pH adjusterStrong alkaliConsumed in use

Sodium Hydroxide

Sodium Hydroxide · INCI Sodium Hydroxide

Sodium hydroxide is caustic soda, and on an ingredients list otherwise made of acids it is the only thing pulling the other way. Used as a pH adjuster it goes into the water phase as a dilute solution, meets the citric acid there, and brings the batch up to its specified acidity. Doing that uses it up.

A strong alkali used to reach a mild acidity, consumed in the reaching. What is left is citrate.

Sodium hydroxide, the white deliquescent solid, weighed dry and dissolved to a dilute solution before it meets a formula.

What it is

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 to a dilute solution 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.

The thing to understand about it is that 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 at all. There is only sodium ion and hydroxide ion. The hydroxide is then taken up by whatever acid is present, which here means citric acid, and the sodium stays behind as the counter-ion of the citrate 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.

One consequence is worth carrying, because most of this page rests on it. 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.

What it does in our formulas

Two things, both done to the formula before it is filled, and neither of them done to your skin.

It brings the batch to its target pH

Sodium hydroxide is a strong base, and the job it is given is to meet citric acid in the water phase and take the batch up to the acidity the formula is specified at. It is dosed to an endpoint rather than to a recipe: the correct quantity is whatever titrates that particular batch to target, which for a leave-on at pH 5 to 6 is a fraction of a percent and often well under a tenth of one. The independent expert panel that reviews cosmetic ingredients describes the inorganic hydroxides as functioning in cosmetics primarily as pH adjusters, and the acid-base chemistry underneath is textbook.

Well characterised

It has no buffering of its own, and it makes the thing that has

This is the part that gets inverted. A strong base has no conjugate pair, so sodium hydroxide contributes no buffer capacity whatever; it is the titrant, not the buffer. What it does is convert citric acid step by step into its sodium salts, and those are what hold the pH steady afterwards. Reaching pH 5 to 6 takes roughly 1.7 to 2.2 moles of sodium hydroxide per mole of citric acid, which puts the finished system close to a disodium citrate buffer, generated in the vessel rather than weighed in. Adding sodium hydroxide to citric acid, and weighing in citric acid alongside pre-formed sodium citrate, produce the same solution at the same pH and the same total citrate. Different orders of addition, not different chemistries.

Well characterised

What the research does not show

Sodium hydroxide attracts claims from two opposite directions, and both make the same mistake. One says it is caustic soda and has no business in a moisturiser. The other says it supports your barrier and your microbiome. Each attaches to the ingredient a property that belongs somewhere else.

Marketed as Lye, in a face cream

It is lye, and it is not in the face cream. Sodium hydroxide is ionic, so in water it dissociates completely and there is no molecular sodium hydroxide at any pH, only sodium ion and hydroxide ion. The hydroxide is then consumed neutralising the acid it was added to, and the sodium stays behind as the counter-ion of the citrate. At pH 5.5 the free hydroxide left in solution is around three billionths of a mole per litre, some seven orders of magnitude below the amount charged, and the same charge stirred into plain water would read pH 12.4. Reading 5.5 instead is itself the evidence. That same panel puts the principle plainly: an inorganic hydroxide used as a pH adjuster should be judged on the free hydroxide remaining in the formulation, not on the concentration used.

Marketed as The chemical that burns skin

True, and worth being exact about rather than waving away. Sodium hydroxide is one of the better-characterised irritants in dermatology, largely because researchers use it as a standard irritant to validate their test methods. Under a 48-hour occluded patch, a 2% aqueous solution produced visible reactions in 29% of subjects and a 4% solution produced them in all of them, and a government occupational assessment treats solutions at pH 11.5 and above as corrosive to skin. Every one of those exposures is free alkali at pH 11.5 or above, held under occlusion, usually with the deliberate aim of provoking a reaction so a method could be validated. There is no irritation dataset for sodium hydroxide at pH 5 to 6, because at that pH there is no hydroxide left to test.

Marketed as Barrier-supporting and microbiome-friendly

Those adjectives are attached to sodium hydroxide in consumer ingredient content, and they do not belong to it. No study has tested sodium hydroxide against any skin or microbiome endpoint, and none is likely to, because the question is not a coherent one. What the research examines is finished pH, and whatever an acidic product does it does on account of where its pH sits, not on account of which base was used to get there. The same pH is reachable with a different alkali, or by weighing in the citrate salt and leaving this ingredient out altogether. If there is a claim in any of it, it is a claim about the product and its pH, not about a fraction of a percent of a titrant.

So: an ingredient that arrives as a dilute solution, meets an acid, and is gone. What it leaves behind is a salt and a number. The number is the part that matters, and it is measured on every batch and again across the shelf life, because a finished pH is the only test this ingredient has.

On the label and in the range

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, as the alkali side of the pH adjustment, correcting a batch that has come in below its target acidity.

This is ordinary practice across cosmetics, food and pharmacy, and it is the reason the name turns up on so many ingredient lists elsewhere. Where a formulator adds more than a trace, it is declared. Where it is used to nudge a batch onto its number, what ends up in the finished product is the salt rather than the alkali.

Is it suitable for me?

The independent expert panel that reviews cosmetic ingredients concluded that the inorganic hydroxides are safe in practices of use and concentration other than hair straighteners and depilatories, when formulated to be non-irritating, and its stated approach is to judge a pH adjuster on the free hydroxide left in the formulation rather than on the amount charged. Under UK and EU cosmetic law sodium hydroxide is a restricted ingredient, with different conditions attached to different jobs; the entry covering its use as a pH adjuster outside depilatories sets no percentage ceiling at all, and controls it entirely through the finished pH, which must not exceed 11. A leave-on formulated between pH 5 and 6 clears that by five to six pH units. It is not a contact allergen: a government occupational review looked for occupational cases, diagnostic patch tests and predictive animal tests and found none of them, and assigned it a corrosive notation rather than a sensitising one, so the honest position is that there is no evidence of sensitisation potential and that the mechanism is irritant and corrosive only. For skin that reacts easily, what matters is that the finished pH lands where it is specified to land, which is checked on every batch and again on stability. It contains nothing of animal origin and suits a vegan routine.

Vegan-friendlySensitive-skin suitable

As with anything new, if your skin reacts easily it is worth patch testing first.

The research behind this page

We review the published literature. We do not conduct the studies below.

National Institute for Occupational Safety and Health. Skin Notation Profiles: Sodium Hydroxide (NaOH). DHHS (NIOSH) Publication No. 2011-150, 2011. →
European Commission Joint Research Centre. European Union Risk Assessment Report: Sodium Hydroxide (CAS 1310-73-2). 2008. →
Angelova-Fischer I, Dapic I, Hoek A-K, et al. Skin barrier integrity and natural moisturising factor levels after cumulative dermal exposure to alkaline agents in atopic dermatitis. Acta Derm Venereol 2014;94(6):640-644. →
Regulation (EC) No 1223/2009 on cosmetic products, Annex III, entry 15a (GB assimilated text). →
Burnett CL, Bergfeld WF, Belsito DV, et al. Safety Assessment of Inorganic Hydroxides as Used in Cosmetics. Int J Toxicol 2021;40(2 Suppl):16S-35S. →

Used in manufacture to set batch pH. Not listed as an ingredient in any product. Shop the range