Oberhelman — Pentavalent Vanadium. 531 



by titration with standard iodine ; but in this case (unlike that 

 of the reduction of the ferric salt) the presence of a catalyzer 

 is necessary to bring about the reduction of the vanadic acid, 

 and the concentration of the free hydrochloric must be slightly 

 larger than that indicated by Norton as applicable in the reduc- 

 tion of the ferric salt. When considerably larger amounts of 

 hydrochloric acid are present the thiosnlphate is decomposed 

 with the liberation of sulphur dioxide, which, of course, entirely 

 vitiates the process. 



On the introduction of thiosulphate into a solution of ammo- 

 nium vanadate containing a little copper sulphate and slightly 

 acidified, the golden yellow changes to green and finally to a 

 pure blue which indicates reduction of the vanadium to the 

 tetravalent state. With iron it is necessary to introduce a little 

 sulphocyanide as an indicator of complete reduction. With 

 vanadium no outside indicator is necessary. When the solu- 

 tion has become a pure blue it is only necessary to add starch, 

 with enough potassium iodide (0*1 to every 100 cm3 of solution) 

 to bring out a sufficiently delicate end-point,* before proceed- 

 ing with the titration by iodine. 



Certain experiments showed plainly that with a certain 

 amount of copper sulphate and hydrochloric acid present 

 the rapidity of reduction depends largely upon the concentra- 

 tion of the acid. It was found that to bring about complete 

 reduction within a reasonable time as much as l*5 cm3 of the 

 (32%) acid should be present in every 400 cm3 of solution and 

 that this concentration of acid does not cause appreciably, at 

 ordinary temperature and during the period of reduction, a 

 secondary decomposition of sodium thiosulphate (resulting in 

 the liberation of sulphur dioxide) provided the excess of this re- 

 agent does not exceed the equivalent of 7£ cm3 of the N/10 solu- 

 tion in the 400 cm3 .f In the quantitative tests of the process 

 detailed below the concentrations of acid and thiosulphate were 

 kept within these safe limits. Moreover, since copper sulphate 

 and potassium iodide react at sufficient concentrations with 

 the liberation of iodine according to the expression 



2CuS0 4 + 4KI ^=> Cu 2 I 2 + 2K 2 S0 4 + I 2 



it is plain that in the reduction of vanadic acid by the thiosul- 

 phate the amount of copper sulphate used as a catalyzer should 

 be kept, if possible, well within the maximum, which may be 

 present without setting up in appreciable degree the reactions 

 by which iodine is set free. The following table contained 

 the results of experiments made to discover the amount of 

 copper sulphate which may be present in a solution of 400 cm3 



*Hale. Am. Chein. Jour., xxviii, 938. 



f Compare Norton, this Journal, (4), vii, 287, 1899. 



