how to determine chromate ion
Chromate Photometric Method (0-30 ppm as CrO4 )
Theory of Test
This test is based on the development of a reddish color produced by the reaction of hexavalent chromium with diphenyl carbazide.
Apparatus Required
Filter photometer complete with assorted laboratory glassware.
Chemicals Required
Chromate Reagent
Chromate Standard
Sulfuric Acid, 10%
Procedure for Test
This procedure uses a wavelength of 540 nm and a minimum light path of 10 mm. Prepare a calibration curve for the photometer using successive dilutions of the chromate standard to adequately cover the anticipated range of chromate in the sample. The dilutions of the standard should be treated in exactly the same manner as that shown below for analysis of the water samples.
Pipet 5.0 ml of sample into a 250 ml glass flask. Add 100 ml of clear, chromate-free water with a 100-ml plastic graduate. Pipet 1.5 ml of 10 per cent sulfuric acid into the flask and swirl to mix. Use a portion of this solution to set photometer at zero absorbance. Pour contents of cell back into flask containing prepared solution. Pipet 1.5 ml of chromate reagent into flask and swirl to mix. After four minutes, measure absorbance on the photometer.
Calculation of Results
Compare the photometer absorbance with the calibration curve and read the results directly in parts per million of chromate at CrO4.
Limitations of Test
This procedure is not affected by the ions normally found in water. The test determines only hexavalent, not trivalent chromium. The above procedure incorporates a 1 to 20 dilution, yielding an effective range of 0 to 30 ppm as CrO4.
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