Heteropoly-Niobates

The oxalo-niobates constitute the only well-defined series of heteropoly-niobates. Indications of the formation of salts of other heteropoly- acids containing complex anions depend on the following observations:

1. Niobium pentoxide yields complex compounds with tungsten trioxide, WO₃.
2. Addition of potassium chromate to a solution of niobium oxytrichloride yields a yellow precipitate of complex constitution.
3. The precipitation of niobic acid is hindered in the presence of titanic acid.
4. Whereas niobic acid is readily precipitated from solutions of niobates by the action of very weak acids, arsenic acid, citric acid, tartaric acid and malonic acid behave like oxalic acid and do not yield precipitates.
5. Addition of sodium phosphate to a solution of niobium oxytrichloride or phosphoric acid to a concentrated solution of an alkali niobate yields a precipitate which contains both phosphoric acid and niobic acid.
6. Many complex titano-niobates, silico-niobates, titano-silico-niobates, uranyl-titanoniobates, silico-zircononiobates, and tantalo-niobates occur naturally.

Oxalo-niobates or niobo-oxalates correspond to the vanado-oxalates, and contain both oxalic acid and niobic acid radicals in the complex anion. The only known series possesses the general formula 3R^•₂O. Nb₂O₅.6C₂O₃.xH₂O, where R stands for an alkali metal. The sodium, potassium and rubidium salts are prepared by fusing one molecular proportion of niobium pentoxide with three molecular proportions of the alkali carbonate in a platinum crucible. The aqueous extract of the melt is poured into hot oxalic acid solution; concentration and cooling, or addition of alcohol or acetone, then brings about precipitation of the complex salt. Comparison of the electrical conductivity measurements of solutions of the alkali oxalo-niobates with those of the alkali hydrogen oxalates determined under the same conditions indicates that the oxalo-niobates are hydrolysed in aqueous solution, and that their anions contain a complex oxalo-niobic acid radical.

Ammonium Oxalo-niobate, 3(NH₄)₂O.Nb₂O₅.6C₂O₃.3H₂O, is prepared by fusing niobium pentoxide with potassium carbonate, dissolving the melt in water and precipitating the solution with hydrochloric acid. The hydrated niobic acid so obtained is then dissolved in a solution of ammonium hydrogen oxalate. Cooling, or addition of alcohol, yields beautiful, glistening crystals which, however, readily undergo hydrolysis in water with precipitation of niobic acid.

Potassium Oxalo-niobate, 3K₂O.Nb₂O₅.6C₂O₃.4H₂O, can be recrystallised unchanged from water, and also separates on the addition of acetone to its aqueous solution. It loses two molecules of water of crystallisation between 60° and 65° C. and the remainder at 150° C., at which temperature decomposition begins. When heated in a stream of chlorine, carbon tetrachloride, or hydrogen chloride, it leaves a residue of potassium chloride and niobium pentoxide. Its solution reacts acid towards phenolphthalein, and yields precipitates with salts of a large number of metals, but not with zinc, manganese or mercuric salts. Attempts to prepare other potassium niobium oxalates have been unsuccessful.

Rubidium Oxalo-niobate, 3Rb₂O.Nb₂O₅.6C₂O₃.4H₂O, is obtained in small needles by precipitation from its aqueous solutions with alcohol. It forms supersaturated solutions very readily.

Sodium Oxalo-niobate, 3Na₂O.Nb₂O₅.6C₂O₃.8H₂O, is similar to the potassium salt, but displays a greater tendency to form supersaturated solutions.

Barium Oxalo-niobate, 5BaO.Nb₂O₅.10C₂O₃.20H₂O, does not belong to the alkali series described above. It is prepared either by the addition of barium chloride to a solution of potassium oxalo-niobate, or by digestion of a mixture of barium oxalate and hydrated niobium pentoxide in a solution of oxalic acid. The crystals are soluble in oxalic acid solution but insoluble in water and in cold hydrochloric and nitric acids.

Attempts to prepare the normal oxalo-niobic acid, Nb₂(C₂O₄)₅, by the action of niobic acid on oxalic acid solutions, were not successful. When a large excess of oxalic acid was employed, crystals of a compound having the composition Nb(C₂O₄H)₅ were only once obtained; when an excess of niobic acid was employed, the compound Nb₂O₅.C₂O₃.3H₂O or 4H₂O was formed. Both of these compounds are unstable in the dry state, but in faintly acid solution oxalo-niobic acid is much more stable than oxalo-tantalic acid, which is preferentially hydrolysed by the addition of tannin. This reaction has recently been recommended for the separation and estimation of niobium and tantalum.