Determination of a-sulphonated ester

The ester cannot be determined by saponification because this slowly removes the sulphonate group as well as hydrolysing the ester group. It can be determined by acid hydrolysis followed by measurement of the a-sulphonated carboxylate salt produced, either by potentiometric titration of the weak acid or by two-phase titration with benzethonium chloride. Both procedures measure the a-sulphonated ester plus the unsulphonated ester. If the latter is present at a significant level, it can be determined (section 5.11.2) and corrected for. 

Weigh a sample containing about 5 g active in a round-bottomed flask. 

Add 25 ml 3 M aqueous hydrochloric acid and heat on a steam bath for 30 min. Attach a reflux condenser and boil for 2h. Cool. 

Add 100 ml neutral ethanol and either titrate potentiometrically with 0.5 M ethanolic sodium hydroxide through end-points at pH about 3.0 and 7.7, or make just alkaline to phenolphthalein with ethanolic 3M sodium hydroxide and titrate with 0.5 M ethanolic hydrochloric acid, first to phenolphthalein and then to bromophenol blue. 

Carry out the procedure described in section 5.11.5, using a 5g sample. 

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