5.1 Crude oils and oil sands bitumen contain naturally occurring acidic species. Acidity of crude oil has been implicated in corrosion of distribution and process systems. The relative amount of these materials can be determined by titrating with bases. The acid number is a measure of this amount of acidic substance in the oil under the conditions of the test.

5.2 Acid number of crude and distilled petroleum fractions has been measured by Test Method D664. Test Method D664 was developed for the analysis of lubricants and biodiesel. The titration solvent used in Test Method D664 does not properly address dissolving difficult samples such as crude oil, bitumen, and high wax samples addressed in this test method. Refer to Appendix X1.

5.3 Test Method D974 is also not applicable to measuring acidity of crudes and highly colored samples because the indicator is not visible or it is difficult to discern a color change to detect the end point of the titration.

1.1 This test method covers the determination of acidic components in crude oil and petroleum products including waxes, bitumen, base stocks, and asphalts that are soluble in mixtures of xylenes and propan-2-ol. It is applicable for the determination of acids whose dissociation constants in water are larger than 10^–9; extremely weak acids whose dissociation constants are smaller than 10^–9 do not interfere. The values obtained by this test method may not be numerically equivalent to other acid value measurements. The range of KOH acid numbers included in the precision statement is 0.1 mg/g to 16 mg/g.

1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.

1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. Some specific hazards statements are given in Section 7 on Safety Precautions.