Concentration Alone Doesn't Tell the Whole Story: pH, Formulation, and Delivery Matter

The number on the bottle is one input

Skincare marketing often leans heavily on the percentage. 10% niacinamide. 20% vitamin C. 2% salicylic acid. The implied logic is that bigger numbers are better, and that comparing two products comes down to which one wins the percent-off.

The reality is messier. For most actives, concentration is one of three or four variables that determine whether the ingredient works on your skin. The others — pH, the rest of the formulation, and the delivery system — often matter as much or more ^{[D1, D2]}.

This is the kind of nuance that ingredient-list marketing tends to flatten and that we try to keep visible.

Why pH is often the bigger lever

Most acidic actives — AHAs, BHAs, ascorbic acid — need to be in a specific pH range to do their characteristic work: AHAs to exfoliate, ascorbic acid to penetrate. Outside that range they may still hydrate, but they won't deliver the labeled effect ^{[D1, D2]}.

• Glycolic and lactic acids need a pH around 3.5–4.0 to exfoliate. At pH 5+, they are present but mostly in their inactive salt form. A 10% glycolic toner at pH 5 does less work than a 5% glycolic toner at pH 3.8.
• Salicylic acid follows the same free-acid principle: because its pKa is low, more of it stays in the penetrating free-acid form at a mildly acidic pH and shifts toward its less-active salt form as pH climbs, so formulators generally keep it in a mildly acidic range.
• L-ascorbic acid needs a pH below about 3.5 to remain stable and to penetrate the outermost skin layer. Above that, it oxidizes faster and does less. A 'gentle 15% vitamin C' formulated at pH 5 is genuinely gentler — partly because much of the active is doing less.

A percentage with no pH information is incomplete. Reputable brands typically publish pH for their acid products; the absence of that information is its own signal.

Why the rest of the formulation matters

Skin is not a beaker. The vehicle — the cream, gel, serum, or solution that the active sits in — changes how the active behaves.

Solubility and partition

A water-soluble active in an oil-heavy vehicle, or an oil-soluble active in a water-heavy vehicle, often does less than the percentage suggests. ^[D18] Some niacinamide products at 10% are functionally lower than they look because the formulation does not effectively present the active to skin.

Co-ingredients

Vitamin C is the canonical example. The Skinceuticals CE Ferulic formulation (15% L-ascorbic acid + 1% alpha-tocopherol + 0.5% ferulic acid at pH ~3.0) became a category leader not because 15% was special but because ferulic acid stabilizes the ascorbic acid and extends its functional life on skin ^[B1]. A bare 15% L-ascorbic acid serum without ferulic acid and without proper pH degrades faster and works less well.

Buffer systems

Many cleansers and toners contain buffering ingredients that shift the working pH up or down from where you would expect. A cleanser that lists glycolic acid at 5% can be deliberately pH-buffered up to a gentler 4.5–5.0 range, sacrificing some exfoliation for tolerability. That's a formulation choice, not a deception, but it means the percentage alone underspecifies the product.

Why the delivery system matters

Delivery is how efficiently an ingredient gets where it needs to go. Two factors dominate:

Penetration enhancers

Formulations can include ingredients that help actives cross the outermost skin layer — propylene glycol, certain glycerin concentrations, ethanol in moderation, dimethyl isosorbide. In principle, a lower-percentage active in a well-designed vehicle can rival a higher-percentage one in a poorly designed vehicle — the vehicle, not just the number, shapes how much of the active reaches its target.

Encapsulation

Retinoids and vitamin C are commonly encapsulated with the aim of slowing their release and easing irritation. Encapsulation changes the effective concentration on a moment-to-moment basis; the general design tradeoff is peak intensity for steadier delivery and tolerability.

Time of contact

Leave-on products spend more time on skin than rinse-offs. A 2% salicylic cleanser at pH 3.5 looks numerically aggressive but in practice sits on skin for ~30 seconds. A 1% leave-on BHA serum at the same pH stays in contact far longer, so contact time — not just the percentage — shapes how much work it can do.

Where higher percentages do meaningfully matter

Higher concentrations are not irrelevant. Within the same well-formulated brand, at the right pH, in the right vehicle, the higher concentration typically does more — and brings more irritation potential. Some specific cases:

• Retinol: at appropriate pH, 0.5% generally does more than 0.25%, and 1% more than 0.5%, with proportionate irritation increases. Diminishing returns above ~1% for most users.
• Niacinamide: meaningful effects start around 2–4%, peak around 5–10%; above 10% the marginal benefit is small and the irritation risk grows.
• AHA / BHA exfoliants: percentage matters, but only at the right pH; outside pH range the percent comparison is moot.
• Hyaluronic acid: above ~1–2%, more is not better — molecular weight matters more than total percentage.

What to read instead of just the percentage

For any active product, useful information to find:

1. The pH (for acidic actives). If not on the label, check the brand's product page or ask their customer support.
2. The vehicle. Serum, gel, cream, oil — affects penetration and tolerability.
3. Co-ingredients. Ferulic acid with vitamin C, fatty alcohols with retinol, ceramides alongside acids — these meaningfully alter performance.
4. Encapsulation or time-release. Often noted on packaging; affects both tolerance and the percentage-comparison.
5. Brand transparency. Brands that publish formulation specifics — pH, exact concentrations, vehicle type — make this analysis possible. Brands that publish only the headline percentage are deliberately underspecifying.

How we approach this in Drop

When Drop compares two products with the same headline active, we try to surface the formulation context — pH where it matters, vehicle differences, well-evidenced co-ingredient combinations ^[D1]. The user-facing question is not 'which has more vitamin C' but 'which formulation has stronger evidence for what you are trying to do.' That is a slower answer, but it is the honest one.

Bottom line

Percentage is one variable in a four- or five-variable equation. pH determines whether acidic actives penetrate skin and do their characteristic work ^{[D1, D2]}. Formulation chemistry determines how the active behaves on skin. Delivery determines how much of the ingredient actually reaches the relevant layer. A higher number on the bottle is not automatically better; sometimes it is genuinely worse ^[D1]. Reading the full picture rather than the headline percentage is the move.