This reaction recalls the formation of a manganate under similar conditions.
Properties of chromates. The chromates are salts of the unstable chromic acid (H2CrO4), and as a rule are yellow in color. Lead chromate (PbCrO4) is the well-known pigment chrome yellow. Most of the chromates are insoluble and can therefore be prepared by precipitation. Thus, when a solution of potassium chromate is added to solutions of lead nitrate and barium nitrate respectively, the reactions expressed by the following equations occur:
Pb(NO3)2 + K2CrO4 = PbCrO4 + 2KNO3,
Ba(NO3)2 + K2CrO4 = BaCrO4 + 2KNO3.
The chromates of lead and barium separate as yellow precipitates. The presence of either of these two metals can be detected by taking advantage of these reactions.
Dichromates. When potassium chromate is treated with an acid the potassium salt of the unstable dichromic acid (H2Cr2O7) is formed:
2K2CrO4 + H2SO4 = K2Cr2O7 + K2SO4 + H2O.
The relation between the chromates and dichromates is the same as that between the phosphates and the pyrophosphates. Potassium dichromate might therefore be called potassium pyrochromate.
Potassium dichromate (K2Cr2O7). This is the best known dichromate, and is the most familiar chromium compound. It forms large crystals of a brilliant red color, and is rather sparingly soluble in water. When treated with potassium hydroxide it is converted into the chromate