The Determination of Alkalinity in Diesel Engine Lubricating Oils and Additives

Abstract

A study was made of important factors in analyzing the base content, i.e., acid neutralization capacity, of additive concentrates used to compound highly basic lubricants and of the lubricants themselves. It was found that the standard analytical method now in general use for determining base content of petroleum products, D 664-58 (corresponding methods, (British) IP-177/64 and (Japanese) JIS K 2502), is not entirely satisfactory.The principal disadvantages of the D method are as follows: (1) With some complex additives, the results give a significantly false measure of the base that neutralizes strong acids, such as sulfuric acid, which are produced during normal diesel engine operation; (2) lengthy titration times are required to get reproducible results; and (3) the end point is often difficult to determine.The use of a back titration as a modification of the D method overcomes some of these difficulties, but unreliable results can still be obtained. The use of mechanical titrimeters likewise can lead to erroneous ASTM D 664 results.The use of sulfuric acid in place of the hydrochloric acid specified in the D method can give reliable results but has the disadvantages of long titration times and unclear end points.The method found to be most satisfactory in this study is based on a perchloric acid titration in glacial acetic acid solvent.This method is rapid, provides sharp end points, and gives measured base numbers that are essentially identical with the theoretical values.The base content as measured by the perchloric acid method was shown to give better correlation with measured wear in laboratory diesel engine tests than did the base content as measured by the D method. The perchloric acid method is recommended.