The Buffer System Design of High-Concentration Acid Serums: How to Balance Exfoliation Power and Skin Tolerance?

I. "Potent Exfoliation" and "Sensitive Skin Safety": Can They Coexist?

High-concentration acid serums are the most controversial category in the skincare efficacy hierarchy: they can significantly improve uneven skin tone, enlarged pores, and post-acne hyperpigmentation within weeks—but if used incorrectly, they can leave the skin tight, red, and with a compromised barrier overnight.

In 2024, the US market alone consumed over 27,000 metric tons of AHA raw materials, and 43% of the best-selling products on online skincare platforms feature AHA as the core ingredient. This means acid serums are no longer a niche professional category, but a true mass-market necessity—and the mass market means a broader range of skin types, including a large number of consumers with varying tolerance levels.

"Potent exfoliation" and "skin tolerance" seem contradictory, but are actually determined by the exact same formulation parameter: the design logic of pH value and buffer systems.

II. The Efficacy Mechanism of Acid Serums: pH is the Core Switch

The exfoliation mechanism of AHAs (Alpha Hydroxy Acids) relies on undissociated acid molecules penetrating the skin, weakening the calcium ion-mediated cell adhesion between stratum corneum cells, and accelerating the shedding of corneocytes. Only the undissociated form of the acid molecule is effective—and the degree of acid dissociation is determined by the pH value:

• Glycolic Acid (pKa = 3.83): At pH 3.5, approx. 67% is in the active undissociated form; at pH 4.5, only approx. 17% is effective.

• Lactic Acid (pKa = 3.86): Similar pH pattern; for every 1-unit increase in pH, the effective active concentration decreases by about 5–7 times.

• Salicylic Acid (pKa = 2.97): Due to its lower pKa, it retains partial activity at slightly higher pH levels.

This means: a 10% glycolic acid serum at pH 3.5 has a significantly higher actual exfoliating power than a 15% glycolic acid serum at pH 4.5. Concentration is not the sole determinant of efficacy; pH is.

III. Regulatory Boundaries for Acid Serums in Major Global Markets

The US FDA has issued guidance that the maximum use concentration for glycolic and lactic acids in OTC cosmetics is 10%, with a pH ≥3.5, and must contain SPF or include a warning on the label to use sunscreen. In 2025, New York and California proposed bills to restrict the sale of products containing such ingredients to individuals under 18.

The EU SCCS (Scientific Committee on Consumer Safety) conducted safety assessments on the two most common AHAs: Glycolic acid can be safely used at a maximum concentration of 4% and pH ≥3.8 for consumers; Lactic acid at a maximum concentration of 10% and pH ≥3.5. In 2024, the EU also included PHAs (Polyhydroxy Acids) in the AHA definition and raised the maximum allowable concentration for consumer products from 10% to 18%.

Regarding Salicylic Acid, in May 2025, the EU SCCS issued a final opinion (SCCS/1675/25): Salicylic acid is not safe when used as a preservative at 0.5% in all cosmetics; for its use limits and pH requirements as an exfoliating active, the SCCS initiated a new round of safety assessment in January 2026, and the final regulatory framework is pending update.

This draws clear operational boundaries for formulation engineers in different target markets:

Ingredient	US FDA Limit	EU SCCS Limit	Minimum Safe pH
Glycolic Acid	≤10% (OTC)	≤4% (Consumer use)	≥3.5 (US) / ≥3.8 (EU)
Lactic Acid	≤10% (OTC)	≤10%	≥3.5
Salicylic Acid	≤2% (OTC)	See EU Annex III	Usually ≥3.0

IV. Buffer Systems: pH Is Not Just a Number, It's a Stability Capability

Setting the formulation pH to 3.5 does not mean the product maintains 3.5 throughout its entire shelf life. Several scenarios can cause pH drift in acid serums:

• Trace alkaline components in raw materials (e.g., carbonate traces carried by certain humectants) gradually neutralize the free acid in the system.

• High temperatures (above 40°C) accelerate esterification side reactions of small-molecule acids like glycolic acid, consuming free acid.

• Trace metal ions (iron, copper) in packaging catalyze redox reactions, shifting the system's pH upward.

If the pH drifts from 3.5 to 4.0, the concentration of active exfoliating molecules of glycolic acid may have lost over 50%. Consumers buy the same bottle, but half a year later feel "the effect has diminished"—the problem is not that the ingredients expired, but that the buffer system wasn't stable enough.

Common Buffer Systems for Acid Serums:

Buffer System	Effective pH Range	Advantages	Precautions
Glycolic Acid / Sodium Glycolate	2.5–4.5	Self-buffering (the acid itself acts as the buffer pair); simple.	High ionic strength at high concentrations; dosage must be controlled.
Lactic Acid / Sodium Lactate	3.0–5.0	Mild, natural origin, COSMOS compatible.	Sodium lactate is slightly hygroscopic; watch the skin feel.
Citric Acid / Sodium Citrate	3.0–6.2	Low cost, good stability.	Citric acid itself has mild exfoliating properties; must be counted in total acid volume.
Tartaric Acid / Sodium Tartrate	3.5–5.0	Natural origin, reinforces AHA stability.	Slightly higher cost; supply chain needs evaluation.

V. Tolerance Design: High Exfoliation ≠ High Irritation, Three Formulation Tools

Efficacy and tolerance are not necessarily opposed. Through the following three formulation dimensions, tolerance can be systematically improved without sacrificing exfoliation efficiency:

① Embedding Anti-Irritant Active Ingredients

The Ordinary's classic AHA 30% + BHA 2% formula added Tasmanian Pepperberry Extract (Tasmannia Lanceolata) as a "known anti-irritant"—this is one of the key formulation engineering decisions that allows this formula to be accepted by the market despite its ultra-high 30% AHA concentration. Common anti-irritant synergistic ingredients include: Panthenol (0.5%), Allantoin (0.2%), Madecassoside (0.1%).

② Time-Release Technology

Encapsulating AHAs in microcapsules or liposomes to control their release rate on the skin surface, transforming an instant high concentration into a slow, sustained low concentration—the total efficacy is equivalent, but the irritation peak per unit time is significantly reduced. Especially suitable for medium-to-high concentration acid products positioned for sensitive skin.

③ Formulation pH Gradient Design

In lotion or serum compounding systems, design a dual-chamber or sequential application design for the AHA layer and the hydrating active layer: first apply the acid layer at pH 3.5 (action time approx. 10–15 minutes), then apply the hydrating repair layer at pH 5.5–6.5 to actively raise the skin's pH close to the physiological value, completing both exfoliation and accelerating the short-term recovery of barrier function.

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