Lactic acid formula: structure, pH, and lactate
The formula for lactic acid is C₃H₆O₃, but it's the acid-lactate pair that truly dictates what a product does on the skin. Two bottles showing the same percentage can behave oppositely depending on how the formula is buffered. This page describes the molecule and, more importantly, how it is used in a cosmetic product.
Empirical formula, structure, and molar mass
The empirical formula is C₃H₆O₃, and the molar mass is approximately 90.08 g/mol. Three carbon atoms, no more.
Its systematic name is 2-hydroxypropanoic acid. The number 2 indicates the position of the hydroxyl group: on the carbon atom immediately adjacent to the carboxyl group. This is exactly what the prefix alpha means in "alpha-hydroxy acid."
The structure thus carries two functions on a tiny molecule: a carboxyl (–COOH) that provides acidity, and a hydroxyl (–OH) that provides affinity for water. This duality on so few atoms explains its dual behavior, as an exfoliant and a humectant.
For comparison, glycolic acid is 76 g/mol with two carbons, and salicylic acid is 138 g/mol with an aromatic ring. Lactic acid falls between the two, and its intermediate size allows for more measured penetration than glycolic acid.
Two enantiomers: L and D
The central carbon atom has four different substituents: it is asymmetric. The molecule therefore exists in two mirror-image, non-superimposable forms.
The L(+) form is produced by human metabolism and by most lactic acid bacteria used in fermentation. The D(−) form is produced by other strains. An equal mixture of both is called racemic, denoted DL.
The choice of microorganism influences the result: some Lactobacillus strains produce almost exclusively L-lactic acid, while others produce a mixture. Chemical synthesis, however, always yields a racemic mixture.
In cosmetics, the L form is preferred: it is the one the skin recognizes, and most tolerance data relate to it. Unfortunately, a label almost never specifies this.
Acid and lactate are not identical
This distinction governs all formulation.
When the carboxyl group gives up its proton, the molecule becomes the lactate ion. Both forms coexist in solution, in a proportion determined by the pH and the pKa, which is approximately 3.86.
At a pH equal to the pKa, exactly half of the molecules are in their acid form. Below this, the acid dominates; above it, lactate dominates.
| pH | Free acid | Lactate | Dominant behavior |
|---|---|---|---|
| 3.0 | ~88 % | ~12 % | marked exfoliation |
| 3.5 | ~70 % | ~30 % | clear exfoliation |
| 3.86 | 50 % | 50 % | balance |
| 4.5 | ~19 % | ~81 % | hydration, weak exfoliation |
| 5.5 | ~2 % | ~98 % | pure humectant |
Only the free acid form exfoliates. Lactate, on the other hand, is part of the natural moisturizing factor and retains water in the stratum corneum. The same amount of raw material thus produces two radically different effects depending on the pH chosen.
What pH changes in a formula
A formulator does not merely measure a percentage: they choose a pH, and this choice largely determines the product's effect.
The total concentration is indicated on the technical sheet. The free acid fraction, however, is deduced from the pH—and it is this fraction that is active. A 10% product at pH 4.5 delivers approximately 1.9% free acid. A 5% product at pH 3.5 delivers 3.5%. The latter therefore exfoliates more than the former, despite a stated percentage that is twice as low.
This is why the mention of pH on packaging is a sign of a brand that provides useful information. Its absence forces you to guess.
A pH that is too low poses another problem: below 3, irritation becomes probable, and the formula falls outside the realm of routine care. Consumer formulations are typically between 3.5 and 4.5.
Neutralization, buffers, and sodium lactate
To raise the pH of a lactic acid solution, a base is added. The reaction produces a salt: sodium lactate if the base is soda, potassium lactate with potash.
On the INCI list, you will then see two distinct entries: Lactic Acid and Sodium Lactate. Their combined presence indicates a buffer system—an acid-base pair that maintains a stable pH even if the formula changes or if the skin slightly alters the environment.
This buffer has a real benefit: without it, the pH drifts, and the product's activity changes over the months. With it, the behavior remains predictable until the bottle is empty.
A product that lists only Sodium Lactate, without free acid, is not an exfoliant: it is a humectant. It is frequently found in moisturizing creams that have no exfoliating purpose.
Reading a cosmetic formula
Four elements are enough to evaluate a lactic acid product.
- The position of Lactic Acid in the list. Ingredients are listed in decreasing order down to one percent: an acid listed after preservatives is present in trace amounts.
- The declared pH. Between 3.5 and 4, the formula exfoliates without excess. Above 5, it mainly hydrates.
- The presence of Sodium Lactate, which indicates partial neutralization and therefore a deliberately gentler formula.
- Associated active ingredients. Ceramides, glycerin, panthenol, or niacinamide significantly improve tolerance compared to an acid alone in an aqueous base.
Two mentions should raise a red flag. Alcohol denat. at the beginning of the list, which dries out the skin and adds to the acid's effect. And fragrance, the primary cause of intolerance on skin already stressed by an exfoliant.
Stability and compatibility
Lactic acid is chemically stable, which simplifies preservation: it does not oxidize like vitamin C and does not degrade in light like retinol. A transparent bottle therefore does not pose any particular difficulty.
However, it is hygroscopic: it absorbs moisture from the air. A poorly sealed bottle slowly dilutes itself, which changes the concentration over time.
Its formulation compatibility is good with most humectants and emollients. It is poor with ingredients that require a high pH: certain peptides and arbutin degrade in an acidic environment, which is why they are rarely found in the same formula.
On the skin, the issue is not the same as in the bottle. Alternating evenings is enough to avoid incompatibilities of use between an acid and a retinoid. See our exfoliation module.
What the chemical formula doesn't tell you
Knowing C₃H₆O₃ doesn't tell you if a product will be gentle. The pH, vehicle, actual concentration, and associated active ingredients weigh much more heavily than the nature of the molecule.
It also says nothing about its origin. The same empirical formula can come from beet fermentation or chemical synthesis: the molecule is identical, but the process is not.
Finally, the formula does not predict individual tolerance. Two skins exposed to the same product react differently, and only progressive use can reveal this. Our pages on lactic acid and skin pH cover these points.
What the percentage doesn't tell you
Two 10% products won't act the same if one is buffered to a higher pH: it's the concentration-pH combination that matters, rarely the highlighted number. A lactic acid AHA peel combines the acid with a humectant precisely to make this combination sustainable over time.
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