L-ascorbic acid is most bioavailable between pH 2.5 and 3.5, where it remains protonated for skin penetration. However, this range can irritate sensitive skin. Most experienced cosmetic chemists target pH 3.0–3.5 as a compromise. Stable derivatives like sodium ascorbyl phosphate or ascorbyl glucoside work effectively at a gentler pH of 5.0–7.0, trading some penetration speed for tolerability and formulation flexibility.
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