106 Refining Used Lubricating Oils
indicator changes color as soon as the acid is completely neutralized. Alternatively,
a potentiometric method may also be used.
The acid number test is performed on oil samples to quantify the acid buildup in
the oil—an increased acid number is a result of oxidation of the oil, perhaps caused
by overheating, overextended oil service, or water or air contamination. The acid
number limits for lubricating oils vary considerably—in some cases, an acid number
exceeding 0.05 is unacceptable, whereas in other cases, an acid number higher than
4.00 may be acceptable. As with all other readings, trend analysis is the best indica-
tion of the health of both the oil and the machinery.
The measurement of the base number (AS ...