It is a water soluble solution of around 80% strength.
Used in CP soapmaking to make a harder and longer-lasting bar.
Use at 10g to 1kg of soap and add to the lye water before blending with the oils.
Lactic acid is a strong alpha-hydroxy acid (AHA) with excellent exfoliant, moisturing and deep cleansing properties. It is reputed to help reduce pigmentation.
It requires a low pH of between 2.0 and 3.3. At a pH 5.5 it is totally ineffective.
Lactic acid is weaker than glycolic acid but the latter is too dangerous to use except for commercial skincare manufacturers.