Procedure for Carboxyl Group Determination

There will be three inflections in the titration curve as illustrated in Fig. 7.5.1 the first inflection, near +200 to +100mV, is assignable to excess hydrochloric acid and strong acids (for example, sulfonic) present in the sample the second inflection, near —350 mV, corresponds to carboxylic acids and the third, near -480 to -520mV, to phenolic hydroxyl groups. These inflections can be resolved in the 500-200 mV (full-scale deflection) ranges. 

The theoretical titer, a, assignable to the internal standard, p-hydroxybenzoic acid, is calculated from the expression 

The correction factor, c, for hydrochloric acid interference is calculated with reference to the titration curve obtained in the blank run described above. The titer corresponding to the second inflection point, near —320 mV, is subtracted from that corresponding to the first inflection point (near +130 mV) to obtain the titer for the carboxyl group in p-hydroxybenzoic acid. Subtraction of the theoretical titer for p-hydroxybenzoic acid, a, from this value affords the HC1 interference correction factor, c  

The phenolic hydroxyl content is calculated as follows mEq phenolic hydroxyl/g sample = 

Note 1. The inflections may be determined by the use of a Metrohm endpoint locator ruler (Brinkmann Instruments, Westbury, NY) 

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