Preparation of Ammonium Chromate

Preparation of Ammonium Chromate

A saturated solution of ammonium chromate is treated with an excess of concentrated ammonium hydroxide solution until the strong smell of ammonia appears and the color of the solution changes from orange to yellow. Then the reaction mixture is cooled in ice water, the obtained yellow crystals of ammonium chromate are filtered, washed with cold concentrated ammonium hydroxide solution and dried at room temperature.

100 grams of chromium trioxide are dissolved in 70 ml of water. The mixture is stirred until solution is complete and then the flask is cooled with ice water. 160 ml of the concentrated (28-30%) ammonium hydroxide solution in 5 ml portions are added dropwise. After each addition, the mixture is cooled and stirred in order to prevent the formation of a solid cake. After addition of 100 ml of the ammonium hydroxide any formed lumps are divided with a glass rod and the remaining 60 ml of ammonium hydroxide are added in four portions.

The mixture is stirred thoroughly, then cooled in ice water and stirred for additional 10 minutes. The obtained solid is filtered. The filtrate is used to rinse out the flask and transfer all the solid to the filter. Any remaining liquor is removed as completely as possible by pressing the ammonium chromate on the filter. Then the crystals are treated with 10 ml of concentrated (28-30%) ammonium hydroxide solution, and after it has stood a few minutes, the suction is applied again.

The obtained filtrate is evaporated to half the original volume, then the flask is placed in ice water, and when the liquid has cooled to about 60° C, 10 ml of concentrated (28-30%) ammonium hydroxide solution are added and stirred for 10 minutes. The second crop of crystals is filtered and treated as before with 10 ml of concentrated (28-30%) ammonium hydroxide solution. If the two crops are the same color, they are combined and dried together. If, however, the second crop is much darker than the first, it should be kept separate. Ammonium chromate is dried at room temperature in a desiccator.

Synthetic inorganic chemistry, by A. A. Blanchard, 326-327, 1936

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