er similar
conditions.

~Properties of chromates.~ The chromates are salts of the unstable chromic
acid (H_{2}CrO_{4}), and as a rule are yellow in color. Lead chromate
(PbCrO_{4}) 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(NO_{3})_{2} + K_{2}CrO_{4} = PbCrO_{4} + 2KNO_{3},

Ba(NO_{3})_{2} + K_{2}CrO_{4} = BaCrO_{4} + 2KNO_{3}.

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 (H_{2}Cr_{2}O_{7}) is
formed:

2K_{2}CrO_{4} + H_{2}SO_{4} = K_{2}Cr_{2}O_{7} + K_{2}SO_{4} + H_{2}O.

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~ (K_{2}Cr_{2}O_{7}). 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

K_{2}Cr_{2}O_{7} + 2KOH = 2K_{2}CrO_{4} + H_{2}O.

When added to a solution of lead or barium salt the corresponding
chromates (not dichromates) are precipitated. With barium nitrate the
equation is

2Ba(NO_{3})_{2} + K_{2}Cr_{2}O_{7} + H_{2}O
= 2BaCrO_{4} + 2KNO_{3} + 2HNO_{3}.

Potassium dichromate finds use in many industries as an oxidizing agent,
especially in the preparation of organic substances, such as the dye
alizarin, and in the construction of several varieties of electric
batteries.

~Sodium chromates.~ The reason why the potassium salt rather than
the sodium compound is used is that sodium chromate and
dichromate are so soluble that it is hard to prepare them pure.
This difficulty is being overcome now, and the sodium compounds
are replacing the corresponding potassium salts. This is of
advantage, since a sodium salt is cheaper than a potassium
salt, so far as raw materials go.

~Oxidizing action of chro