Acid-base equilibria of Cr(III) were summarized in Problem 9-37. Cr(VI) in aqueous solution above pH 6 exists as the yellow tetrahedral chromate ion, CrO42. Between pH 2 and 6, Cr(VI) exists as an equilibrium mixture of HCrO4 and orange-red dichromate, Cr2O72. Cr(VI) is a carcinogen, but Cr(III) is not considered to be as harmful. The following procedure was used to measure Cr(VI) in airborne particulate matter in workplaces.
1. Particles were collected by drawing a known volume of air through a polyvinyl chloride filter with 5- μM pore size.
2. The filter was placed in a centrifuge tube and 10 mL of 0.05 M (NH4)2SO4/0.05 M NH3 buffer, pH 8, were added. The immersed filter was agitated by ultrasonic vibration for 30 min at 35 C to extract all Cr(III) and Cr(VI) into solution.
3. A measured volume of extract was passed through a "strongly basic" anion exchanger (Table 25-1) in the Cl form. Then the resin was washed with distilled water. Liquid containing Cr(III) from the extract, and the wash was discarded.
4. Cr(VI) was then eluted from the column with 0.5 M (NH4)2SO4/0.05 M NH3 buffer, pH 8, and collected in a vial.
5. The eluted Cr(VI) solution was acidified with HCl and treated with a solution of 1,5-diphenylcarbazide, a reagent that forms a colored complex with Cr(VI). The concentration of the complex was measured by its visible absorbance.
(a) What are the dominant species of Cr(VI) and Cr(III) at pH 8?
(b) What is the purpose of the anion exchanger in step 3?
(c) Why is a "strongly basic" anion exchanger used instead of a "weakly basic" exchanger?
(d) Why is Cr(VI) eluted in step 4 but not step 3?