Tantalates Tantalic acid

Tantalic Acid and Tantalates. Tantalic acid [75397-94-3], Ta2Os H20, is the name of the white insoluble precipitate formed by hydrolysis of alkali hydroxide or alkali carbonate fusions containing tantalum, or by adding ammonia to an acidic solution containing tantalum ions. Tantalic acid is characterized by a high surface acidity, affording it potential use as a catalyst. 

Gr. Tantalos, mythological character, father of Niobe) Discovered in 1802 by Ekeberg, but many chemists thought niobium and tantalum were identical elements until Rowe in 1844, and Marignac, in 1866, showed that niobic and tantalic acids were two different acids. The early investigators only isolated the impure metal. The first relatively pure ductile tantalum was produced by von Bolton in 1903. Tantalum occurs principally in the mineral columbite-tantalite. 

Tantalic Acid and Tantalates. Tantahc acid [75397-94-3] Ta20 is the name of the white insoluble precipitate formed by hydrolysis... 

Synonyms tantalum oxide tantalic acid anhydride... 

Although niobic and tantalic acids are extremely difficult to separate, Marignac finally succeeded, not only in separating them, but also in showing that niobium is both tri- and pentavalent, whereas tantalum always has a valence of five. The separation is based on the insolubility of potassium fluotantalate in comparison with potassium fluo-oxyniobate (12, 20). In the United States the element discovered by Hatchett used to be known as columbium, but in Europe most chemists prefer to use the name niobium which Heinrich Rose gave it. 

Tantalium (V) chloride (tantalium pentachloride) [7721-01-9] M 358.2, m 216.2 , 216.5-220 b 239 /atm., d 3.68. Purified by sublimation in a current of CI2. Colourless needles when pure (yellow when contaminated with even less than 1% of NbCls). Sensitive to H2O, even in cone HCl it decomposes to tantalic acid. Sol in EtOH. [J/ICS 80 2952 1958, Handbook of Preparative Inorganic Chemistry (edBrauer) Vol II 1302 7965]. 

Vanadic acid differs also from niobic and tantalic adds in that it forms a number of complex acids with other acids, and a large number of heteropolyvanadates are known. Niobic add and tantalic acid show little tendency to form complex acids oxaloniobates and oxalotanta-lates have, however, been prepared. 

From a consideration of the composition of the halides, and because of the supposed isomorphism of tantalic acid and stannic acid, Rose gave the formulas NbOa and TaOs to anhydrous niobic acid and tantalic acid respectively, but in so doing he repeated an error that had previously been made by Berzelius with regard to vanadium compounds, and overlooked the presence in the halides of an oxygen atom (see p. 24). Blomstrand 8... 

The removal of titanium from mixed niobic and tantalic acids is a difficult matter. Although titanium and niobium compounds display considerable differences in their general behaviour, when the two elements occur together they appear to undergo a change, in consequence of which they become difficult to separate. Niobic acid, for instance, is precipitated from a much more concentrated boiling sulphuric acid solution than is titanic acid but when the two acids are dissolved... 

A method of separation which avoids the preparation of the double fluorides consists in fusing the mixed niobic and t an tali c acids with sodium carbonate and nitrate, the product is digested with warm water and a current of carbon dioxide is passed through the solution. It is claimed that only tantalic acid is precipitated.5 This process has, however, been the subject of adverse criticism.6 Partial separation of niobium from tantalum can be effected by warming the mixed, freshly precipitated, hydrated oxides with a mixture of hydrogen peroxide and hydrochloric acid the niobium dissolves readily, while the tantalum dissolves only sparingly.7... 

The estimation of small quantities of tantalum in niobium compounds is more difficult, and cannot be carried out colorimetrically. The usual method is to convert the material into the potassium double fluoride, and then to take advantage of the fact that a white precipitate of potassium tantalum oxyfluoride, K4Ta405F14 (see p. 132), is thrown down when a solution of potassium tantalum fluoride, KaTaF7, is boiled.7 Powell and Schoeller 8 find this test imperfect, and have modified the procedure (based on the differential hydrolytic dissociation of oxalo-niobic acid and oxalo-tantalic acid in the presence of tannin in slightly add solution) for the detection and estimation of traces of tantalum in niobium compounds. 

A. Wet Reactions.—(1) Both niobic acid and tantalic acid dissolve readily in hydrofluoric acid, but only very slightly in concentrated hydrochloric acid and in hot concentrated sulphuric acid. The residue from the hydrochloric acid solution of niobic acid, however, readily forms a hydrosol on being triturated with water. The sulphuric acid solution of niobic acid remains clear on being diluted with water, whereas the sulphuric acid solution of tantalic acid becomes turbid on being diluted, and reprecipitates the acid. 

Potassium ferrocyanide yields a yellow or reddish-brown precipitate with a hot solution of tantalic acid in hydrochloric acid niobic acid gives a reddish-brown 1 or greyish-green2 precipitate. 

Tannin produces an orange-red or chocolate-red precipitate with an acid solution of niobic acid, and a yellow or light brown precipitate with acid solutions of tantalic acid. Pyrogallol and other polyhydroxy derivatives of benzene behave similarly. 

Addition of ammonium hydroxide or ammonium sulphide to solutions of niobic acid and tantalic acid in mineral acids reprecipitates the niobic and tantalic acids, which may, however, retain some of the ammonia. This test does not distinguish between niobium and tantalum, and it does not proceed in the presence of tartaric acid. 

B. Dry Reactions.—When heated in the reducing flame a bead of microcosmic salt assumes a blue, violet, or brown colour with increasing quantities of niobic acid the heated bead becomes red on the addition of ferrous sulphate. With tantalic acid under these conditions the bead remains colourless. 

Hydrates of Tantalum Pentoxide, Colloidal Tantalum Pentoxide, Tantalic Acid.—When tantalum pentachloride or pentabromide is treated with water, or when a solution of a tantalate is boiled with dilute acids, a gelatinous predpitate of more or less hydrated tantalum pentoxide is thrown down. Insoluble tantalates on fusion with potassium hydrogen sulphate and extraction of the melt with water give the gel. In dealing with double fluorides of tantalum it is necessary to remove all the hydrofluoric add by evaporation with concentrated sulphuric acid, otherwise double fluorides are obtained. 

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