Washing soda (sodium carbonate)

What it is

Washing soda is sodium carbonate, the salt that sodium hydroxide becomes when it takes up carbon dioxide. It is sold in two main forms. The anhydrous powder, called soda ash in industry, is almost pure carbonate and slowly draws water from the air. Soda crystals are the decahydrate, whose water of crystallisation is 62.9 % of their weight, as Simmons and Appleton recorded in 1908; they lose water in dry air and crumble to powder. The EU food-additive specification covers the anhydrous form, the monohydrate and the decahydrate under one entry, E 500 (i).

The difference matters when a recipe is written for one form. Only 37 % of the weight of soda crystals is sodium carbonate, so it takes about 2.7 times as much crystal to do the work of the powder. Arm & Hammer's washing soda is sodium carbonate itself, 60 to 100 % by its safety data sheet; anything sold as soda crystals is the decahydrate. Check the label for which you have.

In laundry powder

Soap powders are old: Simmons and Appleton describe them as powdered mixtures of soap with soda ash or soda crystals. Home soapmakers still grate a cured laundry bar and mix it with washing soda. The carbonate earns its place by dealing with hard water: it takes calcium out as insoluble calcium carbonate, which leaves more of the soap free to clean, and detergent makers call it a precipitating builder. It is not a complete cure. A Lever Brothers patent notes that carbonate alone seldom brings calcium down far enough for good cleaning, and that the chalk it forms can settle on the washed fabric.

It also makes the wash strongly alkaline, pH 11.4 by its safety data sheet, which is why the wool industry advises a mild, neutral detergent for wool, not soap and soda. The laundry soap guide gives the bars, the powder and the reasons we leave borax out.

Handling it

Sodium carbonate is far milder than lye, but it is not harmless. Its harmonised EU classification is serious eye irritation, H319, with the exclamation-mark pictogram and the signal word Warning, and a maker's data sheet adds that repeated or prolonged skin contact is likely to irritate and advises gloves and eye protection. Grate and mix where the dust will not blow about, keep the jar labelled and out of reach of children, and rinse an eye with plenty of water if dust gets in.

Why it cannot replace lye

Washing soda is an alkali, and it looks enough like lye that people ask whether it could make soap. It cannot. Simmons and Appleton put it plainly in 1908: the carbonates of soda and potash, though considerably cheaper than the caustic alkalis, "cannot be used in the actual saponification of a neutral fat". Factories used carbonate only on fatty acids that had already been split from their glycerin. Oils and washing soda stirred together give no soap.

Carbonate inside soap is a different matter. Factories added a little to stiffen and harden soap, and Simmons and Appleton warned that too much made it brittle and caused efflorescence, a white bloom on the surface.

Soda ash: washing soda on a fresh bar

The white film that can form on fresh cold process soap is the same chemical. Wherever unreacted lye at the surface meets carbon dioxide in the air, it becomes sodium carbonate: 2 NaOH + CO₂ → Na₂CO₃ + H₂O. It forms in the first day or two, only where air reaches the soap, and it is harmless. Sodium carbonate dissolves readily, 30.7 g in 100 g of water at 25 °C, so a steamer or a quick rinse takes it off; it does not dissolve in alcohol. Covering the mould, keeping the loaf warm and using a little less water make it less likely, and the soda ash guide goes through each.