534 



Oberhelman — Pentavalent Vanadium. 



of acid and iodide, while the excess of N/10 thiosulphate was 

 placed at 15 cm8 . 



Reduction in Presence of 

 15 cm3 of the N/iO 



an Excess of Thiosulphate amounting 

 Solution in a Volume of 800 cm3 . 



to 



v,o 6 



taken 

 grm. 



v,o 6 



found 

 grm. 



HC1 CuS0 4 .5H,0 Excess of 

 (32*) N/10Na 2 S 2 O, 

 cm 3 mgrm. cm 3 



KI 



grm. 



present 



Polum 

 cm 3 



0-0967 



0*0965 



3 



10 



15 



0-8 



800 



0-096V 



0-0967 



3 



10 



15 



0-8 



800 



0-1935 



0-1932 



3 



10 



15 



0-8 



800 



01935 



0-1937 



3 



10 



15 



0-8 



800 



0-3870 



0-3866 



3 



10 



15 



0-8 



800 



0-3870 



0-3876 



3 



10 



15 



0*8 



800 



In these experiments, obviously excellent, the rapidity of the 

 process was much increased by the limited increase in the con- 

 centration of the thiosulphate. The results seem to justify the 

 conclusion that, if the conditions are carefully controlled, pen- 

 tavalent vanadium may be rapidly and accurately estimated by 

 direct reduction with sodium thiosulphate and titration of the 

 excess. 



According to the procedure recommended, the vanadate 

 solution is brought to a volume of 800 cm3 or 400 cm3 ; 10 mgrm. 

 or 5 mgrm. (respectively) of copper sulphate, CuS0 4 .5H,0, 

 and 3 cm3 or l'5 cm3 (respectively) of hydrochloric acid (32 per 

 cent) are added ; N/10 thiosulphate is slowly run in, with con- 

 stant stirring, until an excess of 15 cm3 or 7'5 cm3 (respectively) is 

 present ; the solution is allowed to stand until its color has 

 become a pure sky blue ; potassium iodide to bring the 

 amount up to 0'1 grm for each 100 cm3 of solution is added ; and 

 the excess of thiosulphate is titrated with N/10 iodine in the 

 presence of the starch indicator. The vanadium blue interferes 

 but slightly with starch-blue end point. 



When the amount of vanadate to be determined is not 

 approximately known, a preliminary determination in which 

 the copper sulphate is used to the amount of 20 mgrm. will 

 quickly give the information required for the proper adjust- 

 ment of the thiosulphate in an exact analysis to follow. 



