The free acid value of a glycolic acid body lotion, governed by pH, determines how much of the labelled concentration is biologically active on skin. It is the most overlooked number in AHA formulation.
Concentration is the headline. pH is the verdict. Inside the narrow window where a body lotion's labelled percentage either does its work, or quietly does nothing at all.
The number nobody prints on the bottle
The labelled percentage on a glycolic acid body lotion is the first thing every buyer reads, and the last thing the chemistry actually depends on. A formula at twelve per cent glycolic acid can outperform a formula at fifteen, or it can fall flat against a formula at eight. The variable that decides which way the comparison goes is not concentration. It is pH, and the free acid value pH produces.
Almost no Australian body lotion brand discloses the number. Some publish it inconsistently. Most omit it entirely. The omission is not accidental.
What free acid value actually means
Glycolic acid in solution exists in two forms. The free acid form, which is the active molecule responsible for the chemical loosening of corneocyte junctions at the surface of the skin. And the salt form, in which the acid is bound to a base and becomes far less meaningful for resurfacing performance. The ratio between the two is governed by the formula's pH relative to glycolic acid's pKa of approximately 3.83.
At pH 3.83, approximately half of the glycolic acid is in its active free form. Below that pH, more of it is free. Above it, more of it is neutralised. By the time a formula sits at pH 4.5, only about a quarter of the labelled glycolic acid remains in the active form. By pH 5.0, the figure collapses below ten per cent. The label still reads twelve per cent. The free acid value reads closer to one.
This is the number that decides whether the lotion does anything.
The narrow window between theatre and chemistry
The clinically meaningful pH range for a glycolic acid body lotion intended for daily use sits between approximately 3.5 and 4.5. Below 3.5 the formulation becomes irritating without supervised application, the territory of in-clinic chemical peels rather than at-home body care. Above 4.5 the free acid value falls far enough that the labelled concentration becomes increasingly theatrical. The narrow corridor between these two boundaries is where serious AHA body formulation lives.
Within The Lotion's six-marker clinical body lotion standard, this is marker two: buffered pH within the narrow window where the active remains biologically meaningful. It is the marker most easily verified by chemistry and most easily fabricated by silence.
Why a labelled twelve per cent can do nothing
The exercise is straightforward. Two glycolic acid body lotions sit on the same shelf. Both declare twelve per cent on the front. One is buffered to pH 3.8, with a free acid value of approximately seven per cent. The other sits at pH 4.8, with a free acid value of approximately two per cent. The first will resurface the corneum, loosen the keratin compaction of keratosis pilaris, and support measurable textural change with consistent use. The second will deposit emollient on the surface, feel pleasant for a few hours, and leave the underlying texture largely untouched.
Both formulations are technically twelve per cent glycolic acid. Only one is twelve per cent glycolic acid in any sense that matters to the skin.
The barrier reads pH before it reads concentration
Skin chemistry is more pH-sensitive than the labels suggest. The acid mantle, the skin's own protective film, sits at approximately pH 4.5 to 5.5. A body lotion buffered into the 3.6 to 4.0 range is acidic enough to support the free acid form, which is the form involved in disrupting corneocyte adhesion in the upper stratum corneum, while remaining tolerable on intact body skin when supported by humectants and barrier lipids.
This is where supporting actives earn their place in a serious formula. Urea softens the corneum and supports hydration. Niacinamide steadies the look and feel of the barrier through the exfoliation cycle. Shea butter and lipid emollients restore comfort where acid activity can temporarily increase dryness. A correctly buffered AHA lotion is never a single ingredient doing one thing. It is a chemistry of relationships, calibrated to keep the active in its working window while the skin holds its integrity.
Why so few brands disclose it
Cosmetic regulation in Australia sits across ingredient, chemical introduction and consumer law requirements, including the Australian Industrial Chemicals Introduction Scheme, the Therapeutic Goods Administration where therapeutic claims are made, and the Australian Consumer Law. The result is a category in which concentration is the loud variable and pH is the quiet one. Brands lead with the number that is easy to advertise and stay silent on the number that is harder to defend.
The reason pH is harder to defend is structural. Buffering a high-percentage AHA formula to active pH is more complex than allowing it to drift upward toward neutral. It requires precise formulation, stable preservation systems that hold pH over the product's shelf life, and packaging that does not compromise the acidified vehicle. Disclosure of pH invites the consumer to ask why one twelve per cent formula behaves differently from another. That is exactly the question the category has avoided for too long.
This is the structural reason that concentration alone is not the indicator of efficacy the industry has trained consumers to read it as.
How to read a label for the missing number
The pH of a finished cosmetic is rarely printed on the box, but the chemistry leaves traces. Brands that disclose pH explicitly, somewhere on the product page or technical sheet, have decided the number is defensible. Brands that publish only concentration have decided otherwise.
The Lotion's 12% glycolic acid body lotion formulated to disclosed active pH sits at a buffered range of 3.6 to 4.0, made in Australia under cosmetic GMP, with supporting actives positioned to hold barrier integrity through the resurfacing cycle. The disclosure is not a marketing decision. It is the chemistry being legible.
The point of the missing number
The reason this matters is not pedantry. A consumer who buys a high-percentage glycolic acid body lotion is paying for an outcome on real body skin: keratosis pilaris, strawberry legs, crepey-looking upper arms, persistent textural roughness. The outcome arrives only when the chemistry meets the threshold. Below the threshold, the same money buys the impression of treatment without the substance of it. The labelled percentage looks identical on both sides of the threshold. The free acid value is the difference.
The Lotion publishes its pH because the chemistry permits it. Other brands may have equally defensible numbers. The question worth asking, of any AHA body lotion on the Australian shelf, is the one the label rarely answers on its own.
Frequently asked questions
What pH should a glycolic acid body lotion be?
Between approximately 3.5 and 4.5. Below 3.5 the formula becomes irritating for daily body application without supervised use. Above 4.5 the glycolic acid molecule is increasingly neutralised, and the labelled percentage no longer corresponds to biological activity on skin. Clinical body formulations sit deliberately inside this window.
What is free acid value in AHA formulation?
Free acid value is the percentage of the total acid concentration present in its active free acid form rather than its neutralised salt form. It is governed by the formula's pH relative to the acid's pKa, which for glycolic acid is approximately 3.83. A high-concentration AHA lotion with high pH can have a low free acid value, which means the labelled percentage is not doing the work it implies.
Can a labelled high percentage glycolic acid lotion be ineffective?
Yes. If the formula is not buffered to active pH, much of the labelled glycolic acid is present in its neutralised salt form, which lacks the resurfacing activity of free glycolic acid. Concentration without disclosed pH is an incomplete claim. Two lotions labelled at the same percentage can have free acid values differing by a factor of three or more.
Why don't most brands disclose pH?
Cosmetic regulation does not mandate pH disclosure on the label, and disclosing it invites scrutiny. A buffered, pH-correct formulation is more expensive to manufacture and stabilise, and many brands prefer to lead with concentration claims that are easier to communicate than chemistry that is harder to defend.
Is low pH harmful to the skin barrier?
Body skin tolerates a wider pH range than facial skin. A buffered glycolic acid lotion at pH 3.6 to 4.0 sits within the daily-tolerable range for intact body skin when supported by humectants such as urea and glycerin and barrier lipids such as shea butter. Below pH 3.5 the formula becomes harsh for daily use without supervised application.
What does buffered actually mean on a glycolic acid label?
Buffered means the formula's pH is stabilised within a defined range. A correctly buffered AHA lotion balances biological activity with skin tolerance, holding pH inside the narrow window where the free acid value remains meaningful while the barrier remains intact.
At what pH is glycolic acid no longer working?
By pH 5.0, the free acid value of glycolic acid collapses below approximately ten per cent of the labelled concentration. By pH 5.5, the figure is lower again. The labelled percentage on such a formula remains accurate, but the proportion of acid in biologically active form is small enough that resurfacing outcomes are unlikely.
References
- Kornhauser A, Coelho SG, Hearing VJ. Applications of hydroxy acids: classification, mechanisms, and photoactivity. Clinical, Cosmetic and Investigational Dermatology. 2010;3:135-142.
- Tang SC, Yang JH. Dual effects of alpha-hydroxy acids on the skin. Molecules. 2018;23(4):863.
- Smith WP. Comparative effectiveness of alpha-hydroxy acids on skin properties. International Journal of Cosmetic Science. 1996;18:75-83.
- Yu RJ, Van Scott EJ. Alpha-hydroxyacids and carboxylic acids. Journal of Cosmetic Dermatology. 2004;3:76-87.
- Therapeutic Goods Administration. Cosmetic and therapeutic goods guidance for advertisers and suppliers.
- Australian Industrial Chemicals Introduction Scheme. Cosmetics and soap guidance.
