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The acidity of an oil sample is related to the deterioration of the sample. The mineral oil insulating fluid is essentially a non-polar saturated hydrocarbon; however, when the sample undergoes oxidative degradation there are formed carboxylic acids, which are acidic in nature. The presence of these acidic materials can be quantitatively determined by a procedure called titration. The amount of a standardized base that is needed to neutralize the acidic materials present in a known quantity of an oil sample is determined. The result is referred to as the acidity or the neutralization number of the sample and is reported in terms of the milligrams of potassium hydroxide per gram of the oil sample. The titration procedure can be done either volumetrically or gravimetrically and the end point can be determined either colorimetrically or potentiometrically.


The details of the entire procedure are given in the ASTM D 974 standard and are only briefly mentioned here. The standard alcoholic potassium hydroxide solution is made up by dissolving 6 g of potassium hydroxide in 1 L of anhydrous isopropyl alcohol. The resulting solution is treated with barium hydroxide and standardized against pure potassium acid phthalate. The titration solvent is a mixture of about equal volumes of isopropyl alcohol and toluene containing about 0.5% water. The colorimetric indicator used is p-naphtholbenzein and the orange sample solution is titrated to a green or green-brown end point.


The mineral oil is a non-polar material while the acidic components formed by the deterioration of the oil are highly polar materials. The result of this large difference in polarity is that the two are mutually insoluble in each other. Since the insulating fluid in a new unit would have little or no acidic materials initially present, as the acidic materials start to form the small amounts present would be soluble. However, as more of the acidic material forms it would reach a saturation point and further formation would result in the separation of solid material. This material would settle to the bottom of the unit as a sludge. Another source of sludge would be from the reaction of the acidic materials in the insulating fluid with various metals present in the unit to give salts, which would also tend to be insoluble in the insulating fluid.

The buildup of sludge inside of a unit can drastically affect the operation of the unit. The presence of the polar materials dissolved in the oil can reduce the dielectric strength of the fluid as well as increasing its dissipation factor. The presence of the solid material can interfere with the circulation by plugging up pumps or cooling radiators, which in turn affect the fluids function as a heat transfer medium.

The IEEE has suggested guidelines for neutralization numbers depending on the type of oil and the unit it is being used in (IEEE C57.106-1991). Some representative values are given below:

Type of Oil/Unit Neutralization Number
Shipments of New Mineral Oil as Received from Refinery Maximum 0.03 mg KOH/g Oil
New Oil for Units Rated at 345 kV and above Maximum 0.03 mg KOH/g Oil
Limits for Continued Use
< or = 69 kV
69 - 288 kV
> 345 kV
Maximum 0.2 mg KOH/g Oil
Maximum 0.2 mg KOH/g Oil
Maximum 0.1 mg KOH/g Oil
Limits for Oil to be Reclaimed
Group II
Group III
Maximum 0.2 mg KOH/g Oil
Maximum 0.5 mg KOH/g Oil
New Oil for Circuit Breakers Maximum 0.03 mg KOH/g Oil

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