ASTM D5770 Test Method for Semiquantitative Micro Determination of Acid Number of Lubricating Oils During Oxidation Testing
4. Summary of Test Method
4.1 A 2.0 mL portion of the titration solution is titrated with a sample using a dropping pipet. The number of drops of sample required to turn the blue-green titration solution to a persistent orange color is noted.
4.2 A second 2.0 mL portion of the titration solution is titrated with an acid number reference solution of known acid number. The number of drops of the reference solution required to turn the blue-green titration solution to a persistent orange color is noted.
4.3 The estimated acid number of the sample is calculated using the acid number of the reference solution and the ratio of the number of drops of the reference solution required to effect the color change to the number of drops of the sample required for the same change.
5. Significance and Use
5.1 This test method provides a semiquantitative estimate of the acid number of lubricating oils obtained from laboratory oxidation tests using smaller amounts of sample than Test Methods D974, D664, or D3339. It has specific application in Test Method D943 and in Test Method D4871. This test method, therefore, provides a means of monitoring the relative oxidation of lubricating oils by measuring changes in acid number, at different time intervals and under various oxidizing test conditions.
5.2 Since this test method is semiquantitative, each laboratory shall develop its own criteria for each oxidation test method for determining when to switch from this semiquantitative test method to a more precise test method for acid number.
