Determination of Cr(III) and Cr(VI) by solvent extraction-atomic absorption spectrophotometry

Abstract

Masking a large amount of Cr(III) with EDTA, micro amounts of Cr(VI) were extracted into methyl isobutyl ketone (MIBK) as diethyl dithiocarbamate (DDTC) complex and analyzed by atomic absorption spectrophotometry.(a) Determination of Cr(VI): Take a sample solution {containing less than 50 μg of Cr(VI)} into a 100 ml beaker, add 10.0 ml of 0.2 M EDTA solution and boil for 5 min. After cooling to room temperature, adjust the pH value to 45. Transfer the solution to a 100 ml separatory funnel, add 2.5 ml of 2% DDTC solution and make the total volume of it to 50 ml with distilled water. Add 10.0 ml of MIBK and shake the solution vigorously for 5 min. Wash the stopper of the separatory funnel with samll quantities of water and stand for 10 min. Separate the layers and measure the Cr concentration in the MIBK extract with a Hitachi model 207 atomic absorption spectrophotometer, using air-acetylene flame (analytical line; 3579 A). The extractability of Cr(VI) was 74.8%. The respective presence of less than 5-fold Bi(III), 20-fold Co(II), 10-fold Fe(III) and 5-fold V(V) of Cr(VI) did not interfere. (b) Determination of total Cr: Take a sample solution (containing less than 20 mg of Cr) into a beaker, add 2 ml of (1+4) H2SO4. Add 0.01 N KMnO4 solution dropwise while heating until the solution becomes faintly pink, and boil for 5 min. After cooling, dilute to constant volume with distilled water. Then, take a given volume (containing less than 40 μg of Cr) of the solution, adjust the pH value to 45 Transfer the solution to a 100 ml separatory funnel and determine amounts of Cr as the procedure (a) after addition of 2% DDTC solution. The extractability of Cr(VI) in this case was 98.1%.Amounts of Cr(III) were calculated from difference between those of total Cr and Cr(VI). The relative error was +3%.