798 



CHROME GEEEN 



The neutral chromate is a yellow salt, prepared by heating chrome iron-ore with a 

 salt of potash. It contains : 



Potash . 

 Chromic acid 



48-0 

 52-0 



100-0 



The value of a solution of chromate of potash, if it be tolerably pure, may be inferred 

 from its specific gravity by the following table : 



At specific gravity 1-28 it contains about 50 per cent, of the salt. 

 1-21 33 



1-18 

 1-15 

 1-12 

 1-11 

 1-10 



25 

 20 

 16 

 14 

 12 



The acid chromate or bichromate (called by modern chemists the dichromate or 

 anhydrochromate of potassium) is a red salt, which by slow cooling, may bo obtained 

 in tie form of square tables, with bevelled edges, or flat, four-sided prisms. They are 

 permanent in the air, have a metallic and bitter taste, and dissolve in about one-tenth 

 of their weight of water at 60 F., but in one half of their weight of boiling water. 

 The composition of bichromate of potash is: potash, 3T6 ; chromic acid, 68 '4. 



In making the red bichromate of potash from solutions of the yellow salt, nitric acid 

 was at first chiefly used ; but, in consequence of its relatively high price, sulphuric, 

 muriatic, or acetic acid has been frequently substituted upon the large scale. 



(For the preparation of both these salts refer to CHHOMB IRON.) 



These salts are much employed in CALICO PRINTING and in DYEING. The bichromate 

 of potash is also used in the photographic process known as Carbon-printing ; a sensitive 

 surface being obtained by bichromate of potash and gelatine. Another modern use 

 of the bichromate is in a convenient form of galvanic cell, where it serves to oxidise 

 the hydrogen set free by the action of an acid upon zinc. 



Chromate of Potash, Adulteration of, to detect. The chromate of potash has the 

 power of combining with other salts up to a certain extent without any very sensible 

 change in its form and appearance ; and hence it has been sent into the market 

 falsified by very considerable quantities of sulphate and muriate of potash, the pre- 

 sence of which has often escaped observation, to the great loss of the dyers who use 

 it so extensively. The following test process has been devised by M. Zuber, of 

 Miilhouse : Add a largo excess of tartaric acid to the chromate in question, which 

 will decompose it, and produce in a few minutes a deep amethyst colour. The 

 supernatant liquor will, if the chromate be pure, afford now no precipitate with the 

 nitrates of baryta or silver ; whence the absence of the sulphates and muriates may 

 be inferred. We must, however, iise dilute solutions of the chromate and acid, lest 

 bitartrate of potash be precipitated, which will take place if less than 60 parts of 

 water bo employed. Nor must we test the liquid till the decomposition be complete, 

 and until the colour verge rather towards the green than the yellow. Eight parts of 

 tartaric acid should be added to one of chromate to obtain a sure and rapid 'result. 

 If nitrate of potash {saltpetre) is the adulterating ingredient, it may be detected by 

 throwing it on burning coals, when deflagration will ensue. The green colour is a 

 certain mark of the transformation of the chromic acid partially into the chrome oxide; 

 which is effected equally by the sulphurous acid and sulphuretted hydrogen. Here 

 this metallic acid is deoxygenated by the tartaric, as has been long known. The tests 

 which I should prefer are the nitrates of silver and baryta, having previously added 

 so much nitric acid to the solution of the suspected chromate as to prevent the pre- 

 cipitation of the chromate of silver or baryta. The smallest adulteration by sulphates 

 or muriates will thus be detected. 



A mixture of sulphate of soda and chloride of sodium tinged with strong solution 

 of chromium is sometimes sold for pure bichromate of potash. H. M. N. 



CHROIVIATYPE. A photographic process. See PHOTOGRAPHY. 



CHROIYIATYPOGRAPHY. A French process for printing letter-press in 

 colours. 



CHROME AXiTTM. A double sulphate of sesquioxide of chromium and potash. 

 It is obtained as a by-product in the preparation of the aniline colours, and is some- 

 times used in dyeing. See ALUM. 



CHROME CREEIT. This term is applied both to the green oxide (sesquioxide) 

 of chromium', and to a green pigment prepared by mixing chrome yellow with Prussian 

 blue. 



