Chromium Oxides and Hydroxides

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26-11-2018

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Chromium (II) oxide

Chromium (II) oxide, CrO, is basic. It is found in the form of an insoluble black powder.

Chromium (III) oxide

Chromium (III) oxide, Cr₂O₃ is the main oxide of chromium. It is amphoteric and while it is insoluble in water, it will dissolve in acid. It is found in nature in the form of a rare mineral, eskolaite. It is used as a pigment, producing a dark green color.

Chromium dioxide

Chromium dioxide or chromium (IV) oxide, CrO₂, in its natural state looks like black crystals. It exhibits ferromagnetic properties and was once widely used as a synthetic magnet in magnetic data tape such as audio cassette tapes. It was considered to be one of the most perfect magnetic pigments for recording tapes because of its thin, long, glass rod like crystals. This amorphous solid can be formed through the thermal decomposition of dichromate complexes.

Chromium trioxide

Chromium trioxide or chromium (VI) oxide, CrO₃ is acidic oxide, or acidic anhydride of chromic acid. It will react with water to form chromic acid and will react with a base to form a chromium salt. In solid form, it is a dark red-orange granular complex. It is used in chrome-plating as a strong oxidizer, however, it is extremely toxic.

Chromate and Dichromate

Chromate, CrO₄^2-, is a salt of chromic acid. This salt is associated with a yellow color in basic conditions, for example potassium chromate. Dichromate, Cr₂O₇^2-, is a salt of dichromic acid. This salt is associated with a strong orange color in acidic conditions, for example potassium dichromate. However, compounds of chromate or dichromate with heavy metals usually display a red color. Dichromate is a strong oxidizing agent but it is a bad precipitating agent. Chromate on the other hand is used as a precipitating agent but it is a poor oxidizing agent. Chemical equilibrium is displayed when either anion is in an aqueous solution.