Abstract: A reagent for pH-metric acid number determination in a petroleum oil with low acid numbers (AN≦0.03 mg KOH/g oil) includes a salt of a strong base and a weak acid, and, an alkali in a mixture of water and an alcohol. The salt of a strong base and a weak is, preferably, sodium benzoate in a concentration of 0.02 to 0.1 M. The alcohol is, preferably, isopropanol in a concentration of from 50% to 80% by volume. The alkali is, preferably, an hydroxide and, most preferably, potassium hydroxide. The alkali should be adjusted to provide a pH′o, which is the pH of an equivalence-point on a titration curve of a strong acid added to a test portion of sodium benzoate, water and isopropanol.