The precipitate formed by sulphuretted hydrogen may contain the black mercuric, lead, and copper sulphides, dark-brown bismuth sulphide, yellow cadmium and arsenious sulphides, orange-red antimony sulphide, brown stannous sulphide, dull-yellow stannic sulphide, and whitish sulphur, the last resulting from the oxidation of sulphuretted hydrogen by ferric salts, chromates, &c. Warming with ammonium sulphide dissolves out the arsenic, antimony and tin salts, which are reprecipitated by the addition of hydrochloric acid to the ammonium sulphide solution.
In aqueous solution the free acid acts as an oxidizing agent, bleaching indigo and liberating iodine from potassium iodide, or it may act as a reducing agent since it readily tends to pass into nitric acid: consequently it discharges the colour of acid solutions of permanganates and chromates.
Chromic acid and its salts, the chromates and bichromates, can be detected by the violet coloration which they give on addition of hydrogen peroxide to their dilute acid solution, or by the fact that on distillation with concentrated sulphuric acid and an alkaline chloride, the red vapours of chromium oxychloride are produced.
The yellow colour of normal chromates changes to red on the addition of an acid, but goes back again to yellow on making the solution alkaline.
Normal chromates on the addition of silver nitrate give a red precipitate of silver chromate, easily soluble in ammonia, and with barium chloride a yellow precipitate of barium chromate, insoluble in acetic acid.
Reducing agents, such as sulphurous acid and sulphuretted hydrogen, convert the chromates into chromic salts.
Chromium forms three series of compounds, namely the chromous salts corresponding to CrO, chromous oxide, chromic salts, corresponding to Cr203, chromium sesquioxide, and the chromates corresponding to Cr0,, chromium trioxide or chromic anhydride.
Soret has confirmed, for the ultra-violet rays, Dr Gladstone's conclusions with regard to the identity of the absorption spectra of different chromates.
The chromates of sodium, potassium and ammonium, as well as the bichromates of potassium and ammonium, were found to give the same absorption spectrum.
Nor is the effect of these chromates confined to the blocking out simply of one end of the spectrum, as in the visible part, but two distinct absorption bands are seen, which seem unchanged in position if one of the above-mentioned chromates is replaced by another.
Other instances of the isomorphism of thallous with potassium salts are the nitrates, phosphates, hydrazoates, sulphates, chromates, selenates, and the analogously constituted double salts, and also the oxalates, racemates and picrates.
The selenates are isomorphous with the chromates and sulphates.
BICHROMATES AND CHROMATES.
Chromium trioxide dissolves readily in water, and the solution is supposed to contain chromic acid, H 2 CrO 4; the salts of this acid are known as the chromates.
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.
- The alkaline chromates are usually obtained by fusion of a chromium compound with an alkaline carbonate and an oxidizing agent, such for example as potassium nitrate or chlorate.
The majority of the chromates are yellow in colour, and many of them are isomorphous with the corresponding sulphates.
The alkaline chromates are soluble in water, those of most other metals being insoluble.
The chromates are stable towards heat; they are poisonous, and may be recognized by the yellow precipitates they give with soluble barium and lead salts.
Very insoluble chromates, such as lead, remove all the color from the supernatant liquid.
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