Acid Number and Naphthenic Acids by Colorimetric Titration

This method is for determining the acid number of petroleum products, petroleum distillates, and other hydrocarbons by colorimetric titration. Inorganic acids, organic acids, mercaptans and thiophenols respond to this analysis, but their respective salts do not. This method is limited to light colored distillates. For dark colored samples, use the potentiometric procedure in UOP Method 565. The typical range for acid number detection is 0.004 to 0.5 mg KOH/g of sample, although higher levels can be accommodated.

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 detection for naphthenic acids is 5 to 250 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.