Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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cipitate was dissolved with excess EDTA, and the unreacted EDTA was back titrat- ed with standardized magnesium chloride. This back titration method is regard- ed as an accurate procedure for the determining sulfate. [3] However, one performing the procedure [4] could have serious errors: incom- plete forming and precipitating of barium sulfate, (after adding 10% BaCl2, solution with precipitate BaSO4 was maintained only for 5min, at 75°C) and there could be possible loss of the precipitate during separation of precipitated bari- um sulfate by means of decanting. In order to overcome these problems and to improve the accuracy of the method, this work suggested the following changes in the procedure: 1. Precipitate BaSO4 is maintained for 2 hours at 75°C, while during the first hour the suspension is stirred. 2. In order to separate the solution in the centrifuge tube without losing any of the precipitated BaSO4, a vacuum device was used. (Fig.1,2) 3. According [3] the reduction of Cr (VI) to Cr (III) during the determina- tion of the sulfate in the presence of a high concentration of hexavalent chromium, should be performed, using glycerin. We replaced the reduc- ing agent hydroxyl-amine hydrochloride on the reducing agent glycerin with HCI to reduce Cr (VI) to Cr (III). Glycerin with hydrochloric acid rapidly and effectively reduce the chromium (VI), thus preventing the co- precipitation with barium sulfate. Hydrochloric acid provides the envi- ronment for complete precipitation of BaSO4. 4. In order to guarantee the complete dissolution of precipitate BaSO4 into EDTA + NH4OH mixture, centrifuge tubes were heated in an aluminum heating block with stirring, until all precipitate was dissolved. The dis- solution takes few minutes. These improvements enabled the determi- nation of sulfate in the chromium plating bath with a good accuracy. The EDTA method is faster than the gravimetric method for the determi- nation of sulfate and is quite accurate. EXPERIMENTAL Reagents and apparatus. All the materials were reagent grade and were used without further purification. Distilled and deionized water were used in the preparation of all solutions. Solutions were stored in glass and polyethylene bot- tles. Used deionized water (m-1-2 microSiemens). Indicator Eriochrom Black T: powder 2g EBT+198g NaCl. Standard solution EDTA 0,1M (37,224g C10H14N2Na2O8 x 2H2O +5g NaOH ) transfer into volumetric flask of 1liter and dilute to mark with deionized water(µ- 1-2). EDTA 0.02M. 200ml 0.1M EDTA transfer to volumetric flask 1liter and dilute with deionized water to mark. Solution of magnesium chloride. Solution 1: Fixanal 8,362g MgCl2 x 6H2O (1g of magnesium) was transfer to vol- 444

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