TAN - Total Acid Number

Transformer oils are oxidized under the influence of excessive temperature and oxygen resulting in an increase in Acid Number.  Continued reaction can result in sludge and varnish deposits.  This may cause blockages and the transformer will not cool efficiently, which promotes further oil breakdown.  An increase in the acidity also has a damaging effect on the cellulose paper whereas an increase in Acid Number often goes hand in hand with a decrease in dielectric strength.  The sample is dissolved in a mixture of toluene and 2-propanol (which contains a small amount of water).  The resulting solution is then titrated potentiometrically with alcoholic KOH solution using non-aqueous electrodes with alcoholic saturated lithium chloride as the electrolyte.  The meter readings are plotted automatically against the respective volumes of titrating solution and the end points are taken only at well defined inflections in the resulting curve.  Results are quoted in mg KOH/g.

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