AHA / BHA Free Acid Value (FAV) & Effective pH Calculator
Determine the true bioavailable exfoliating strength of your chemical peel or acid toner using the Henderson-Hasselbalch equation and chemical pKa constants.
Load Cult Product Presets:
2. Stated Concentration7.0%
Range: 0.5% – 30%3. Formulation pHpH 3.60
Range: pH 2.0 – 5.5Calculated Effective Free Acid Value (FAV)
4.41%active un-ionized acid
Un-ionized Bioavailable Fraction62.9%(Remaining 37.1% is inactive salt)
Dermatological Potency Classification
CLINICAL POTENT
At pH 3.6, 62.9% of Glycolic Acid (AHA) (pKa 3.83) remains in its un-ionized, bioavailable free-acid state. A 7% label concentration delivers an effective Free Acid Value of 4.41%.
Recommended Frequency: 2 to 3 alternate evenings per week (never same night as Retinoids)
The Pharmacological Henderson-Hasselbalch Equation
Free Acid % = Total Stated % / (1 + 10^(pH - pKa))
Only un-ionized, protonated acid molecules can cross the lipophilic stratum corneum. Once formulated above the acid's pKa, the molecule dissociates into a charged salt that cannot easily penetrate cellular junctions.
This explains why a 10% Glycolic toner buffered to pH 4.5 delivers less active acid (1.76%) than a 7% toner formulated at pH 3.6 (4.45%).