Determination of Epoxy Equivalent

Epoxy resins are characterised by two or more epoxy groups in the structure. To cure an epoxy resin with a suitable hardener, accurate estimation of epoxy groups is crucial. Epoxy equivalent is defined as the amount of resin that contains one mole of epoxy. Epoxy equivalent of an epoxy sample is determined by a standard titration method [6] using hydrogen bromide solution in acetic acid. The method is briefly described next. 

About 100 mg of anhydrous sodium carbonate, 20 ml of chlorobenzene and 10 ml of glacial acetic acid are taken in an Erlenmeyer flask and magnetically stirred until all the carbonate dissolves. Four drops of 0.1% solution of crystal violet in acetic acid are added. The purple-coloured solution is titrated with hydrogen bromide solution (prepared by diluting about 6 ml of commercial reagent with 250 ml of acetic acid). The titre value is noted at the blue-green end point. About 300 mg of epoxy sample is titrated in a similar way. A blank titration is also carried out without any sample. 

The epoxy equivalent (Z) is calculated from the following formula  

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