As fluid is used, acidic compounds can be formed. Normally controlled at 0.2 mg KOH/g by purification media such as fuller's earth. Too high at any time can lead to later problems and shortened fluid life. This is not the same as pH.
a chemical counting of how many carboxyl groups have not reacted with alcohol groups
A measure of the amount of KOH needed to neutralize all or part of the acidity of a petroleum product.
In chemistry, acid number (or "neutralization number" or "acid value" or "acidity") is the mass of potassium hydroxide (KOH) in milligrams that is required to neutralize one gram of chemical substance. The acid number is a measure of the amount of carboxylic acid groups in a chemical compound such as a fatty acid. In a typical procedure, a known amount of sample dissolved in organic solvent is titrated with a solution of potassium hydroxide with known concentration and with phenolphthalein as a color indicator.