[658] Vide, e.g. D. R. P. 189364, 218316, 115016, 207001; F. 438908, etc.
[659] Sprechsaal, 1911, 44, 72.
These two oxides find employment to a small extent in the manufacture of ‘Siloxide’ quartz glass.[660] Quantities up to 1·5 per cent., added to the molten silica, reduce the difficulty of working the material. Exhaustive tests carried out by Thomas[661] indicate that the vessels made from this material are, on the whole, to be preferred to ordinary quartz glass, resisting high temperature better, and showing less tendency to become crystalline and therefore brittle when maintained for considerable times at high temperature.
Much work has been carried out during the last few years with the object of utilising titanium compounds for the ‘fixation’ of nitrogen.
The metal combines very vigorously with the gas at about 800°C. (vide p. 224), forming the nitride. If the gas, or air, be passed over a heated mixture of the dioxide with powdered coke, formation of the cyanonitride occurs at comparatively low temperatures (1100°-1300°C.) if a small quantity of an alkali salt be present,[662] the action being apparently catalytic; if excess of carbon is used, considerable quantities of the cyanide may be formed. Numerous experiments carried out by the chemists of the Badische Anilin- und Soda-Fabrik have shown that at high temperatures, the action of water and a suitable oxidising agent, or in the presence of metallic compounds, the action of steam alone, will liberate considerable quantities of ammonia from both these derivatives,[663] whilst in the presence of platinum compounds, if air be pumped in, the higher oxides of nitrogen are formed. One or two examples may be given:
(1) Ti₂N₂ + 4NaOH + H₂O + 2CuO = 2NH₃ + Cu₂O + 2Na₂TiO₃—autoclave at 180°C.
(2) 2Ti₂N₂ + 2H₂SO₄ + 6H₂O + O₂ = 4TiO₂ + 2(NH₄)₂SO₄—autoclave at 120°-140°C.
(3) Ti₂N₂ + 3H₂O = Ti₂O₃ + 2NH₃—steam at 500°-600°C.
[662] Vide Bosch, U. S. P. 957842, May, 1910.
[663] Vide, e.g. D. R. P. 202563 and 203748 of March, 1907; 204204 and 204475 of November, 1908; E. 2414, 1908; F. 387002 of June, 1908; U. S. P. 957843 of May, 1910, gives a résumé of all the processes.
In the second case, the oxygen is derived from air pumped into the apparatus, and ferrous sulphate is used as a catalyst. In the third case, a metallic salt, oxide, or hydroxide is required as a catalyst.
In view of the success of the cyanamide method for the fixation of atmospheric nitrogen, these processes, though of considerable theoretical interest, do not seem likely to become of practical importance.
One or two minor uses have been suggested for titanium dioxide. Small quantities are fused with bauxite, silica, and ferric oxide in the preparation of abrasives,[664] whilst a mixture with carbon is suggested as a refractory body for linings, crucibles, etc., surface heating of this forming a layer of highly resistant carbide.[665] An interesting American patent protects the use of the dioxide for the preparation of phosphorus pentoxide from bone-ash or natural calcium phosphate.[666] The pulverised mixture of the phosphate and oxide is introduced at the upper end of an inclined rotating furnace, by means of a hopper and screw feed; fuel is fed in at the lower end, and an outlet is provided for the periodic removal of the calcium titanate, etc., formed. The silica and alumina of the impure phosphate, together with the titanium dioxide introduced, displace the phosphorus pentoxide, which, being volatile, escapes continuously through a special pipe; there is left a mixture of silicate, aluminate and titanate of calcium, which may be used as a source of titanium compounds.
[664] Saunders, U. S. P. 954766, 954777, and 954778.
[665] Becket, U. S. P. 1038827, September, 1912.
[666] Peacock, U. S. P. 995897, June, 1911.
—Owing to the difficulties of the separation from the acidic oxides, silica, zirconia, and the pentoxides of columbium and tantalum, and from the basic oxides, alumina and the oxides of iron and tin, the estimation of titanium in a mineral or a steel is usually a difficult and tedious process. Gravimetric as well as volumetric methods are employed. In the former, the element is isolated and weighed in the form of the dioxide; in the latter, standard solutions of suitable oxidising agents are employed, advantage being taken of the ease with which the element can be transformed from the trivalent to the tetravalent condition.
The mineral or steel in which the element is to be estimated is usually fused with sodium hydrogen sulphate, which forms the sulphate. If thorium, uranium or rare earths are present, treatment in the cold with hydrofluoric acid is often more suitable; the acidic oxides are taken into solution, leaving the more positive elements in the form of the insoluble fluorides. Trautmann finds that steels or ferro-titaniums of high silicon content are attacked only very slightly by fused sodium bisulphate; he recommends[667] ignition to the oxides, evaporation with hydrofluoric acid to remove silicon as the volatile tetrafluoride, and fusion of the residue with bisulphate.
[667] Zeitsch. angew. Chem. 1911, 24, 877.
The bisulphate melt, after cooling, is leached with water, and the whole boiled under a reflux condenser for several hours; this treatment should throw down the oxides of titanium, columbium and tantalum, leaving zirconium and aluminium in the form of the sulphates in the acid solution; the addition of ammonia may be necessary to effect complete hydrolysis. The acidic oxides may also be precipitated if the solution be diluted and treated with excess of acetic acid before boiling. In both cases, a considerable quantity of iron is thrown down. The precipitated oxides are dissolved in the cold by dilute sulphuric acid to which hydrogen peroxide has been added.
For volumetric estimation, separation from iron is not generally necessary. If gravimetric methods are to be employed, separation may be effected in several ways. Titanium dioxide may be precipitated in a fairly pure condition by reducing the solution with sulphur dioxide, and boiling until the titanium sulphate has been completely hydrolysed. According to Barneby and Isham,[668] this method gives low results; these authors prefer to remove iron completely from the solution, and then effect complete hydrolysis by addition of ammonium acetate and acetic acid to the boiling solution. For this purpose, they dissolve the mixed oxides in hydrochloric acid, and remove ferric chloride by ether extraction. Bornemann and Schirmeister[669] precipitate titanium dioxide completely by means of ammonia, holding iron in solution as ferrocyanide; for this purpose, iron is completely reduced to the ferrous state by means of sodium hydrogen sulphite, and solutions of potassium cyanide and ammonia are added together to the warm liquid, which is afterwards heated nearly to the boiling-point to effect the precipitation.
Iron may also be removed by the ordinary methods, if some reagent be previously added to hold titanium in solution. For this purpose, tartaric acid and its salts are commonly used; none of the ordinary precipitants will throw down the element if this reagent be present. After addition of ammonium tartrate, iron is removed by means of ammonium sulphide. After filtering, tartaric acid may be removed by means of potassium permanganate, the manganese dioxide formed being reduced with sulphur dioxide. According to Thornton,[670] evaporation with a mixture of sulphuric and nitric acids is a more convenient method of destroying the organic acid; titanium dioxide is then thrown down by diluting and boiling in the usual way.
[670] Amer. J. Sci. [iv.], 1912, 34, 214.
Bourion[671] describes a method of separating the oxides by the action of a mixture of hydrogen chloride and sulphur monochloride at a suitable temperature. The ferric chloride which is formed sublimes, leaving titanium dioxide unattacked.
[671] Compt. rend. 1912, 154, 1229.
For volumetric estimation of small quantities of titanium in solution, colorimetric methods are generally employed. Addition of hydrogen peroxide to such a solution gives an intense reddish-yellow colouration, which is compared with the colourations obtained with solutions containing known quantities of the element. Wells[672] finds that under suitable conditions, an accuracy of about 2 per cent. is to be expected with this method. Lehner and Crawford[673] find that in concentrated sulphuric acid solution, thymol gives a red colouration which is at least twenty-five times as intense as the colour given by hydrogen peroxide, and they accordingly propose thymol as a suitable reagent for the colorimetric estimation. Fenton[674] has shown that a very intense colouration is obtained when a solution of a titanium salt is treated with dihydroxymaleic acid; this reaction has been shown by Mellor[675] to be well adapted for the colorimetric estimation and for the estimation of titanium and vanadium together in a solution.
[672] Zeitsch. anorg. Chem. 1911, 70, 395.
[673] J. Soc. Chem. Ind. 1912, 31, 956.
[674] Trans. Chem. Soc. 1908, 93, 1064.
[675] Abstr. Chem. Soc. 1913, 104, ii. 627.
The volumetric methods for the estimation of larger quantities require complete reduction to the trivalent condition. This is best effected by means of zinc and hydrochloric acid, or, where potassium permanganate is to be used, by zinc and sulphuric acid. Precautions must be taken to ensure that reduction is complete; an apparatus suitable for rapid estimations has recently been described by Shimer and Shimer.[676] Where potassium permanganate is employed (Pisani’s method), the iron must be estimated separately by means of a standard solution of titanium trichloride. Knecht and Hibbert[677] titrate directly, after reduction, with a standard solution of a ferric salt, using potassium thiocyanate as indicator; here no correction has to be applied for iron originally present in the solution. The same advantage attaches also to the method of titration by means of methylene blue,[678] a dye reduced to the colourless leuco-base by salts of trivalent titanium, but not affected by ferrous salts.
[676] J. Soc. Chem. Ind. 1912, 31, 955.
[677] Ber. 1903, 36, 1549.
[678] See Hibbert, J. Soc. Chem. Ind. 1909, 28, 190.