Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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control, analysis, and testing 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 sepa- rated from precipitated barium sulfate by means of vacuum suction. The precipitate is dis- solved 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 meth- ods and etc. [1,2,3] The centrifugation is used in plating shops for the rapid determination of sul- fate in chromium plating bath, (sulfate–as barium sulfate). H`owever the deter- mination of sulfate by centrifugation is rapid but relatively inaccurate com- pared to classic analytical methods (gravimetric and volumetric). In the gravimetric method the determination of sulfate –barium sulfate is pre- cipitated, washed, dried and weighed. The gravimetric methods are generally accu- rate 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 con- suming and may be less accurate in case of chromium plating bathes, because co- precipitation chromium (III) and other ions with barium sulfate (the color of pre- cipitate 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 con- centration 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 deter- mination 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 elec- trolytes has few advantages, namely speed, accuracy, and selectivity. [2] W.W. White and M.C. Henry [4] were described the method, "The determi- nation 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 separat- ed from the mother solution via centrifuge, followed by decantation. The pre- 442

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