クロム鍍金液中のケイフッ化物の光度滴定

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Usual methods of determination of the concentration of sodium silicofluoride (510 g/<I>l</I>) in chromium plating baths require a complicated procedure to remove chromic acid before the estimation. The authors have devised a rapid method for silicofluoride determination, using a direct photometric titration with a standard thorium nitrate solution without separation of chromic acid. It was found that ferric salicylate in a monochloroacetic acid buffer solution of pH 2.8 gave maximum absorption at 515 mμ, while Cr (III) showed minimum absorption at about 490 mμ; chromic acid gave almost no absorption in a region longer than 510 mμ. The chromium plating solution, adjusted to about pH 3 with NaOH, is treated with a monochloroacetic acid buffer solution of pH 2.8, a small amount of ferric salicylate or sodium salicylate as an indicator, and Emasol 1130 (Kao Soap Co.), in the order named. The solution is titrated with a standard thorium nitrate solution using a 510 mμ or 545 mμ interference filter. The measurement range was 2. 612 mg/ 50 m<I>l</I> with error ± 2 %, and the time required for the determination was 20 minutes.
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