168 Gooch and Norton — lodometric 



Art. XVI. — The lodometric Determination of Molybdenum} 

 by F. A. Gooch and John T. Norton, Jr. 



[Contributions from the Kent Chemical Laboratory of Yale University, LXXII ] 



A process for the iodometric determination of molybdic 

 acid, which consists in treating a soluble molybdate in a Bun- 

 sen distillation-apparatus with potassium iodide and hydro- 

 chloric acid, has been advocated by Friedheim and Euler.* 

 According to this process the molybdate, containing from 0*2 

 grm. to 0*3 grm. of molybdenum trioxide, is treated with from 

 0*5 grm. to 0*75 grm. of potassium iodide and enough hydro- 

 chloric acid, of sp. gr. 1*12, to fill two-thirds of the flask of 

 the apparatus. The liquid is warmed until heavy vapors of 

 iodine till the flask and then boiled until iodine vapor is no 

 longer visible and the color of the liquid residue is a clear 

 green. The iodine liberated is collected in the distillate and 

 titrated with sodium thiosulphate, every atom of iodine found 

 indicating presumably the reduction of a molecule of molybdic 

 acid to the condition of the pentoxide Mo 2 5 . 



It was pointed out in. a former article from this laboratory,f 

 that greater precaution than was taken by Friedheim and 

 Euler is necessary in order that the reduction may proceed 

 according to theory, and that the iodine collected may serve 

 as a reliable measure of the molybdic acid. It was found that 

 the green color of the liquid comes gradually and that it mav 

 develop distinctly before the molybdic acid is fully reduced. 

 It was found, also, that since even a trace of oxygen liberates 

 iodine from the hot mixture of potassium iodide and hydro- 

 chloric acid of the strength employed, it is not sufficient to 

 rely upon the volatilization of iodine to expel the air originally 

 in the apparatus, but that it is essential to conduct the dis- 

 tillation in an atmosphere devoid of oxygen. The suggestion 

 was made therefore that the operation should be carried on in 

 a current of carbon dioxide and that the mixture, constituted 

 definitely, should be boiled between stated limits of concen- 

 tration which were determined by experiment. It was found 

 that when amounts of a soluble molybdate containing less than 

 0*3 grm. of molybdenum trioxide are treated with potassium 

 iodide, not exceeding the theoretical proportion by more than 

 0*5 grm., and 40 cm3 of a mixture of the strongest hydrochloric 

 acid and water in equal parts, the reduction proceeds with a 

 fair degree of regularity and is practically complete when the 

 volume has diminished to 25 cm3 . If the operation is pro- 



* Ber. d. d. Chem. Gesell., xxviii, 2066. 



f Gooch and Fairbanks, this Journal. IV, ii, 156. 



