Free Acid Value Explained

When you buy a glycolic acid product, the percentage on the label tells you how much glycolic acid was put into the formula. What it does not tell you is how much of that glycolic acid is actually active. A 10%(Low concentration) glycolic acid product can deliver anywhere from less than 1% to nearly 9% active acid depending on its pH. The concept that captures this reality is called free acid value - and understanding it changes how you evaluate every acid product you use [1].

What Is Free Acid Value?

Free acid value (FAV) is the percentage of an alpha-hydroxy acid that exists in its protonated (retaining its hydrogen atom), un-ionized form - the form that can penetrate the skin and produce biological effects. It is calculated by multiplying the listed concentration by the fraction of acid molecules that are in their active state at the product's pH.

In simple terms: free acid value = what the product actually delivers, as opposed to what the label says was put in.

The concept exists because glycolic acid, like all acids, exists in equilibrium between two forms in solution:

• Protonated form (HA): The active form. The molecule retains its hydrogen atom, remains un-ionized, and can penetrate the lipid-rich environment of the stratum corneum. This is the form that exfoliates, stimulates cellular turnover, and produces the effects you are looking for.
• Deprotonated form (A-): The inactive form. The molecule has donated its hydrogen atom, becoming a negatively charged glycolate ion. This ionized form cannot easily penetrate the skin's lipid barrier and has substantially reduced biological activity.

The ratio between these two forms is entirely determined by the pH of the solution relative to glycolic acid's pKa of 3.83. This is not a design choice by the manufacturer - it is a fundamental consequence of acid-base chemistry [2].

Why Free Acid Value Matters More Than Listed Concentration

Consider two products sitting next to each other on a shelf:

Product A: "10%(Low concentration) Glycolic Acid Serum" - pH 3.0

• 87.1% of the glycolic acid is in its active form
• Free acid value: 8.7%
• This is a potent product that will produce noticeable tingling, effective exfoliation, and carries meaningful irritation potential

Product B: "10%(Low concentration) Glycolic Acid Serum" - pH 4.5

• 17.6% of the glycolic acid is in its active form
• Free acid value: 1.8%
• This is a mild product with minimal tingling, gentle exfoliation, and low irritation risk

Both labels truthfully state "10% glycolic acid." Both contain the same total amount of glycolic acid molecules. But Product A delivers nearly five times more active acid to your skin than Product B. They are, for practical purposes, completely different products despite identical label claims.

This is not a niche corner case. This scenario plays out across the glycolic acid market every day. Brands are not required to disclose pH, and most do not. The consumer is left comparing percentages that, without pH information, are meaningfully incomplete [3].

The Chemistry: How pH Determines Free Acid Value

The relationship between pH and free acid value is governed by the Henderson-Hasselbalch equation, one of the most fundamental equations in acid-base chemistry:

pH = pKa + log([A-] / [HA])

Where:

• pH is the measured acidity of the product
• pKa is 3.83 for glycolic acid (the pH at which 50% is protonated)
• [A-] is the concentration of deprotonated (inactive) glycolic acid
• [HA] is the concentration of protonated (active) glycolic acid

Rearranging to solve for the fraction of active acid:

Fraction active = 1 / (1 + 10^(pH - pKa))

And then:

Free acid value = Listed concentration x Fraction active

For a complete explanation of pH chemistry and how it applies to skincare, see our pH science article.

Worked Example: 8% Glycolic Acid at pH 3.5

Let us walk through a real-world calculation.

1. Start with the Henderson-Hasselbalch equation: pH - pKa = 3.5 - 3.83 = -0.33
2. Calculate the ratio: 10^(-0.33) = 0.468, meaning for every 1 part active acid there are 0.468 parts inactive
3. Calculate the fraction active: 1 / (1 + 0.468) = 0.681, or 68.1%
4. Multiply by the listed concentration: 8% x 0.681 = 5.4% free acid value

So this "8% glycolic acid" product actually delivers 5.4% active acid to the skin.

Free Acid Value Reference Table

This table shows the free acid value for common glycolic acid concentrations across the pH range used in skincare products:

| pH | % Active | FAV from 5% | FAV from 7% | FAV from 10% | FAV from 15% | FAV from 20% | |----|----------|-------------|-------------|-------------|-------------|-------------| | 2.0 | 98.5% | 4.9% | 6.9% | 9.9% | 14.8% | 19.7% | | 2.5 | 95.5% | 4.8% | 6.7% | 9.6% | 14.3% | 19.1% | | 3.0 | 87.1% | 4.4% | 6.1% | 8.7% | 13.1% | 17.4% | | 3.5 | 68.1% | 3.4% | 4.8% | 6.8% | 10.2% | 13.6% | | 3.83 | 50.0% | 2.5% | 3.5% | 5.0% | 7.5% | 10.0% | | 4.0 | 40.3% | 2.0% | 2.8% | 4.0% | 6.0% | 8.1% | | 4.5 | 17.6% | 0.9% | 1.2% | 1.8% | 2.6% | 3.5% | | 5.0 | 6.3% | 0.3% | 0.4% | 0.6% | 0.9% | 1.3% |

Several patterns emerge from this table:

1. Below pH 3.0, differences are small. The jump from pH 3.0 to pH 2.0 only increases the active fraction from 87% to 99%. Most of the acid is already active.
2. Between pH 3.0 and 4.5, changes are dramatic. This is the range where pH has the greatest practical impact - and it is the range where most consumer products are formulated.
3. Above pH 4.5, very little acid is active. A 10%(Low concentration) glycolic acid product at pH 5.0 delivers only 0.6% active acid - less than a 5%(Low concentration) product at pH 3.0.
4. Concentration matters less than you think at high pH. A 20% product at pH 4.5 (FAV: 3.5%) delivers less active acid than a 5%(Low concentration) product at pH 3.0 (FAV: 4.4%).

How Brands Manipulate Perception

Understanding free acid value reveals a marketing practice that is not illegal but is, at minimum, misleading.

High Percentage, High pH

Some brands formulate products with high glycolic acid percentages (15%, 20%, even 30%(High concentration)) at relatively high pH values (4.0–5.0). The label displays an impressive percentage, suggesting a potent product. But at high pH, most of that glycolic acid is in its inactive, ionized form. The product is much gentler than the percentage implies - which may be fine if that is what you want, but deceptive if you are choosing it because you think "30%" means "very strong."

A 30%(High concentration) glycolic acid product at pH 4.5 has a free acid value of approximately 5.3%. A 10%(Low concentration) product at pH 3.0 has a free acid value of approximately 8.7%. The product with one-third the listed concentration is actually delivering substantially more active acid.

The "Professional Strength" Claim

Some products marketed as "professional strength" achieve high percentages at relatively high pH. Without knowing the pH, "professional strength" is a meaningless label. Professional chemical peels are potent not because of high percentage alone, but because of the combination of high percentage and low pH - typically 20–70% at pH 1.5–3.0 [4].

Partially Neutralized Formulations

Some brands partially neutralize their glycolic acid by adding a base (sodium hydroxide, ammonium hydroxide, or arginine). This raises the pH, converting active glycolic acid into glycolate salts. The total glycolic acid percentage on the label may include both the active acid and its neutralized salt - meaning the label percentage overstates the active ingredient.

This is not always manipulation. Partial neutralization is a legitimate formulation strategy that improves tolerability while retaining some efficacy. The Narda et al. (2021) study demonstrated that glycolic acid adjusted to pH 4 stimulated collagen production without inflammatory markers, showing that partially neutralized formulations have genuine value [5]. The problem is not partial neutralization itself - it is the failure to communicate it to consumers.

How to Estimate Free Acid Value When pH Is Not Listed

Most consumer glycolic acid products do not list their pH on the label. Here are strategies for estimating it:

Contact the Brand

The most direct approach. Many brands will provide pH information if asked. Contact their customer service by email and ask for the formulated pH of the specific product. Reputable brands should know this number.

Use pH Test Strips

Consumer pH test strips (available at pharmacies and online) can give you an approximate reading. Apply a small amount of product to the strip and compare the color change to the reference chart. These strips typically have an accuracy of plus or minus 0.5 pH units, which is useful for categorization (is this product below 3.5 or above 4.0?) but insufficient for precise free acid calculations.

Look for Sensory Clues

While not a substitute for measurement, the sensory experience provides rough guidance:

• Immediate tingling or stinging that fades within minutes: Likely pH below 4.0 with meaningful free acid.
• Very mild or no tingling: Could indicate higher pH (above 4.0) or low concentration.
• Intense stinging or burning: Likely very low pH (below 3.0) or very high free acid. If uncomfortable, the product may be too potent.

These are imprecise indicators and should not replace actual pH data, but they offer a real-world check on product potency.

Check Independent Reviews and Testing

Some skincare review platforms and chemistry-focused content creators test product pH values. Look for sources that use calibrated pH meters rather than test strips for the most accurate data.

Free Acid Value and Skin Irritation

Free acid value is directly related to both efficacy and irritation potential. Understanding this relationship is essential for finding the right product.

The Dose-Response Relationship

Higher free acid value means:

• More glycolic acid penetrating the stratum corneum
• More desmosome (cell-to-cell bond) disruption (stronger exfoliation) [6]
• More keratinocyte (skin cell) stimulation (stronger cellular turnover)
• More pH-dependent antibacterial activity against C. acnes [7]
• More irritation potential (stinging, redness, dryness)

This is a genuine trade-off. You cannot have the benefits of high free acid value without accepting some level of increased irritation risk. The goal is not to maximize free acid value but to find the level that delivers the results you want without exceeding your skin's tolerance.

Evidence for the Dose-Response Relationship

Decker and Graber (1996) demonstrated histologically that skin irritation increases with concentration at a given pH, and increases as pH decreases at a given concentration. This establishes the dual-variable model: both total concentration and pH (which determines free acid value) independently contribute to both efficacy and irritation potential [1].

The Edraki et al. (2022) RCT added a crucial nuance: a well-formulated 5%(Low concentration) glycolic acid complex outperformed a 20% glycolic acid formulation for acne across multiple endpoints including inflammatory lesion count, skin hydration, and patient satisfaction. This suggests that when formulation quality is high, lower free acid value can be more effective than brute-force higher concentration - likely because the gentler product can be tolerated better and used more consistently [8].

The Sweet Spot: Balancing Efficacy and Tolerability

Based on the clinical evidence and practical experience, different free acid value ranges suit different needs:

FAV Below 2%: Very Gentle

Best for: Beginners, sensitive skin, maintenance after achieving results, daily use. Typical products: 5%(Low concentration) to 7% glycolic acid at pH 4.0–4.5. What to expect: Minimal tingling. Gradual improvement over 8–12 weeks. Very low irritation risk. May be insufficient for stubborn acne or significant texture concerns.

FAV 2–5%: The Practical Sweet Spot

Best for: Most regular users, mild-to-moderate concerns, daily-to-alternate-day use. Typical products: 7% to 10%(Low concentration) glycolic acid at pH 3.5–4.0. What to expect: Mild tingling for a few minutes. Visible results within 4–8 weeks. Manageable irritation for most skin types. This range aligns with the Abels et al. (2011) study that showed significant acne improvement at 10% concentration, pH 4.0 (FAV approximately 4.0%) [9].

FAV 5–8%: High Potency

Best for: Experienced users with established tolerance, more stubborn concerns. Typical products: 10%(Low concentration) to 15% glycolic acid at pH 3.0–3.5. What to expect: Noticeable tingling. Faster results. Higher irritation potential. May not be tolerable for daily use - every other day or several times per week may be more sustainable.

FAV Above 8%: Professional/Peel Range

Best for: Professional supervision only. Typical products: High-concentration peels at low pH. What to expect: Significant stinging. Visible peeling. Rapid but aggressive results. Risk of chemical burns if contact time is not controlled. This level of free acid requires professional expertise [4].

Comparing Products by Free Acid Value

Let us apply this concept to some hypothetical product comparisons that illustrate why free acid value is the metric that matters:

Scenario 1: Which Is Stronger?

• Product X: 15% glycolic acid, pH 4.2 -> FAV = 15% x 0.299 = 4.5%
• Product Y: 7% glycolic acid, pH 3.2 -> FAV = 7% x 0.810 = 5.7%

Product Y is stronger despite having less than half the listed glycolic acid. The lower pH makes the difference.

Scenario 2: Switching Brands

A consumer has been using a 10%(Low concentration) glycolic acid product at pH 4.0 (FAV = 4.0%) and switches to a 10% product from another brand at pH 3.2 (FAV = 8.1%). Same percentage on the label, but the new product delivers twice the active acid. If the consumer does not know this, they may experience unexpected irritation.

Scenario 3: "Upgrading" Concentration

Someone using a 5%(Low concentration) glycolic acid at pH 3.5 (FAV = 3.4%) decides to "upgrade" to 20% glycolic acid at pH 4.5 (FAV = 3.5%). Despite quadrupling the listed concentration, they have barely changed the free acid value. The "upgrade" is essentially no change in potency at all - just a higher percentage of inactive glycolate.

These scenarios are not hypothetical edge cases. They represent real situations that consumers encounter regularly. Without understanding free acid value, product choices become guesswork.

Limitations of Free Acid Value

Intellectual honesty requires noting what free acid value does not tell you.

Formulation Matters Beyond pH

The vehicle (cream, gel, serum, solution), additional ingredients (emollients, humectants, penetration enhancers), and overall formulation quality all affect how glycolic acid performs on the skin. Two products with identical free acid values but different vehicles may perform differently. The Edraki et al. (2022) finding - that a 5% complex outperformed a 20% formulation - suggests formulation sophistication can be more important than raw free acid value [8].

Skin Buffering Capacity

Your skin is not a passive surface. It actively buffers against pH changes, meaning the pH (and therefore free acid value) at the skin surface is not identical to the pH measured in the bottle. The product's pH shifts upward as it contacts the skin's buffering system, reducing the effective free acid value somewhat. This is one reason clinical outcomes do not always correlate perfectly with in-bottle free acid calculations.

Limited Clinical Validation

While the chemistry of free acid value is established and non-debatable, clinical trials have not directly compared products matched on free acid value to prove it is a better predictor of outcomes than nominal concentration. The principle is scientifically sound and consistent with available evidence, but this is an honest limitation to acknowledge [1].

Contact Time Matters

Free acid value is a snapshot of what the product contains, not a complete description of total acid exposure. A wash-off product (cleanser) with high free acid value delivers less total exfoliation than a leave-on product (serum) with lower free acid value, because contact time dramatically affects how much acid penetrates the skin.

Practical Recommendations

1. Prioritize free acid value over label percentage when comparing products. Use our free acid calculator to compute it from concentration and pH.

2. Ask brands for their product's pH. If they will not or cannot tell you, that is information in itself. Brands that formulate carefully know their pH.

3. Start with products in the 2–4% FAV range if you are new to glycolic acid. This corresponds to approximately 7–10% glycolic acid at pH 3.8–4.2. Our concentration guide provides detailed guidance for different experience levels.

4. When switching brands, calculate FAV for both products to ensure you are not inadvertently doubling or halving your active acid exposure. You can compare products in our database to see their free acid values side by side.

5. Remember that higher FAV is not always better. The sweet spot is the free acid value that produces results you can see without irritation you cannot tolerate. For many people, this falls in the 3–5% range.

6. Account for contact time. A leave-on serum at 3% FAV delivers more total exfoliation than a rinse-off cleanser at 6% FAV. Choose your product format based on how much exfoliation you actually need.

7. Pair with sun protection. At clinically effective free acid values, glycolic acid increases UV sensitivity. Daily broad-spectrum SPF 30 or higher is non-negotiable [10]. See our side effects and safety guide for full details.

References

1. 1. Decker LC, Graber EM. (1996). Clinical and histological effects of glycolic acid at different concentrations and pH levels. Dermatol Surg. doi:10.1111/j.1524-4725.1996.tb00341.xClinical study
2. 2. Green BA, Yu RJ, Van Scott EJ. (2009). Clinical and cosmeceutical uses of hydroxyacids. Clin Dermatol. doi:10.1016/j.clindermatol.2009.06.023Review
3. 3. Andersen FA. (1998). Final Report on the Safety Assessment of Glycolic Acid, Ammonium, Calcium, Potassium, and Sodium Glycolates.... Int J Toxicol. doi:10.1177/109158189801700101Safety assessment
4. 4. Sharad J. (2013). Glycolic acid peel therapy - a current review. Clin Cosmet Investig Dermatol. doi:10.2147/CCID.S34029Review
5. 5. Narda M, Trullas C, Brown A, et al. (2021). Glycolic acid adjusted to pH 4 stimulates collagen production and epidermal renewal without affecting levels of proinflammatory TNF-alpha in human skin explants. J Cosmet Dermatol. doi:10.1111/jocd.13570Ex vivo study
6. 6. Fartasch M, Teal J, Menon GK. (1997). Mode of action of glycolic acid on human stratum corneum: ultrastructural and functional evaluation of the epidermal barrier. Arch Dermatol Res. doi:10.1007/s004030050212Mechanistic study
7. 7. Yang AJ, et al. (2020). pH-Dependent Antibacterial Activity of Glycolic Acid: Implications for Anti-Acne Formulations. Sci Rep. doi:10.1038/s41598-020-64545-9Mechanistic study
8. 8. Edraki K, et al. (2022). Effect of 5% glycolic acid complex and 20% glycolic acid on mild-to-moderate facial acne vulgaris. Chin Med J (Engl). doi:10.1097/CM9.0000000000002287RCT
9. 9. Abels C, Kaszuba A, Michalak I, et al. (2011). A 10% glycolic acid containing oil-in-water emulsion improves mild acne: a randomized double-blind placebo-controlled trial. J Cosmet Dermatol. doi:10.1111/j.1473-2165.2011.00572.xRCT
10. 10. Kornhauser A, Wei RR, Yamaguchi Y, et al. (2009). The effects of topically applied glycolic acid and salicylic acid on ultraviolet radiation-induced erythema, DNA damage and sunburn cell formation in human skin. J Dermatol Sci. doi:10.1016/j.jdermsci.2009.03.011Controlled study

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