Metal Finishing Guide Book

2012-2013

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troubleshooting, testing, & analysis VOLUMETRIC COMPLEXOMETRIC METHOD TO DETERMINE SULFATE CONTENT IN CHROMIUM PLATING SOLUTION BY DR. V. PERSITS, ISRAEL AEROSPACE INDUSTRIES, PETAH, TIKVA Abstract. The complexometric method involves the use of glycerin, in the presence of HCI (1:1), to reduce chromium (VI) to chromium (III). Sulfate is precipitated by barium in the presence of Cr (III) and the sediment separated by centrifuging, when the solution is separated from precipitated barium sulfate by means of vacuum suction. The precipitate is dissolved with excess EDTA in ammonium environment, using heating block, and unreacted EDTA is back titrated with standardized magnesium chloride. Concentration of sulfate in chromium plating solutions can be determined by various methods: by centrifugation, gravimetrically, with some volumetric methods and etc. [1,2,3] The centrifugation is used in plating shops for the rapid determination of sulfate in chromium plating bath, (sulfate���as barium sulfate). H`owever the determination of sulfate by centrifugation is rapid but relatively inaccurate compared to classic analytical methods (gravimetric and volumetric). In the gravimetric method the determination of sulfate ���barium sulfate is precipitated, washed, dried and weighed. The gravimetric methods are generally accurate and are often used for the check work when a substance is determined more than one method. (*Israel Aerospace Industries LTD.) However, gravimetric method for determination of sulfate is very time consuming and may be less accurate in case of chromium plating bathes, because coprecipitation chromium (III) and other ions with barium sulfate (the color of precipitate BaSO4, precipitated from chromium plating solutions, is yellow-green or white-green, but not white; the ratio CrO3 : SO4 in the solution is ~100:1. The concentration of iron and other metals could be up to 15g/l.) The sulfuric acid in the plating solution serves as the ���catalyst��� that is essential if any plating is to take place. It must be present within very narrow limits (2.5���3 g/l) to control the quantity and character of the deposit. Therefore, it is desirable to have more than one method (in addition to the gravimetric method) for the determination of sulfate in chromium plating solutions. This provides an accurate method to determine the sulfate (2.5 ���3 g/l) in the presence of 100-fold excess of chromic acid and other metals, presented in chromium baths (Fe, Cu, Ni, Mg). Volumetric-complexometric method for the determination of sulfates in electrolytes has few advantages, namely speed, accuracy, and selectivity. [2] W.W. White and M.C. Henry [4] were described the method, ���The determination of sulfate in chromium plating baths by EDTA titration.���[4] They used hydroxylamine hydrochloride to reduce chromium (VI) to chromium (III). Barium sulfate was precipitated in the presence of chromium (III) and separated from the mother solution via centrifuge, followed by decantation. The pre504

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