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. INDEX Absorption Spectra, ~148~ Acetate process, ~304~ Acetylacetone derivatives, ~135~ Actinium, 100 Aenigmatite, 8, 55 Aeschynite, 8, ~65~ Aldebaranium, 205 Allanite, 8, 36, ~39~, 91 Alshedite, 54 Alvite, 8, 59 Anatase, 8, ~78~ Ancylite, 8, 81 Anderbergite, 8, 49 Annerödite, 9, 61 Arc spectra, ~151~ Arfvedsonite, 9, 51 Arizonite, 9, 59 Arrhenite, 9, 70 Astrophyllite, 9, 55 Auer mantles, history of, ~270~ Auerbachite, 9, 31 Auerlite, 9, 51 Baddeleyite, 10, ~75~ Bagrationite, 10, 45 Bastnäsite, 10, 81 Beckelite, 10, 51