Analysis of Hypochlorite in Commercial Liquid Bleaches by Coulometric Titratlon Experiments in undergraduate analytical chemistry courses can be more interesting a s well as instructive if they are made relevant and practical by involving the study of real-life samples. This report describes a n experiment that involves the analysis of hypochlorite, the active ingredient in most commercial liquid bleaches, by coulometric titration. The hypochlorite content is determined by reacting the liquid bleach solution with.iodide and treating the iodine produced with a measured excess of thiosulfate solution previously standardized coulometrically. The excess thiosulfate is analvzed hv titration with iodine ~ .e n e r a t e deoulometrieallv. The end ooint is determined visuallv and wotentiametrically. A Sargent Model IV Coulometer with two platinum electrodes is employed as the constant current source. To s 4M)-ml beaker, add 100 ml of distilled water, 10 ml glacial acetic acid, 3 g potassium iodide, and 2 ml of a 2% starch solution. Immerse the two platinum foil electrodes and generate iodine until the solution takes on s light blue color which will be reproduced in all subsequent titrations in this same beaker. Pipet exactly 5 ml of a n approximately 0.02 N sodium thiosulfate solution into this beaker and standardize i t by the coulometric generation of iodine. Weigh out accurately about 5 g of a commercial bleach solution and dilute to 250 ml in a volumetric flask. Pipet 5 ml of the diluted bleach solution into the same beaker containing the titrated thiosulfate solution. Add exactly 10 ml of the standardized thiosulfate solution. (This will be sufficient to react with all of the iodine produced by the bleach-iodide reaction and provide a n excess of thiosulfate if the original bleach contains less than 7% sodium hypochlorite by weight.) Titrate coulometrically to the light blue end point. Other brands of bleach can be determined in an identical fashion, titrated in the same reaction vessel. A Beekman Zeromatic SS-3 pH meter fitted with a platinum and a calomel electrode together with a Beckman 5-in. recorder provides another means far the detection and determination of the equivalence point. The sharp break 12M)-300 mV) in the oxidation-reduction titration curve obtained eomsoonds to the end m i n t of the titration and eerees velv e n d ,~~ ~ o i nobtained t with the use of~~~~~-~~ the starch indicator. ~"~~~ ,well with the .~~~~~~ .~~~~~~~~~~~ The enperimenr rs rapid, bemg e a d g completed in one laboratory period, and the results are quite reproducible. The sodium hypochlorite content found in some commercial blcache, was as lolhws: Pure.. 6 . 3 7 ~\manufacturer's listed value. 6%):Clorox, 5 390 ,manufacturer's listed value. 5.25901; and Iris. 5.5% ~manufaclurer'sh t e d value, 3.25% ~~
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California State University Long Beach, California 90840
Van T. Lieu Gene E. Kalbus
Volume 52, Number 5. May 1975 / 335
