Licence: Public Domain Mark
Credit: Qualitative chemical analysis / by C. Remigius Fresenius. Source: Wellcome Collection.
Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.
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![^^^l^^\f''''Sf towards the end of the operation to blue; on addm^ water the red colour is restored. ^ 4. Sulphuretted hydrogen does not precipitate solutions of cobalt salts of strong acids, if they contain free acid; from neutral solutions n ?ro eJ^ precipitates part of the cobalt as black sulphide of cobalt i^oto H-ob]. Acetate of protoxide of cobalt is not precipitated, or only to a very slight extent, in presence of free acetic acid, but in the absence of fi'ee acid it is precipitated completely, or ahnost completely It the solution contain enough of an acetate of an alkali, sulphuretted hydrogen will precipitate the whole of the cobalt from the hot solution even ^vhen excess of acetic acid is present, ' 5. Sulphide of ammonium precipitates from neutral, and sul- phuretted hydrogen from alkaline solutions, the whole of the metal as black hydrated sulphide of cobalt, CoS [CoS]. Chloride of ammo- nium promotes the precipitation most materially. Sulphide of cobalt IS uasoluble m alkalies and sulphide of ammonium, almost insoluble in acetic acid, very sparingly soluble in hydrochloric acid, and if precipi- tated from hot solutions almost insoluble in the latter. Nitric acid or nitro-hydrochloric acid, however, readily dissolves it on appHcation of 6. Potassa and soda produce blue precipitates ofbasicsalts of cobalt, which, if the solutions are sufficiently dilute, are insoluble in excess of the precipitants; the precipitate on exposure to the air absorbs oxygen and turns dii ty green; this subsequently passes to a grayish-yeUow, and on boHing is converted into the pale red hydrate of protoxide of cobalt, which contains alkali, and generally appears rather discoloured from sesquioxide formed in the process. If alcohol is added before boiling, the precipitate is rapidly converted into the dark- brown hydrate of the sesquioxide. If sufficient chloride of ammonium is present, caustic alkalies do not produce a precipitate. Neutral carbonate of ammonia dissolves the washed precipitates of basic salt or hydrated protoxide completely to an intensely violet-red solution, in which a somewhat larger proportion of potassa or soda produces a blue precipitate, the liquid still retaining its violet colour. If a very concentrated solution of potassa in excess is added to a solution of a salt of cobalt, or if hydrated oxide of cobalt is heated with a very little water and some soHd potash, the whole of the cobalt dissolves to a blue liquid containing cobaltate of potassa. 7. Ammonia produces the same precipitate as potassa, but it redis- solves in excess of ammonia to a reddish solution, which turns brownish- red on exposure to the air; the addition of potassa or soda to this throws down a portion of the cobalt as blue basic salt. Ammonia pro- duces no precipitate in solutions containing ammonia salts or a free acid. 8. If potassa or soda is added to a solution of a cobalt salt and then bromine water, a solution of chloride of soda or iodine, and the mixture is boiled, the whole of the cobalt is precipitated as the brownish- black hydrated peroxide of cobalt, provided the solution is suf- ficiently dilute. This precipitate is insoluble in a mixture of ammonia with chloride of ammonium solution, and in cyanide of potassium. If, however, a large quantity of hydrated peroxide of nickel is present, the cobalt oxide dissolves in cyanide of potassium along with the nickel oxide. N.B. The presence of non-volatile organic acids or sugnr interferes with or prevents the precipitation of cobalt as oxide or peroxide.](https://iiif.wellcomecollection.org/image/b21966953_0150.jp2/full/800%2C/0/default.jpg)
