot, not guessed from the assay

Calcium hypochlorite is an oxidising substance, and the UN Model Regulations carry several entries for it. The assay alone does not decide which one applies — form and corrosivity decide it too, and mis-declaring is a detention, fine and insurance problem, not a paperwork one.

Bleaching powder at 32–34% is normally UN 2208. But because calcium hypochlorite carries a harmonised corrosive classification, a dry mixture that meets Class 8 criteria must go as UN 3486, calcium hypochlorite mixture, dry, corrosive — Class 5.1 with subsidiary risk 8, and hydrated material takes UN 2880. Read the entry off the safety data sheet for the lot; do not copy a UN number from a 65% pool-chemical sheet onto a 33% consignment.

Pack size is a carriage variable in its own right. The carriage guidance issued by the International Group of P&I Clubs and CINS recommends a package limit of 45 kg net in drums and a maximum of 14 tonnes net per container for this material. A 50 kg drum is unremarkable for domestic road movement but sits outside that guidance for containerised export, so raise pack size at the enquiry stage if the load is going into a box.

Who carries the duty in India

Road transport of dangerous goods in India runs on the Central Motor Vehicles Rules, 1989, rules 129 to 137. Under rule 131 the consignor must ensure the goods carriage is validly registered to carry the goods, that it is equipped with the necessary first-aid, safety equipment and antidotes, that the transporter has full and adequate information about the load, and that the driver is trained in handling the dangers it poses. The vehicle carries a TREM card in the cab and an emergency information panel showing the technical name, the class label and the consignor's own name and emergency telephone number.

Those are the consignor's duties, not the buyer's. Where Velora consigns the load, they are Velora's; where you collect ex-works or nominate your own transporter, they become yours. Settle which it is in the purchase order, because the emergency panel has to carry somebody's telephone number.

When sodium hypochlorite is the better buy

If the material is going into water treatment and you have dosing equipment, sodium hypochlorite solution is often easier: it is already in solution, so there is no dissolving and no insoluble lime residue to settle out and block a dosing line. It has the same problem in a different shape, though — it also loses available chlorine on storage, faster in heat and light, and it is typically supplied at 10–14% available chlorine, so you are moving a great deal more water per kilogram of chlorine delivered.

Two things to settle before comparing the two. First, the basis of the hypochlorite number: Indian trade quotes it as % available chlorine w/v (grams per 100 mL), as % w/w, or as grams per litre, and those are different numbers — roughly, a 14% available chlorine solution is about 14.7% NaOCl w/w. Ask which basis the offer is on. Second, the dangerous-goods class changes: you move from UN 2208, Class 5.1, PG III to UN 1791, hypochlorite solution, Class 8, PG II — different placarding, different bunding and different storage material (vented HDPE or FRP, not stainless steel). For most sites that is the deciding factor.

Sodium hypochlorite solution above 5% concentration by weight is listed in Schedule I to the Gujarat Poisons (Regulation and Control of Sale and Use of Acid) Rules, 2014. Selling it or holding it for sale in Gujarat requires a licence in Form A, and every sale is recorded against the purchaser's identity in the prescribed registers. Commercial hypochlorite at 10–14% available chlorine is well above that threshold. Bleaching powder itself is not in Schedule I. Confirm the licensing position of any supplier you buy hypochlorite from, including us.

The trade-off is a familiar one: bleaching powder is concentrated and cheap to freight but awkward and slower to lose strength; hypochlorite solution is easy to dose but heavy to move, quicker to fade, and a corrosive rather than an oxidiser. Which is cheaper depends on your haul distance and how fast you turn stock over.

HSN for bleaching powder is 2828 10 10 — in the Indian tariff, “commercial calcium hypochlorite (bleaching paste or powder)”. The six-digit subheading 2828.10 covers commercial calcium hypochlorite and other calcium hypochlorites, and India splits it: 2828 10 10 for the commercial bleaching paste or powder, 2828 10 90 for other calcium hypochlorites. Sodium hypochlorite is not in this subheading — it falls under 2828 90. Cross-check the code against your own purchase invoices with your tax adviser.

HSN verification: services.gst.gov.in and cbic-gst.gov.in were both unreachable when this article was written, so the eight-digit description above is mirror-verified against ITC(HS) databases rather than confirmed against the primary CBIC source.