This method is for determining the acid number of petroleum products, petroleum distillates, and similar materials by potentiometric titration. Inorganic acids, organic acids, mercaptans and thiophenols respond to this analysis, but their respective salts do not. For naphthenic acids, it is frequently desired to include the salts in the measurement. Therefore, a procedure is also included for the determination of sodium naphthenate salts. The typical range for acid number determination is 0.002 to 5 mg KOH/g of sample, although higher concentrations can be accommodated.

There are three procedures included in the method. The acid number of a sample can be determined on an as-received basis or on a mercaptan- and thiophenol-free basis. The method can also be used to determine naphthenic acids including sodium naphthenates (soaps) in caustic-washed hydrocarbons. For an estimated relative molecular mass of 130, the range of quantitation for naphthenic acids is 5 to 500 mass-ppm. The latter two procedures apply almost exclusively to light kerosines and gas oils where it is assumed that the organic acids are entirely naphthenic.

If no potentiometric titrator is available, and the sample is light in color, the titration may be performed colorimetrically; see the APPENDIX.