This method is for determining the acid number of petroleum products, petroleum distillates, and other chemicals by potentiometric titration. Inorganic acids, organic acids, mercaptans and thiophenols respond to this analysis, but their respective salts do not; a procedure is also included for sodium naphthenate salts. The typical range for acid number detection is 0.002 to 5 mg KOH/g of sample, although higher levels can be accommodated.