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The evidence for the formation of complex heteropoly-acids with tantalic acid is very comparable to that set forth in the case of niobic acid. Solutions of tantalates are readily hydrolysed in aqueous solution by boiling, and even more readily by the addition of mineral acids, acetic acid or succinic acid; in the presence, however, of arsenious acid, arsenic acid, tartaric acid or citric acid no precipitation of tantalic acid takes place. Again, tincture of galls yields a yellow precipitate with solutions of tantalates which have been rendered feebly acid with sulphuric acid; this reaction does not, however, take place in the presence of ordinary tartaric acid, racemic acid or citric acid. Tartaric acid also prevents the formation of the precipitates which are thrown down on the addition of potassium ferrocyanide or potassium ferricyanide to faintly acid solutions of tantalates, and hinders the precipitation of tantalic acid from solutions in inorganic acids by the action of ammonia. In all these cases it is assumed that complex acids or their salts are produced, in consequence of which the usual reaction does not take place.

None of these complex acids has, however, been isolated, and only one well-defined salt, namely, potassium oxalo-tantalate, 5K2O.Ta2O5.10C2O3, is known. This compound has been prepared by fusing tantalic acid with potassium carbonate, dissolving the product in water, and adding oxalic acid. It undergoes hydrolysis more readily than potassium oxalo-niobate.

Tantalic acid dissolves in oxalic acid, but not so freely as niobic acid; the solution, which presumably contains oxalo-tantalic acid, is also more readily hydrolysed than solutions of oxalo-niobic acid.

Among naturally occurring complex tantalates the following have been observed, but exact analyses are not always available: titano-tantalates, silico-tantalates, stanno-tantalates, antimonyl-tantalates, urano-tantalates, urano-titanotantalates.

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