Hi all. Does anybody know when Strong Acid Number is applicable? Sometimes when i do Acid Number testing, i get negative results (usu. for chiller comp. oil). The instrument supplier suggested that i try SAN.

It sounds like you may have a problem with your blank if you are getting negative numbers on an acid titration. The difference between a strong acid versus weak acid titration is where you set the end point of the titration.

The strong acid titration end point would be set at a lower pH to measure acids with a lower pka. This will miss measuring any acids with higer pka in the sample.

Refrigeration oil samples may have very low acid numbers (less than 0.005 mg/g KOH), or may be very high (greater than 1 mg/g KOH) depending on the formulation. Chlorinated refrigerant decomposition may generate strong acids (like HCl) in the oil. Newer POE lubricants used with chlorine free HFC refrigerants will generate weaker organic acids as decomposition products.

Which instrument is being used for testing acid number?