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.
Another method which can be carried out in acid solution is based on the insolubility of the hypophosphite, ThP₂O₆,11H₂O, in dilute acids.[516] To the boiling acid solution, an aqueous solution of sodium hypophosphate, Na₂H₂P₂O₆,6H₂O, is added drop by drop. The precipitate, which contains any titanium and zirconium present in the original solution, is best treated with a mixture of sulphuric and fuming nitric acids; the phosphates produced by the oxidation are freed from nitric acid by evaporation, dissolved in water, with addition of sulphuric acid, and thorium precipitated as the oxalate, which is then ignited as usual. This method has been suggested for the technical separation of thorium from monazite (vide [p. 278]). Since the precipitations by means of sodium hypophosphate and potassium iodate can be carried out with solutions obtained directly from the product of the action of sulphuric acid on the mineral, these two methods are probably more suitable for the rapid and accurate estimation of thorium for technical purposes than any of the others mentioned.
[516] Wirth, Zeitsch. angew. Chem. 1912, 25, 1678; see also Koss, Chem. Zeitg. 1912, 36, 686, and Rosenheim, ibid. p. 821.
CHAPTER XIX
THE MANUFACTURE OF MANTLES FROM COTTON AND RAMIE
The fabric chosen for the manufacture of the original Welsbach mantles was a specially selected cotton, woven from threads of a specified thickness. The oxide skeleton left after burning off the impregnated fabric, however, showed many serious defects. Gradual shrinkage occurred during use, so that the mantle was gradually withdrawn from the hottest zone of the flame; the contraction also resulted in crumpling, which caused the fragile fabric to fall to pieces. The light-giving power showed a gradual but continuous diminution, so that after a hundred hours, the decrease sometimes amounted to thirty per cent. of the original intensity. Lastly, owing to the fragility due to the torsion introduced by the twisting together of so many short fibres in the spinning of the fabric, the life of these mantles was very short, and their susceptibility to shock very great.