532 Oberhelman — Pentavalent Vanadium. 



or a little more in presence of l-5 cm3 of 32^ hydrochloric acid 

 and potassium iodide in varying amounts. The starch reaction 

 was observed . after a little more than the lapse of time which 

 would occur in the titration of the excess of thiosulphate by 

 iodine in the vanadic acid determination. 







Table I. 







The Action c 



>/ Copper Sulphate upon Potassium Iodide 



with Liberatu 







of Iodine. 







CuS0 4 .5H 2 



HC1(32$) 



KI 



Volume 



Starch 



grm. 



cm 3 



gm. 



cm 3 



reaction 



o-oioo 



1-5 



0-4 



410 



None 



o-oioo 



u 



0-8 



420 



None 



o-oioo 



(( 



1-2 



430 



None 



0-0200 



a 



1-2 



440 



None 



0-0300 



« 



1-2 



450 



None 



0-0300 



u 



1-6 



460 



None 



0-0300 



<( 



2-0 



400 



Blue 



0-0300 



a 



2-4 



400 



Blue 



0-0200 



a 



2-4 



400 



None 



o-oioo 



a 



2*4 



400 



None 



From the results given in Table I it would appear that in 

 presence of l*5 cm3 of (32 per cent) hydrochloric acid in the 

 water solution of approximately 400 cm3 there is no appreciable 

 liberation of iodine when so much as 30 milligrams of the 

 copper sulphate and 1*5 grm. of potassium iodide are present. 

 When, however, even smaller amounts of the copper salt and 

 iodide are present during the reduction of vanadic acid by the 

 thiosulphate taken in slight excess (2 cm3 -5 cmS ) an appreciable 

 over-use of thiosulphate appears. 



In studying the conditions for the quantitative estimation of 

 vanadium, the solutions used were made from ammonium 

 vanadate and standardized by means of the Holverscheit 

 method (Inaug. Diss. Berlin, 1890), according to which the 

 vanadate is treated with potassium bromide and hydrochloric 

 acid and the liberated bromine determined by absorption in a 

 solution of potassium iodide and titration of the liberated 

 iodine. The thiosulphate solution, approximately N/10, was 

 standardized against N/10 arsenite made from twice sublimed 

 arsenic trioxide. The amounts of copper sulphate were 

 restricted to a maximum of 10 milligrams in a volume of 400 cm3 

 and the iodide to about 0'1 grm. for every 100 cm3 of solution. 

 Qualitative experiments showed that the reduction of the 

 vanadic acid might be considerably accelerated by the presence 

 of an excess of the comparatively insoluble cuprous iodide, 

 used as the catalyzer in place of copper sulphate, but the quan- 



