Sodium Hydroxide

Sodium hydroxide is a strong inorganic base. It arrives as a white deliquescent solid, made by passing electricity through brine in what is called the chloralkali process, and it is the same material sold in hardware shops as caustic soda or lye. Most people who have met it before met it in soap making, where it turns fat into soap and the finished bar sits somewhere around pH 9 to 10. That is a different job from this one. As a pH adjuster it is dissolved in water first and stirred into a well-agitated water phase, never added neat, because neat addition creates brief pockets of very high pH that can damage the things around them before the batch has settled.

It does not survive contact with the formula. Sodium hydroxide is ionic, and in water it dissociates completely, which means there is no molecular sodium hydroxide in an aqueous solution at any pH. There is only sodium ion and hydroxide ion. The hydroxide is then taken up by the acid present in the batch, and the sodium stays behind as the counter-ion of the salt that results. At pH 5.5 the free hydroxide left in solution is around three billionths of a mole per litre, roughly seven orders of magnitude below the amount that went in. The same charge stirred into plain water would read pH 12.4. Reading 5.5 instead is itself the evidence that the hydroxide has gone.

Sodium hydroxide is dosed to an endpoint rather than to a recipe. The right amount is whatever brings that particular batch to target, which is why the quantity is small, why it varies, and why the control on it is a measured finished pH rather than a percentage.

Two things keep each bottle sound. The first is the preservative, a ready-made blend of sodium benzoate and gluconolactone. Sodium benzoate does the preserving, and gluconolactone helps it work. The second is the pH. Sodium citrate buffers the cream at pH 5 to 5.5, close to the skin's own. At the end of making, the pH is checked and fine-tuned with a little sodium hydroxide or citric acid.

Sodium hydroxide does not appear on a Biotaderm ingredients list, because it is not added as a formula ingredient. It is used during manufacture, in very small quantities, to fine-tune the pH, raising the pH of a batch that has come in too acidic.

This is ordinary practice across cosmetics, food and pharmacy, and it is the reason the name turns up on so many ingredient lists elsewhere. When it is used to adjust a batch's pH, what ends up in the finished product is a salt rather than the alkali.