Niobic acid

Concentrated sulfuric acid (97 wt %) at 300—400°C has been used to solubili2e niobium from columbite and pyrochlore (18,19). The exothermic reaction is performed in iron or siUcon-iron cmcibles to yield a stable sulfato complex. The complex is filtered free of residue and is hydroly2ed by dilution with water and boiling to yield niobic acid which is removed by filtration as a white coUoidal precipitate. 

Fusion with caustic soda at 500—800°C in an iron cmcible is an effective method for opening pyrochlores and columbites (20). The reaction mixture is flaked and leached with water to yield an insoluble niobate which can be converted to niobic acid in yields >90 wt% by washing with hydrochloric acid. 

Another solvent extraction scheme uses the mixed anhydrous chlorides from a chlorination process as the feed (28). The chlorides, which are mostly of niobium, tantalum, and iron, are dissolved in an organic phase and are extracted with 12 Ai hydrochloric acid. The best separation occurs from a mixture of MIBK and diisobutyl ketone (DIBK). The tantalum transfers to the hydrochloric acid leaving the niobium and iron, the DIBK enhancing the separation factor in the organic phase. Niobium and iron are stripped with hot 14—20 wt % H2SO4 which is boiled to precipitate niobic acid, leaving the iron in solution. 

Niobium Pent chloride. Niobium pentachloride can be prepared in a variety of ways but most easily by direct chlorination of niobium metal. The reaction takes place at 300—350°C. Chlorination of a niobium pentoxide—carbon mixture also yields the pentachloride however, generally the latter is contaminated with niobium oxide trichloride. The pentachloride is a lemon-yeUow crystalline soHd that melts to a red-orange Hquid and hydrolyzes readily to hydrochloric acid and niobic acid. It is soluble in concentrated hydrochloric and sulfuric acids, sulfur monochloride, and many organic solvents. 

Niobic Acid. Niobic acid, Nb20 XH2O, includes all hydrated forms of niobium pentoxide, where the degree of hydration depends on the method of preparation, age, etc. It is a white insoluble precipitate formed by acid hydrolysis of niobates that are prepared by alkaH pyrosulfate, carbonate, or hydroxide fusion base hydrolysis of niobium fluoride solutions or aqueous hydrolysis of chlorides or bromides. When it is formed in the presence of tannin, a volurninous red complex forms. Freshly precipitated niobic acid usually is coUoidal and is peptized by water washing, thus it is difficult to free from traces of electrolyte. Its properties vary with age and reactivity is noticeably diminished on standing for even a few days. It is soluble in concentrated hydrochloric and sulfuric acids but is reprecipitated on dilution and boiling and can be complexed when it is freshly made with oxaHc or tartaric acid. It is soluble in hydrofluoric acid of any concentration. 

Nloha.tes, Niobic acid is amphoteric and can act as an acid radical in several series of compounds, which are referred to as niobates. Niobic acid is soluble in solutions of the hydroxides of alkaH metals to form niobates. Fusion of the anhydrous pentoxide with alkaH metal hydroxides or carbonates also yields niobates. Most niobates are insoluble in water with the exception of those alkaH metal niobates having a base-to-acid ratio greater than one. The most weU-known water-soluble niobates are the 4 3 ad the 7 6 salts (base acid), having empirical formulas MgNb O c, (aq) and M24Nb2202y (aq), respectively. The hexaniobate is hydrolyzed in aqueous solution according to the pH-dependent reversible equiHbria (130), when the pH is ca 9. 

The 7 6 salts are the acid salts of the normal 4 3 hexaniobates. The formulas can be written as M2H(Nb 02C)) (aq). Further hydrolysis can take place. At pH ca 4.5, the irreversible precipitation of niobic acid occurs. 

There is a real opportunity to reduce biodiesel production costs and environmental impact by applying modem catalyst technology, which will allow increased process flexibility to incorporate the use of low-cost high-FFA feedstock, and reduce water and energy requirement. Solid catalysts such as synthetic polymeric catalysts, zeolites and superacids like sulfated zirconia and niobic acid have the strong potential to replace liquid acids, eliminating separation, corrosion and environmental problems. Lotero et al. recently published a review that elaborates the importance of solid acids for biodiesel production. ... 

Out of the metal oxides, sulfated titania and tin oxide performed slightly better than the sulfated zirconia (SZ) catalyst and niobic acid (Nb205). However, SZ is cheaper and readily available on an industrial scale. Moreover, it is already applied in several industrial processes (7,8). Zirconia can be modified with sulfate ions to form a superacidic catalyst, depending on the treatment conditions (11-16). In our experiments, SZ showed high activity and selectivity for the esterification of fatty acids with a variety of alcohols, from 2-ethylhexanol to methanol. Increasing... 

H-ZSM-5, Y and Beta) Sulfated metal oxides (zirconia, titania, tin oxide) Niobic acid (Nb205)... 

Chemicals and catalysts - Double distilled water was used in all experiments. Unless otherwise noted, chemicals were purchased from commercial companies and were used as received. Dodecanoic acid 98 wt% (GC), methanol, propanol and 2-ethylhexanol 99+ wt% were supplied by Aldrich, niobic acid by Companhia Brasileira de Metalurgia e Mineragao (CBMM), zirconil chloride octahydrate 98+ wt% by Acros Orgartics, 25 wt% NH3 solution and H2SO4 97% from Merck. Zeolites beta, Y and H-ZSM-5 were provided by Zeolyst, and ion-exchange resins by Alfa. 

Ninhydrin, 22 101 Ninhydrin-color reaction amino acids, 2 570 Niobates, 27 152-153 24 315 Niobia-phosphate catalytic aerogels, 2 763t Niobic acid, 27 152 Niobic salts, 27 152-153 Niobium (Nb), 27.132-157 24 313, 315. See also Nb-Ti entries Niobium compounds Niobium metal analytical methods for, 27 142-144 dissolution methods for, 27 142 economic aspects of, 27 140-142 effect on stainless steel corrosion resistance, 7 809... 

However, the story does not end there. It was not until 1844 when Heinrich Rose (1795-1864) rediscovered the element by producing two similar acids from the mineral niobic acid and pelopic acid. Rose did not reahze he had discovered the old columbium, so he gave this new element the name niobium. Twenty years later, Jean Charles Galissard de Marignac (1817—1894) proved that niobium and tantalum were two distinct elements. Later, the Swedish scientist Christian Wilhelm Blomstrand (1826—1899) isolated and identified the metal niobium from its similar twin, tantalum. 

After preparing niobic acid by Marignac s method, he mixed eighty-two parts of it with eighteen of sugar carbon, moistened the mixture slightly with turpentine, and pressed it into the form of a cylinder, which he heated in his electric furnace, using six hundred amperes under fifty volts. A violent reaction took place in accordance with the equation ... 

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 more recent process, which avoids the difficulties associated with Marignac s method, is based on the solubility of niobium pentoxide and the comparative insolubility of tantalum pentoxide in a mixture of equal volumes of selenium oxychloride, SeOCI8, and concentrated sulphuric acid.8 The tantalum pentoxide is left in the residue, and hydrolysis of the extract after dilution yields niobic acid. 

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. 

The detection of niobium and tantaluA ideg nfli almost entirely on the reactions given by niobic acid and tanxanc acid. All the common niobium and tantalum compounds are hydrolysed on bdng boiled in acid solutions, and yield precipitates of the respective adds. Natural minerals are previously fused with potassium hydrogen sulphate, and the aqueous extract of the melt usually precipitates the mixed acids... 

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. 

The hydrofluoric acid solution of niobic acid does not yield a precipitate on the addition of potassium fluoride (potassium niobium fluoride, K2NbF7, which is formed, being soluble in about 12-5 parts of water), whereas the hydrofluoric acid solution of tantalic add yields colourless, rhombic needles of potassium tantalum fluoride, K2TaF7 (which is soluble in about 150 parts of water under the same conditions), when treated with a saturated solution of potassium fluoride, carefully evaporated and cooled slowly. After removal of the tantalum, and with further concentration, any niobium present separates in pdates of potassium niobium oxyfluoride, K2Nb0F5.H20, if the hydrofluoric acid is not in excess, and in needles of potassium niobium fluoride, K2NbF7, if the hydrofluoric acid is in excess. 

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. 

General.—The compounds of niobium are not so numerous as those of vanadium. The following oxides,1 Nb203, Nb02, Nb205, are known, but only the pentoxide gives rise to salts, viz. the niobates.2 The acid character of niobium pentoxide or niobic acid is very weak the niobates are decomposed, for instance, by carbon dioxide, and are readily hydrolysed to the pentoxide. Niobic acid is, in fact, very comparable in its method of preparation and behaviour to silicic acid and stannic acid. 

Niobic acid displays a much less pronounced tendency than vanadie acid to form heteropoly-compounds with other acids, but oxaloniobates are known. It reacts with hydrogen peroxide to form perniobic add, HNb04.a,H20, salts of which are known. The double niobium oxy-fluorides also take up active oxygen. 

Cadmium Niobium Fluoride, 3NbF5.5CdF2.5HF.28H20 or NbsCd5F25. 5HF.28HaO, is obtained in long, transparent prisms by the action of cadmium carbonate on a solution of niobic acid in concentrated hydrofluoric acid. It is insoluble in water. 

Caesium Niobium Fluoride, CsF.NbFs or CsNbF6, is obtained in fine needles by repeated crystallisation of csesium niobium oxyfluoride, CSjNbOFj, from hydrofluoric acid.2 Another csesium niobium fluoride having the probable composition 7CsF.NbFs or Cs7NbF5 has been prepared by the action of a solution of caesium hydroxide in hydrofluoric add on niobic acid in the same solvent.3... 

Ferrous Niobium Fluoride, 2NbF5.3FeF2.4HF.19H20 or NbaFe3F16. 4HF.19H20, is obtained in greenish-yellow, thin prisms by dissolving iron and niobic acid in equivalent proportions in hydrofluoric acid. 

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