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Tantalum pentoxide is insoluble in solutions of caustic alkalis and alkali carbonates, but on being fused with these substances reacts to produce the alkali tantalates. Tantalates of the metals are obtained by double decomposition, using a soluble alkali tantalate and a soluble salt of the metal. Tantalates of the alkaline earths have also been obtained by fusing tantalum pentoxide with the chloride of the alkaline earth.

The tantalates display wide variations in composition, the proportions of basic oxide to acid oxide ranging from 1:3 to 5:1, but, as in the cases of the vanadates and niobates, it is probable that many of those which have been prepared and described are really isomorphous mixtures of simpler compounds. According to Marignac, the only true chemical compounds are the 1:1 or meta-salts, of the general formula R2O.Ta2O5, and the 4:3 or hexabasic salts, of the general formula 4R2O.3Ta2O5. According to a recent investigation, the members of the latter series are more correctly represented as 7:5 salts, for example 7K2O.5Ta2O5.24H2O, and not as 4:3 salts; they can be alternatively written as derivatives of the hypothetical ortho-acid, H3TaO4, in which each of the four oxygen atoms has been replaced by a co-ordinated (TaO3) group, thus H7[Ta(TaO4)4]; for example, K7[Ta(TaO4)4].12H2O. This class of alkali tantalates is of importance because it includes the only soluble tantalates known, other alkali tantalates and the tantalates of all other metals being insoluble in water.

The soluble alkali tantalates undergo ready hydrolysis; when their aqueous solutions are boiled, precipitation of a more acid salt takes place and some alkali base passes into solution. Separation of the base in this manner is naturally favoured by the presence of acids, and even so weak an acid as carbon dioxide or hydrogen sulphide precipitates tantalic acid or an acid salt; with the stronger acids (sulphurous acid, sulphuric acid, hydrochloric acid, nitric acid, etc., but not with hydrofluoric acid) precipitation of tantalic acid takes place readily, but excess of the strong mineral acid redissolves the precipitate. Potassium chloride and ammonium salts also precipitate tantalic acid from solutions of tantalates. Arsenious acid, arsenic acid, hydrocyanic acid, tartaric acid and citric acid do not, however, hydrolyse solutions of alkali tantalates; this difference in behaviour is attributed to the formation of soluble salts of heteropoly-acids. Addition of caustic soda or concentrated solutions of sodium salts to a solution of potassium tantalate yields a precipitate of sodium tantalate which is insoluble in the presence of a high concentration of sodium ions.

Measurements of the electrical conductivity of solutions of alkali tantalates have been made, but no definite conclusions are deducible as to the complexity of the ions present.

Known Tantalates

Ammonium Tantalate, (NH4)2O.3Ta2O5.5H2O

Ammonium Tantalate, (NH4)2O.3Ta2O5.5H2O, is obtained as a flocculent precipitate by the addition of ammonium chloride to a solution of 4:3 sodium tantalate. An ammonium potassium tantalate, (NH4)2O.K2O.4Ta2O5.5H2O, has also been obtained by the same reaction, using potassium tantalate.

Barium Tantalate, 4BaO.3Ta2O5.6H2O

Barium Tantalate, 4BaO.3Ta2O5.6H2O, results from the action of a barium salt in solution on a solution of 4:3 sodium or potassium tantalate.

Cesium Tantalates

Fusion of tantalum pentoxide with caesium carbonate and extraction with water yields monoclinic crystals of the 4:3 salt, 4Cs2O.3Ta2O5.14H2O; addition of alcohol to its aqueous solution precipitates the 7:6 salt, 7Cs2O.6Ta2O5.38H2O.

Calcium Metatantalate, CaO.Ta2O5

Calcium Metatantalate, CaO.Ta2O5, is obtained as a crystalline mass by fusing calcium chloride with tantalum pentoxide. Fusion of this salt with more calcium chloride gives crystals of the 2:1 or pyro-salt, 2CaO.Ta2O5. Calcium pyrotantalate is present in some of the natural tantalum ores, for example in fergusonite and mikrolite.

Cobalt Tantalate, CoO.Ta2O5

Cobalt Tantalate, CoO.Ta2O5. - A compound having this probable composition has recently been prepared by heating the two oxides together at high temperatures.

Iron Tantalates

The meta-salt, FeO.Ta2O5, has been obtained more or less pure by fusing tantalum pentoxide and ferrous fluoride with excess of potassium chloride. Iron tantalates occur naturally in niobites and tantalites. The ore tapiolite has the approximate composition FeO.Ta2O5; the tantalum may be partially substituted by niobium.

Lithium Tantalate, 7Li2O.5Ta2O5.40H2O

Lithium Tantalate, 7Li2O.5Ta2O5.40H2O, is precipitated in six-sided plates when lithium hydroxide solution is added to a solution of 7:5 potassium tantalate.

Magnesium Tantalates

The 4:3 salt, 4MgO.3Ta2O5.9H2O, separates out on the addition of a soluble magnesium salt to a solution of a tantalate. Fusion of magnesium chloride and tantalum pentoxide has given large hexagonal plates of the 4:1 salt, 4MgO.Ta2O5.

Mercurous Tantalate, 4Hg2O.3Ta2O5.5H2O

Mercurous Tantalate, 4Hg2O.3Ta2O5.5H2O, is a greenish-yellow compound which is precipitated by mixing a mercurous salt and a tantalate in solution.

Potassium Tantalates

The 4:3 salt, 4K2O.3Ta2O5.16H2O, is prepared by fusing tantalum pentoxide with excess of caustic potash in a silver crucible; the aqueous extract is evaporated in a vacuum. It forms monoclinic prisms, a:b:c = 0.7164:1:0.5475; β = 95°19'. It is stable in air and can be crystallised from its aqueous solutions unchanged, but boiling the solution precipitates a more acid salt. According to the more recent investigations of Jander and Schulz, the crystals obtained by the foregoing reaction are six-sided prisms of the 7:5 salt, 7K2O.5Ta2O5.24H2O; varying proportions of tantalum pentoxide and caustic potash were employed with the same result, but evaporation was effected either over phosphorus pentoxide or on the water-bath, and the crystals were washed with alcohol.

Potassium Metatantalate, K2O.Ta2O5

Potassium Metatantalate, K2O.Ta2O5, is produced when the 4:3 salt is heated and the product washed with water. It is insoluble in water. The 2:3 salt, 2K2O.3Ta2O5.6H2O, is precipitated out by the action of carbon dioxide on the aqueous extract of a fused mixture of tantalum pentoxide and potassium carbonate. Continued boiling of this salt with water yields the 1:2 salt, K2O.2Ta2O5.3H2O. The anhydrous 3:7 salt has also been reported.

Rubidium Tantalate, 4Rb2O.3Ta2O5.14H2O

Rubidium Tantalate, 4Rb2O.3Ta2O5.14H2O, is obtained in transparent, colourless, monoclinic prisms by fusing tantalum pentoxide with rubidium carbonate and extracting the melt with water. It is isomorphous with the corresponding niobium salt and with 4:3 caesium tantalate.

Silver Tantalate, 4Ag2O.3Ta2O5.3H2O

Silver Tantalate, 4Ag2O.3Ta2O5.3H2O, is a yellowish-white substance formed by the action of a silver salt on a tantalate in solution.

Sodium Tantalates

4:3 Sodium tantalate, 4Na2O.3Ta2O5.25H2O, is prepared by fusing tantalum pentoxide with caustic soda in a silver crucible; excess of caustic soda is removed by washing with water, and the residue is crystallised from hot water. Another method consists in adding a concentrated solution of sodium chloride to the aqueous extract from a fused mixture of tantalum pentoxide and potassium carbonate, when the salt separates as a dense, micro-crystalline powder. It forms six-sided plates, a:c = 1:1.0167. It loses most of its water of crystallisation between 105° and 110° C., but the remainder (apparently five molecules) appears to be water of constitution, as its removal takes place only at much higher temperatures, with decomposition. It is soluble in 493 parts of water at 13.5° C. and in 162 parts of boiling water, but at the latter temperature hydrolysis takes place, with precipitation of a salt which is richer in acid. The solution is alkaline to litmus. According to Jander and Schulz, the composition of the foregoing salt when crystallised at ordinary temperatures is 7Na2O.5Ta2O5. 40H2O or Na7[Ta(TaO4)4].20H2O; when crystallised at 100° C. it forms needles, the composition of which is 7Na2O.5Ta2O5.22H2O or Na7[Ta(TaO4)4].11H2O.

Sodium Metatantalate, Na2O.Ta2O5

Sodium Metatantalate, Na2O.Ta2O5, is obtained from the 4:3 salt either by heating the latter strongly and washing the residue with water, or by the addition of alcohol to its aqueous solution. In the latter case the metatantalate contains two molecules of water when dried at 100° C. When an aqueous solution of the 4:3 salt is treated with carbon dioxide or hydrogen sulphide, the 1:3 salt, Na2O.3Ta2O5.5H2O, or the 2:7 salt, 2Na2O.7Ta2O5.10H2O, is obtained.

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