The acid is purified by dissolving the Na salt in H2O, acidifying to Congo Red paper with HCl and adding a concentrated solution of FeCl3 whereby Fe sulfinate precipitates. Collect the salt, wash it with a little H2O, drain, suspend it in H2O and add a slight excess of 1.5M aqueous NaOH. The Fe(OH)3 precipitates, it is filtered off, the sulfinic acid in the aqueous solution is extracted with Et2O, the extract is dried (Na2SO4) and evaporated to give colourless crystals of benzenesulfinic acid m 84o which are stored under N2 in the dark, as it slowly oxidises in air to the sulfonic acid. [Beilstein 11 H 2, 11 I 3, 11 II 3, 11 III 3, 11 IV 3.]
