(Equivalent Tests: IP 431 and ISO 7537)
EXPLANATION
This test method measures the acid number of oils obtained from laboratory oxidation test (Test Method D943) using smaller amounts of samples than those used in other acid number tests for Test Methods D664 or D974. It is applicable for the determination of acids having dissociation constants larger than 10(-9). Extremely weak acids or salts whose dissociation and hydrolysis constants, respectively, are 10(-9) do not interfere. No general relationship between corrosion and this acid number is known. Dark colored oils may be more difficult to analyze by this method because of the difficulty in detecting color change. In such cases Test Method D664 may be used if sufficient sample is available. The values obtained by Test Methods D3339 and D664 may or may not be numerically the same but they should be of the same order of magnitude. However, the values obtained by Test Methods D3339 and D974 have been found to be the same within the precision of the two test methods.
TEST SUMMARY
An oil sample is dissolved in a solvent mixture of toluene, isopropyl alcohol, and a small amount of water. The solution is then titrated at room temperature under a nitrogen atmosphere with standard KOH in isopropyl alcohol to the stable green color of the added indicator p-naptholbenzein.
TEST PRECISION
The test has no known bias.