Traditional Bulgarian roasting lamb and pork barbecue Stock Photo Alamy The Ordinary Lactic Acid 5% Face serum 30ml. Lactic Acid 5% + HA is a mild alpha hydroxy acid solution that offers gentle yet effective exfoliation targeted at the skins surface. It is supported with purified Tasmanian pepperberry extract, which is a known anti irritant. This water based serum is ideal for dry and sensitive skin. This pH of this formula is approximately 3. 8. Lactic Acid has a pKa of 3. 8 and pKa is the most important aspect to consider in formulating with acids. pKa implies acid availability. When pKa is close to pH, there is an optimal balance between salt and acidity, optimizing the delivery of the acid form to ensure it provides an effective exfoliation that can be used daily.