Oxalo niobates

The oxalo-niobates constitute the only well-defined series of hetero-poly-niobates. Indications of the formation of salts of other heteropolyacids containing complex anions depend on the following observations ... 

Oxalo-niobates or niobo-oxalates correspond to the vanado-oxalates, and contain both oxalic acid and niobic add radicals in the complex anion. The only known series possesses the general formula 3R aO. Nb 205.6C203.a H20, 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 jjs poured into hot oxalic add solution concentration and cooling, or addition of alcohol or acetone, then brings about precipitation of the complex salt. Comparison of the dectrical 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.6... 

Potassium Oxalo-niobate, 8K2O.NbgOg.6CjjOg.4HgO, can be reerystal-... 

Rubidium Oxalo-niobate, 3Rb20.Nb205.6C203.4H20, is obtained in small needles by precipitation from its aqueous solutions with alcohol. It forms supersaturated solutions very readily. 

Sodium Oxalo-niobate, 3Na20.Nb205.6C20s.8H20, is similar to the potassium salt, but displays a greater tendency to form supersaturated solutions. 

Barium Oxalo-niobate, 5BaO.Nb2Os.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. 

Oxysulphides.—The action of carbon bisulphide vapour or hydrogen sulphide on niobium pentoxide, sodium niobate or niobium oxytrichloride gives rise to a black powder which assumes a metallic appearance on being rubbed, and which conducts electricity well. This is probably an oxysulphide of niobium, Nba.OJ,Sz, but its exact composition varies with the experimental conditions, and its individuality is a matter of doubt.5 Oxysulphides of niobium of doubtful composition, mixed with potassium sulphide, are also produced by the action of carbon bisulphide vapour on potassium oxalo-niobate. ... 

None of these complex adds has, however, been isolated, and only one well-defined salt, namely, potassium oxalo-tantalate, 5K20.Ta205. 10C2O3, is known. This compound has been prepared by fusing tantalic add with potassium carbonate, dissolving the product in water, and adding oxalic add.3 It undergoes hydrolysis more readily than potassium oxalo-niobate. 

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