Neutralization Number
Standard Test Method for Acid and Base Number by Color-Indicator Titration, ASTM D974
Neutralization Number – either Acid Number or Base Number – is frequently determined for many different petroleum products using ASTM D974. For transformer oil, the concern is with the acid number. Impurities in new oil and in in-service oil will react with the reagent used in acid number determination (potassium hydroxide – KOH). The relative amount of these impurities is quantified as the acid number (which is sometimes referred to as “total acid number”). Acid number is reported as milligrams of potassium hydroxide per gram of sample (mg KOH/g).
Acid number is a chemical property of the oil, related to its purity. Highly refined new oils have very little in the way of impurities in them and a correspondingly low acid number. A typical new oil specification is a maximum acid number of 0.015 mg KOH/g. Acid number is also a useful in-service oil test because it is a direct measure of the extent of oil oxidation. Many of the oxidation products that are formed in oil as it ages react with potassium hydroxide and are measured as a group by the acid number determination.
ASTM D974 is a manual method of titration – adding the potassium hydroxide solution to an oil sample which has had a color indicator added to it. The color indicator changes color when free potassium hydroxide is present. As long as there is no color change, the added potassium hydroxide reagent is reacting with aging compounds in the oil. When a color change is noted, the sample size, volume of added KOH, and concentration of the KOH added are used to calculate the acid number of the oil specimen.
Some laboratories have modified ASTM D974 to allow use of an automatic titrator. In this case, electrodes that measure pH are used instead of the color indicator. This can be an acceptable method, but there can be difficulties in calibrating the instrument properly.
Neutralization Number – either Acid Number or Base Number – is frequently determined for many different petroleum products using ASTM D974. For transformer oil, the concern is with the acid number. Impurities in new oil and in in-service oil will react with the reagent used in acid number determination (potassium hydroxide – KOH). The relative amount of these impurities is quantified as the acid number (which is sometimes referred to as “total acid number”). Acid number is reported as milligrams of potassium hydroxide per gram of sample (mg KOH/g).
Acid number is a chemical property of the oil, related to its purity. Highly refined new oils have very little in the way of impurities in them and a correspondingly low acid number. A typical new oil specification is a maximum acid number of 0.015 mg KOH/g. Acid number is also a useful in-service oil test because it is a direct measure of the extent of oil oxidation. Many of the oxidation products that are formed in oil as it ages react with potassium hydroxide and are measured as a group by the acid number determination.
ASTM D974 is a manual method of titration – adding the potassium hydroxide solution to an oil sample which has had a color indicator added to it. The color indicator changes color when free potassium hydroxide is present. As long as there is no color change, the added potassium hydroxide reagent is reacting with aging compounds in the oil. When a color change is noted, the sample size, volume of added KOH, and concentration of the KOH added are used to calculate the acid number of the oil specimen.
Some laboratories have modified ASTM D974 to allow use of an automatic titrator. In this case, electrodes that measure pH are used instead of the color indicator. This can be an acceptable method, but there can be difficulties in calibrating the instrument properly.