ACID NUMBER: D5770
ACID NUMBER BY SEMI-QUANTITATIVE MICRO DETERMINATION OF ACID NUMBER OF LUBE OILS DURING OXIDATION TESTING: D5770
EXPLANATION
This test method 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. It gives a semi-quantitative estimate of the acid number using smaller amounts of sample than those required in Test Methods D664, D974, or D3339. This test has specific applications in Test Methods D943 and D4871. This test is a micro version of Test Method D974 and both methods give similar results. The method should, however, not be used as a replacement for higher precision test methods such as D664 or D974. It shall not be used to monitor oils in-service. The test is applicable to turbine oils, hydraulic oils, and other circulating oils whose acid numbers fall in the range 0.02 to 1.0 mg of KOH per gram of sample.
TEST SUMMARY
The method is similar to Test Method D974. A 2.0-mL portion of the solution is titrated with samples 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. A second 2.0-mL portion of the titration solution is then titrated with an acid number reference solution of known acid number the same way as the first time. From the ratio of number of drops required for color change in above two titrations, the acid number is calculated.
TEST PRECISION
In the acid number range of 0.06 to 1.1 mg KOH/g of sample, the repeatability and reproducibility were found to be 0.20 acid number and 0.70 acid number, respectively. The procedure is not known to have any bias.
