The sodium salt of lactic acid is hygroscopic and, as a hydrating substance, is part of the skin's own natural moisturising factor (NMF) together with glycerin, urea, lactic acid and other organic acids. In addition, sodium lactate forms a buffer system with lactic acid that stabilises the pH of the skin's own acid mantle at 5.5. For precisely these reasons, sodium lactate is also used in cosmetic formulations - namely as a humectant and to stabilise the pH value. At a pH of 5.5 and using 0.5% of 80% lactic acid, about 1.7% of 60% sodium lactate should be used.
Use concentration: 
1-4 %