Bismuthyl Nitrite, 2BiONO₂.H₂O

Bismuthyl Nitrite, or Basic Bismuth Nitrite, 2BiONO₂.H₂O, is obtained as a yellowish-white precipitate when sodium nitrite is added to an aqueous solution of normal bismuth nitrate and mannitol. The anhydrous salt can be obtained by drying over sulphuric acid in a vacuum desiccator. The salt decomposes when heated above 60° C., with evolution of nitrogen tetroxide.

Although attempts to prepare pure normal bismuth nitrite have not proved successful, a number of complex nitrites, or bismuthinitrites, have been obtained. They may be prepared by precipitation from a strong solution of alkali nitrite by bismuth nitrate or a mixture of the nitrates of a third metal and bismuth. Two groups of salts have been prepared, the simple bismuthinitrites, for which the general formula is M₃Bi(NO₂)₆, and the mixed bismuthinitrites, having a general formula M₂M'Bi(NO₂)₆, where M represents ammonium, potassium, rubidium, caesium or thallium, and M' represents one of the three metals lithium, sodium or silver. All possible compounds of the first type, except that of ammonium, have been obtained, as also have all possible compounds of the second type. All the salts are highly crystalline, the simple type forming orange or yellow hexagonal plates, but the mixed type pale yellow to red crystals, probably octahedral. The simple bismuthinitrites of caesium and thallium differ somewhat in constitution from the normal type, the caesium compound, Cs₃Bi(NO₂)₆.Bi(NO₂)₃, containing an additional molecule of bismuth nitrite, while that of thallium, Tl₃Bi(NO₂)₆.TlNO₂.H₂O, contains an additional molecule of thallous nitrite. All compounds that have been prepared are readily hydrolysed by water; the simple bismuthinitrites are generally less stable and more soluble than the mixed salts. Attempts to prepare compounds containing two metals of the M group, or two of the M' group, have proved unsuccessful; so also have attempts to prepare salts of the type M₃'Bi(NO₂)₆, thus, sodium bismuthinitrite has not been obtained in the solid form, but is probably produced in solution when bismuth nitrate is added to a solution of sodium nitrite. The reagent prepared in this way has been suggested for use in the detection and separation of rubidium and caesium, while the solution obtained by adding the nitrates of bismuth and caesium to a solution of potassium nitrite has been used for the estimation of sodium, owing to the formation of sodium ccesium bismuthinitrite, which is soluble only with difficulty. This substance, however, appears to vary in composition according to the method of preparation.

In addition, a number of bismuthinitrites containing nickel have been prepared by the addition of a salt of any of the metals of the M group to a solution containing nickel nitrate, bismuth nitrate, and sodium or lithium nitrite. The composition of these compounds has not in all cases been definitely established.

By the addition of a saturated solution of bismuth nitrate, in varying proportions, to a saturated solution of sodium cobaltinitrite, three distinct bismuth oxycobaltinitrites, or bismuthyl cobaltinitrites, have been obtained, ranging in colour from brick-red to yellow and having the compositions (BiO)₃Co(NO₂)₆, (BiO)₃Co(NO₂)₅, and (BiO)₃Co(NO₂)₄, respectively. These compounds are all very hygroscopic. They are decomposed by water on standing, are readily decomposed by acids with evolution of nitrogen peroxide, and by solutions of sodium and ammonium hydroxides. They are insoluble in ether and only slightly soluble in alcohol. The following co-ordination formulae have been suggested: [Co(NO₂)₆]3BiO, [Co(NO₂)₅(BiO)]2BiO, and [Co(NO₂)₄(BiO)₂]BiO.