Chromium identification

Chromium confirmatory tests

Chromium is a group 3 cations. If you are interested in group 3 analysis click the link!

1) Addition of lead acetate (CH3COO)2Pb

This test is exactly the opposite of the one we saw to confirmate the presence of lead during group 1 analysis!

We need to set up a slightly acidic pH in order to avoid the precipitation of lead (Pb2+) as lead hydroxide((Pb(OH)2)) and the transformation of chromate in dichromate. The acetic acid buffer is perfect to fix the pH around 5 (should be verified with litmus test).For further confirmation, just check the solubility in strong bases (the precipitate dissolves because lead is moved to plumbate). For more details → lead confirmation.

2) Formation of chromium (VI) oxide peroxide

It's a very interesting and specific essay to identify chromium compounds.

We use nitric or sulfuric acid in order to have a pH around 3. After that, we add a few mL of diethyl ether or isoamyl alcohol to the test tube (the goal is to form a two-phases system) and then a few drops of hydrogen peroxide. Plugging the top of the tube with the finger we shake it to encourage the extraction of the new compound (chromium oxide peroxide) in the organic layer. The test is extremely sensitive but requires the operator to be extremely carefull. The pH is fundamental and it also has to be performed without heating and very quickly. Chromium oxide peroxide is indeed very unstable and could easily decompose in the acqueous phase even before we start shaking.

CrO42− + 2 H2O2 + 2 H+ → CrO5 + 3H2O