Tetrahydroquinone indicator

Any sulfate ions resulting from oxidation of sulfite should be removed otherwise they will be converted to sulfuric acid in the subsequent procedure and destroy sultone. Ordinarily, 12-13 g. of barium chloride dihydrate is required. The end point of the addition is conveniently determined with tetrahydroquinone indicator used in spot tests on filter paper [cf. Ind. Eng. Chem., Anal. Ed., 9, 331 (1937)]. 

Place 25 mL of the decolorized filtrate obtained from the Sodium Chloride test (above) into a 125 mL Erlenmeyer flask, and add 1 drop of a 0.5% phenolphthalein solution in 50% ethanol. Add 0.05 N sodium hydroxide, dropwise, until the solution is alkaline to pH paper, and then add 0.002 N hydrochloric acid until the indicator is decolorized. Add 25 mL of ethanol and about 0.2 g of tetrahydroquinone sulfate indicator. Titrate with 0.03 N barium chloride solution to a red endpoint. Make a blank determination. 

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