The total acidity of the oil is expressed as neutralistion number which is defined as the number of milligrams of potassium hydroxide required to neutralise completely the acids present in one gram of oil. The acids produced by Oxidation of mineral oils are soluble in alcohol and if a fixed quantity of oil is mixed with a fixed quantity of alcohol in mixture may be acidic, neutral of alkaline depending upon the acid present. If a fixed quantity of universal indicator is added to this mixture the colour will indicate the PH from which the acidity may be calibrated.

A. 1.1 ml portion of the insulating oil (approximately 1 gm) to be tested is accurately pipetted into a clean dry test tube. To this is added 1 ml of rectified spirit (ethyl alcohol) and the mixtures is gently shaken. One ml of solution of 0.0085N sodium carbonate is then added. After shaking the test tube again five drops of universal indicator are added. The resulting mixture develops a colour depending upon the PH value of the mixture. Oil having total acidity of 0 to 0.5 shall show transition colours from prussian blue to yellow. Whereas those having an add number of 0.5 and above shall show a change from yellow to vermillion. The chart supplied with the Kit gives the colours obtaind by using this test on oils having neutralisation (total acidity) from 0 to 1.0.