Why caustic soda lye is quoted on a 100% NaOH basis

The answer, up front

Caustic soda lye is an aqueous solution of sodium hydroxide, and the water in it is worth nothing. So the trade prices and contracts it by the tonne of 100% NaOH contained — the “dry basis”, “solid basis” or “100% basis” — rather than by the tonne of liquid that actually moves. A quote of “₹X per MT on 100% basis” for 48% lye means you will receive roughly 2.08 tonnes of liquid for every tonne you are billed for.

That single convention is the source of most of the confusion in buying caustic. A price per wet tonne of 48% lye and a price per tonne of caustic soda flakes are not comparable numbers, and neither is a 48% quote against a 50% quote. Put both on a 100% basis first, then compare.

The arithmetic

The conversion is only a division, but it has to be done before anything else.

• Mass of NaOH in a consignment = wet tonnes × concentration. 10 MT of 48% lye contains 4.8 MT NaOH.
• Wet tonnes needed for one tonne of NaOH = 1 ÷ concentration. At 48%, 2.083 MT; at 50%, 2.000 MT; at 32%, 3.125 MT.
• Flakes are a solid. IS 252:2013 specifies 99.50% NaOH minimum on a dry basis, and Indian flakes are traded predominantly at 99% minimum, with 98% a lower traded grade. One tonne of 99% flakes carries 0.99 MT NaOH — close enough to a dry tonne that flakes are often quoted as-is, which is exactly why the two get compared carelessly. Use the assay on your lot, not the round number.

Worked through: if 48% lye is quoted at ₹A per wet tonne and 99% flakes at ₹B per tonne, the lye costs ₹A ÷ 0.48 per tonne of NaOH and the flakes ₹B ÷ 0.99. Only those two numbers belong next to each other.

Why freight breaks the comparison again

Freight is charged on what the tanker actually carries, which is wet tonnes. Every tonne of NaOH delivered as 48% caustic soda lye brings 1.08 tonnes of water along with it, and you pay to move that water. This is why:

• Long-haul and imported lye is almost always shipped at the strongest practical concentration — 48% or 50% — while 32% lye tends to be a local, short-haul or point-of-use dilution.
• A 50% quote that looks marginally dearer than a 48% quote on a 100% basis can still land cheaper once freight is added, because you move 4% less wet tonnage per tonne of NaOH. Freight is billed on the wet tonnage, so 4% is the number that reaches the invoice.
• Flakes carry almost no water at all, which is part of why they travel to distant and smaller consumers while lye serves large plants within tanker reach.

So the cost that matters is the delivered cost per tonne of 100% NaOH: ex-works price on a dry basis, plus freight computed on the wet tonnage, plus handling.

What limits how strong lye can be

You cannot simply buy the most concentrated lye available and save on freight, because caustic soda solution solidifies at ordinary ambient temperatures — and, importantly, not in a straight line with strength. The solidification point tracks the hydrate that separates out, so it rises and falls across the range. Published figures:

So 40% lye freezes at a higher temperature than 50%, and 32% is not freeze-proof either — +7 °C is reached in a north Indian winter. The curve does climb steeply from 45% to 50%, which is why 50% needs insulated and heated or steam-traced tanks, lines and tankers, while 48% — three to four degrees lower — is the common all-India compromise.

If a consignment has partially crystallised in transit, the answer is gentle, uniform heating. Do not heat one spot: localised heating creates trapped liquid pockets behind solid plugs, and the pressure they build can rupture a line or erupt when the plug clears. The solid that separates from 50% liquor is NaOH·2H₂O at about 52.6% NaOH, so the residual liquid gets more dilute, not more concentrated — the hazard is mechanical, not a runaway concentration.

What the COA should tell you

For lye, the assay is the commercial quantity, so the certificate of analysis is not a formality. Expect at minimum:

• NaOH, % w/w — the number the invoice is built on, determined by acid titration.
• Na₂CO₃, % w/w — sodium carbonate formed when the solution picks up carbon dioxide from air (2 NaOH + CO₂ → Na₂CO₃ + H₂O). Every 1% Na₂CO₃ represents about 0.755% NaOH consumed, so a lye stored open or handled badly assays lower. Carbonate is not inert — it is an alkali in its own right and counts in a total-alkalinity titration — but it is a much weaker one and does not substitute for free hydroxide, besides contributing insolubles and scale. This is why IS 252 specifies NaOH and Na₂CO₃ separately, and why you should read both lines.
• NaCl, % w/w — residual chloride from the brine feed. Indian chlor-alkali capacity is now predominantly membrane cell; the mercury-cell plants that preceded it were down to roughly 5% of national capacity by 2011 and were committed to phase-out under CREP. Membrane-cell lye runs low in chloride. Chloride matters wherever it attacks stainless steel or contaminates a product.
• Fe, ppm — iron, which colours the solution and matters in rayon, textiles and some chemical synthesis.

A quote given without a stated basis and a stated assay is not a quote you can compare.

Handling and classification

Caustic soda lye is a Class 8 corrosive. Sodium hydroxide solution ships under UN 1824, Class 8, packing group II or III depending on concentration; solid caustic soda — flakes, pearls, prills — ships under UN 1823, Class 8, packing group II. The packing group follows the concentration, not the mode of transport, and the lot safety data sheet governs. Sodium hydroxide at 5% and above carries the harmonised classification H314, “causes severe skin burns and serious eye damage” — every grade discussed here is well above that — and eye injury from caustic can be permanent. Dilution is strongly exothermic: always add caustic to water, never water to caustic, and never into a closed or restricted vessel.

Carbon steel is the normal material for 50% lye storage at ambient temperature, but caustic stress corrosion cracking — “caustic embrittlement” — sets in from about 46 °C up to boiling in carbon steel under stress. That threshold is lower than people expect: solar gain on an uninsulated tank in an Indian summer will reach it, and so will the exotherm from diluting concentrated lye. Design heated caustic service against the Caustic Soda Service Chart in NACE/AMPP SP0403, and stress-relieve the welds. Aluminium, zinc, tin, lead, galvanised surfaces and zinc-bearing alloys such as brass and bronze are all attacked by caustic and evolve hydrogen; keep them out of the system entirely.

HSN: caustic soda lye 2815 12 00 (sodium hydroxide in aqueous solution); caustic soda flakes 2815 11 10 (solid, flakes — other solid forms fall to 2815 11 90). Confirm the classification on your own invoices with your tax adviser.

Both HSN codes were verified against ITC(HS) and GST tariff mirrors rather than the primary CBIC source, which was not reachable when this article was written. Both match the codes carried on the live caustic product pages.