Preparation and Analysis of Cr II Compounds

3 PREPARATION AND ANALYSIS OF Cr(II) COMPOUNDS As Fig. 11.1 shows, Cr(ll) compounds are readily oxidised to Cr(lll). Even traces of oxygen can oxidise the compounds. Hence they should be prepared in special apparatus to exclude oxygen. One of the most stable Cr(Il) compounds is the acetate (ethanonate) which is in fact a dimer containing short Cr-Cr multiple bonds. 

This can be obtained by reducing Cr(III) solutions eg by Zn and acid. When a Cr(II) solution is run into an excess of Fe(III) solution, the latter is reduced to Fe(II), which is equivalent to Cr(Il) and may be determined eg by titration against standard K2Cr207 solution, using a redox indicator. Fe(II) solutions are only slowly oxidised in air. 

When a solution of CrCl2, prepared by reducing Cr(III) chloride by zinc (as above), is mixed with sodium acetate, a precipitate of the Cr(II) acetate is obtained. This can be analysed as above. 

Assemble in the glove box all the deaerated solutions and all the glassware required and displace the air by nitrogen as described in Sec.2.9.1. Deaerate the following solutions 10 g chromium (III) chloride in HCl solution (25 cm cone. HCl + IS cm water) 15 g sodium ethanoate in 35 cm water. Add inside the box 10 g Zn to the Cr(III) solution, allow the reaction to proceed until the maximum blue colour is attained. Fill a vacuum 1 cm spectrophotometer cell with the supernatant solution. Then filter the remaining suspension into the ethanoate solution, which is being shaken, when a red precipitate is formed. Filter the precipitate in a Buchner assembly, wash on the filter with 20 cm water, 20 cm ethanol and finally 20 cm ether. Dry by pressing between filter paper. 

Record the spectrum of the reduced Cr solution over the range 800-200 nm, using water as a blank. 

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