Ammonium Chromate and Dichromate

Ammonium Chromate and Ammonium Dichromate, (NH4)2Cr04 and (NH4)2Cr207 

The raw material for this preparation is the chromic anhydride from the preceding preparation, which contains at least a small amount of sodium acid sulphate. This dissolves with the chromic anhydride, and it should remain in the mother liquor (as neutral sulphate) from which the ammonium chromate is crystallized. The second and third crop of crystals of the latter, however, are likely to be contaminated with sodium sulphate. An excess of ammonium hydroxide is used to insure the formation of the neutral chromate. 

To convert the ammonium chromate to dichromate it is dissolved in acetic acid. Calculate the volume of 6N acetic add required to convert the weight taken of dry ammonium chromate according to the equation  

Actually use 100 cc. of 6 AT acetic acid for each 45 grams of ammonium chromate. Pour this volume of 6 N acetic acid into a 500-cc. flask and add the solid ammonium chromate. Insert a funnel in the neck of the flask and heat the mixture until it just boils. Keep it below the boiling temperature until solution is complete. If solution is not complete in 10 minutes, add a little more 6N acetic acid through the funnel. When solution is complete, place the flask containing the solution in a pan of ice water and shake for 10 minutes. Filter off the ammonium dichromate, using suction. Remove the liquor as completely as possible by pressing the solid 

Heap up 10 grams of ammonium dichromate in a small mound on a porcelain plate, and apply the flame to the top of the mound until a reaction starts. Write the equation. What element is oxidized and what one reduced Show that the algebraic sum of the valence changes is zero. 

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Ammonium chromates.—The modes of preparation and the properties of ammonium chromate and dichromate are considered in Vol. VII., Part III. 

Both Cr(III) and Cr(VI) form compounds whose water solubilities range from very high to very low. Many compounds of industrial importance, such as the sodium, potassium, and ammonium chromates and dichromates, chromium trioxide (CrOs), and the hydrated Cr(III) nitrate, chloride. 

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