How to determine total chromium count
Total Chromium Photometric Method (O-30 ppm as CrO4 )
Chromium (Total)
Hexavalent chromium (Cr. vi) is an effective corrosion inhibitor in aqueous systems. Trivalent chromium (Cr. iii) is not an effective corrosion inhibitor. In open recirculating cooling water systems, contamination by reducing agents, such as hydrogen sulfide will reduce the hexavalent chromium to trivalent chromium, destroying effectiveness as a corrosion inhibitor. Certain chromate removal wastewater treatment system rely on reduction of chromate (vi) to chromium
(iii) and precipitation as chromic hydroxide [Cr(OH)3] to reduce chromium concentration in the plant effluent.
The total chromium test described below is used to monitor chromium levels in a stream or a system; it measures the sum of Cr (iii) and Cr (vi). Total chromium minus chromate (vi) yields the amount of trivalent chromium, Cr (iii)
Theory of Test
Chromium (iii) is oxidized to chromium (vi) by an acidic ceric solution. The resulting hexavalent chromium is determined by the diphenyl carbazide colorimetric test.
Apparatus Required
Filter photometer complete with assorted laboratory glassware.
Chemicals Required
Chromate Reagent
Chromate Standard
Chromium Reagent
Sulfuric Acid, 10%
Procedure for Test
This procedure uses a wavelength of 540-nm and 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 analyses of the water sample.
Into a 250 ml glass flask, first pipet 5.0 ml of sample and then 0.5 ml of chromium reagent. Swirl the solution and wait 10 minutes before proceeding. Add 100 ml of chromate-free water to the flask with a 100-ml plastic graduate. Pipet 2.0 ml of 10 per cent sulfuric acid into the flask, swirl the solution and adjust the temperature to 70 to 75F. Use a portion of the prepared solution to set the photometer at zero absorbance. Pour the contents of the cell back into the flask containing the prepared solution and then pipet into the flask 1.5 ml of chromate reagent. Swirl the solution and wait exactly three minutes before measuring the absorbance of the photometer.
Calculation of Results
Compare the photometer absorbance with the calibration curve and read the results directly in parts per million of total chromium as CrO4.
To find the trivalent chromium present, subtract Cr(vi) as CrO4 from the total chromium as CrO4 and multiply the result by 0.45 = chromium as Cr+3.
Limitations of Test
This test is not affected by the ions found in most waters. The procedure incorporates a 1 to 20 dilution, which yields an effective range of 0 to 30 ppm as CrO4.
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