Hi, I am using 20 mM lactic acid to induce macrophages polarisation. I have L-(+)-lactic acid from Sigma and in their protocol the solubility of the powder is 100 mg/ml H2O. I am using PBS instead of water, but I have to add 1.8% of PBS to the growth medium to get the final concenctration of 20 mM. I am afraid this dilution can affect macrophages viability. I found out that the solubility of lactic acid is very high, up to 860 mg/ml. I tried to dissolve 200 mg in 1 ml and I had no problem with that. The solution was clear. Do you have any idea why Sigma reccommend to dissolve only 100 mg in 1 ml? How concentrated lactate stocks you are using?