The methods of bleaching by oxygen include all those which aim at the bleaching by exposure to the air and to sunlight (as in the case of artists' linseed-oil), or where oxygen or ozone is introduced in the form of gas or is evolved by chemicals, as manganese dioxide, potassium bichromate or potassium permanganate and sulphuric acid..
The series will thus possess the following general formulae M 2 CrO 4 M2Cr207 M2Cr30,0 &c. (M =one atom of a normal chromate bichromate trichromate monovalent metal.) Chromates.
Potassium chromate, K2Cr04, may be prepared by neutralizing a solution of potassium bichromate with potassium carbonate or with caustic potash.
They are readily decomposed by heat, leaving a residue of the normal chromate and chromium sesquioxide, and liberating oxygen; ammonium bichromate, however, is completely decomposed into chromium sesquioxide, water and nitrogen.
After the calcium sulphate has settled, the potassium chromate solution is converted into bichromate by the action of sulphuric acid, and the salt is allowed to crystallize.