GENERAL INTRODUCTION TO BASE NUMBER
New and used petroleum products may contain basic constituents that are present as additives. The relative amounts of these materials can be determined by titrating with acids. The neutralization number expressed as base number, is a measure of this amount of basic substance in the oil always under the conditions of the test. The neutralization number is used as a guide in the quality control of lubricating oil formulations. It is also sometimes used as a measure of lubricant degradation in service; however, any condemning limits must be empirically established.
There are four different ASTM test methods for the determination of base numbers: D664, D974, D2896, and D4739. Test Method D664 has always given poor precision.
Hence, ASTM has cancelled that base number part of Test Method D664, and replaced it with Test Method D4739. The results obtained with the latter method, however, are not necessarily identical to those obtained with Test Methods D664 or D2896. It needs to be emphasized that:
(1) Different base number methods may give different results for the same sample.
(2) The test method should not be arbitrarily changed, since the new values may not be compatible with the historical data.
(3) Test Method D2896 remains the preferred base number method due to its superior precision, definitive end point and large database.
