[668] J. Amer. Chem. Soc. 1910, 32, 957.
[669] Metallurgie, 1910, 7, 723.
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.