Concentration & pH: Why Disclosure Matters
Why the labelled percentage of a peel does not predict its potency — free-acid value, pH and buffering do — and why a peel whose full concentration and pH are disclosed can be dosed predictably while an undisclosed formula cannot.
The single most important — and most often misunderstood — fact in peeling is this: the percentage on the label does not tell you how strong the peel is. Two AHA peels both labelled "30% glycolic" can behave like completely different products, because real potency is set by the free-acid value and the pH, not the headline concentration. A peel buffered to a high pH is gentle even at a high percentage; an unbuffered peel at a low pH is aggressive even at a modest one. This is not a pedantic distinction — it is the difference between a peel you can dose predictably and one that surprises you. And it is the reason full disclosure of concentration and pH matters: you cannot titrate what the manufacturer will not show you.
Free acid: the fraction that actually works
An acid in solution exists in two forms: ionised (dissociated, inactive at the skin) and un-ionised (protonated, free — the form that penetrates and does the work). The proportion in the active free form is governed by the relationship between the solution's pH and the acid's pKa:
- At pH equal to the pKa, the acid is half free, half ionised.
- As pH falls below the pKa, the free (active) fraction rises — the peel gets stronger.
- As pH rises above the pKa (e.g. by buffering or partial neutralisation), the free fraction falls — the peel gets milder.
Glycolic and lactic acid have a pKa around 3.8; mandelic around 3.4; salicylic around 3.0. So the same labelled percentage at pH 2 versus pH 4 represents two very different free-acid loads — and therefore two very different peels.
Buffering: why 50% can be gentler than 30%
Buffering (or partial neutralisation) deliberately raises the pH of a peel to lower its free-acid fraction, trading peak potency for control and tolerability. The consequence is counter-intuitive but clinically decisive:
A 50% glycolic buffered to pH 4 can be milder than a 30% glycolic at pH 2, because the buffered version has a smaller free-acid load despite the higher number on the label.
This is why two products with the same percentage can produce wildly different reactions, and why "percentage" is a marketing number, not a dosing number. The variables that actually set the dose are:
| Variable | What it controls | Why it matters |
|---|---|---|
| Labelled % | Total acid present | Sets the ceiling — but not the active fraction |
| pH | Free vs ionised ratio | The real lever; lower pH = more free acid = stronger |
| Free-acid value | The active, penetrating fraction | The actual dose delivered |
| Buffering / vehicle | How fast and evenly it penetrates | Trades potency for control |
| Contact time | How long the active fraction acts (AHA) | Time is the dose dial for non-self-neutralising acids |
Why undisclosed formulas are unpredictable
If a manufacturer discloses only a percentage — and not the pH, the buffering, or the free-acid value — you are flying blind:
- You cannot predict the reaction, because the same percentage can be mild or aggressive depending on hidden pH.
- You cannot titrate safely, because you do not know your starting point on the potency scale.
- You cannot compare products or build a coherent depth ladder across a series.
- Worst of all in Fitzpatrick IV–VI, you cannot judge the inflammatory load you are about to impose — and inflammation is the upstream trigger of PIH.
Predictable dosing requires a known free-acid load and pH. An opaque "30% peel" is, clinically, an unknown dose.
Disclosure as a clinical instrument
Full disclosure is not a marketing nicety — it is what makes a peel a clinical instrument rather than a black box. When concentration and pH (and therefore free acid) are published for every product, you can:
- Dose to a target, not to a label.
- Titrate up or down precisely across a series.
- Compare and combine agents with a known additive load (essential when priming with Jessner before TCA, or sequencing depigmenters with a pigment peel).
- Hold the inflammatory load where Fitzpatrick IV–VI skin can tolerate it.
Key takeaway
A peel's potency is set by its free-acid value and pH, not its labelled percentage — buffering can make a high-percentage peel mild, and a low pH can make a modest one aggressive. That means an undisclosed formula is an unknown dose: you cannot predict its reaction, titrate it safely, or judge the inflammatory load it imposes — the last especially dangerous in Fitzpatrick IV–VI, where inflammation drives PIH. Full disclosure of concentration and pH turns a peel from a black box into a clinical instrument you can dose to a target, titrate across a series, and combine with a known additive load.
Frequently asked questions
Why does the percentage on a peel label not tell me how strong it is?
Because potency is set by the free-acid value and pH, not the total acid present. An acid works in its un-ionised, free form, and the proportion in that active form depends on the solution's pH relative to the acid's pKa — the lower the pH below the pKa, the more free acid and the stronger the peel. A peel buffered to a higher pH has a smaller free-acid load, so a 50% buffered glycolic can be milder than a 30% glycolic at a very low pH. The percentage sets the ceiling, but pH and free acid set the actual dose.
What does buffering do to a peel?
Buffering, or partial neutralisation, raises the pH of the peel, which lowers the fraction of acid in its active free form. This trades peak potency for control and tolerability — the peel penetrates more slowly and evenly and is gentler for a given percentage. It is a deliberate safety lever, particularly useful in reactive or Fitzpatrick IV–VI skin. The catch is that buffering makes the labelled percentage even less informative on its own: you need the pH or free-acid value to know how the buffered product will actually behave.
Why does it matter whether a peel's full concentration and pH are disclosed?
Because you cannot dose what you cannot see. If only a percentage is disclosed and not the pH, buffering or free-acid value, you cannot predict the reaction, titrate safely, compare products, or build a coherent depth ladder — and you cannot judge the inflammatory load you are imposing, which in Fitzpatrick IV–VI is the upstream trigger of PIH. Full disclosure of concentration and pH lets you dose to a target rather than to a label, titrate precisely across a series, and combine agents with a known additive load, turning the peel into a predictable clinical instrument.
