Ammonium acid formate iron sulphate

Colorimetric Methods are used only for the estimation of very small percentages of vanadium, e.g. in vanadium steels and alloys. The most important depend on the intensity of the reddish-brown colour produced by the action of hydrogen peroxide on an acid vanadate solution.3 If chromium is present, an equal amount must be introduced into the standard vanadium solution under the same conditions of temperature, acid concentration, etc. Phosphoric acid is added to destroy any yellow colour due to ferric iron, and either hydrofluoric acid or ammonium fluoride to destroy any colour produced by titanium.4 A colorimetric method for the simultaneous estimation of small quantities of titanium and vanadium has also been worked out.5 Other colorimetric processes are based on (a) the formation of a yellow to black coloration, due to aniline black, in the presence of aniline hydrochloride and potassium chlorate or other oxidising agent,6 and (b) the orange coloration finally produced when an acid solution of a vanadate is brought into contact with strychnine sulphate.7... 

Nitroso derivatives of a more complex and more stable character than the foregoing were discovered by Roussin in 1858.2 This investigator observed that a black voluminous precipitate is obtained when a mixture of ammonium sulphide and alkali nitrite is added to an aqueous solution of ferrous sulphate. On boiling, the precipitate passes into solution. The liquid is filtered, and, upon cooling, black crystals separate out, the composition of which has been the subject of considerable discussion. The reaction does not proceed in perfectly neutral solution, a green liquor only being produced, consisting of sulphides of iron and sodium, entirely free from any nitroso derivative. The presence of a small quantity of acid, however, results in the formation of the nitroso derivative, probably because it liberates nitrous acid, which acts direct upon the ferrous salt. 

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