Engine oils contain additives to neutralize acids as they build up. This ability to neutralize acid is measured by the Total Base Number, or TBN test.
The test is similar to the TAN test, except the titrant used is 0.1N Hydrochloric acid, and the solution is titrated to an end point of pH 3. Either a color indicator or pH meter can be used to measure the end point.
As lube oil breaks down, a variety of acids are formed. These acids cause corrosion to any metal component that is subject to corrosion, especially babbitt bearings. Acids are often introduced into lube oils as contaminants. The Total Acid Number titration accounts for all the acids present, either introduced or formed. Some lube oils, especially synthetics, have a base line TAN of as much as 0.7 or greater mg KOH/gram of oil. Most mineral oil lubricants will have 0.0 or 0.1 as a base line.
Two to five grams of oil are mixed with 100 mls of solvent. The solvent consists of 50% toluene, 49.5% isopropanol, and 0.5% water. This mixture is then titrated with 0.1N potassium hydroxide to an end point of 11. A color indicator, p-Naphtholbenzein, changes from orange to green at pH 11. The amount of titrant needed to reach this end point is measured and reported as milligrams KOH/gram of oil.