To maintain the acid number within specified level is the key of quality control of the insulating oil. Acid number is expressed by potassium hydroxide in mg required to neutralize free fatty acid in 1g of oil.
Measurement of acid number of insulating oil is specified in some test standards, such as GB7599 Test Method for Acid Number in Operational Transformer Oil and Turbine oil, ASTM D974 Standard Test Method for Acid and Base Number by Color Indicator Titration etc.
The oil sample is titrated with 0.05mol/L potassium hydroxide ethanol as reagent and with toluene ethanol as solvent and with alkali blue 6B ethanol, and the endpoint is detected on titratioin curve. The acid number is calculated from the titration volume of reagent reaching the endpoint.
