An account has recently been published[513] of a volumetric method for the estimation of thorium. The mixed oxides are dissolved in concentrated acetic acid, and the solution titrated with a standard solution of ammonium molybdate. This reagent effects complete precipitation of thorium, but does not react with compounds of the cerium elements; excess of the molybdate is shown by a solution of diphenyl carbazide, CO(NH·NH·C₆H₅)₂, used as an external indicator. The carbazide, which is obtained by the action of phenyl hydrazine on urea, has the property of producing definite, though evanescent, colourations with compounds of many of the metallic elements; a drop of the working solution, brought into contact with a drop of the carbazide solution, shows a deep rose colouration when excess of ammonium molybdate is present.[514] [513] Metzger and Zons, _J. Ind. Eng. Chem._ 1912, ~4~, 493. [514] Vide Skinner and Ruhemann, _Trans. Chem. Soc._ 1888, ~53~, 554; also Cazeneuve, _Compt. rend._ 1900, ~131~, 346. The iodate process of Meyer and Speter[515] has the great advantage that it is carried out in a strongly acid solution, so that here the tedious purification from phosphoric acid is no longer necessary. After decomposition of the mineral with sulphuric acid, the sulphates are extracted with water, and a suitable quantity of nitric acid added; the solution is then treated with a nitric acid solution of potassium iodate, and the thorium iodate which separates is dissolved in concentrated nitric acid, and re-precipitated to remove traces of the cerium elements. The iodate, after washing, is dissolved in hydrochloric acid, and reduced by sulphur dioxide; the hydroxide is then precipitated by ammonia. Since zirconium is also thrown down under these conditions, the hydroxide is dissolved in hydrochloric acid; pure thorium oxalate is precipitated from this solution by oxalic acid, and is ignited and weighed as oxide, in the usual manner. Since ceric iodate is also insoluble in dilute nitric acid, it is necessary to reduce any ceric compound which may be present before the iodate treatment by the usual methods. [515] _Chem. Zeitg._ 1910, ~34~, 306. See also _Zeitsch. anorg. Chem._ 1911, ~71~, 65.