530 Oberhelman — Pentavalent Vanadium. 



Art. XXXIII. — The Estimation of Pentavalent Vanadium by 

 Means of Sodium Thiosulphate ; by Gr. O. Oberhelman. 



[Contributions from the Kent Chemical Laboratory of Yale Univ. — celxvi.] 



As is well known, standardized sodium thiosulphate in ex- 

 cess may be employed under certain definite conditions as a 

 reducer of ferric chloride, the amount of the thiosulphate which 

 disappears by transformation to tetrathionate (determined by 

 the idiometric determination of the excess) serving as the 

 measure of the ferric salt reduced. In this process, originally 

 proposed by Scherer,* and studied by many investigators, there 

 are three chief sources of error which may appear, according 

 to conditions : viz., incompleteness in the reduction of the 

 ferric salt, decomposition of the thiosulphate by the action of 

 free acid, and progressive oxidation by the action of the air. 

 Nortonf has shown that these sources of error may be made 

 to disappear by the use of a sufficient excess of sodium thiosul- 

 phate, by regulation of the time of action, and by attention to 

 the concentrations of free acid, thiosulphate, and the ferric 

 salt. According to Norton's procedure the volume of the solu- 

 tion should be at least 400 cm3 for each 0*1 grm. of iron oxide 

 present, the quantity of free hydrochloric acid present should 

 not exceed l cm3 of the strongest aqueous acid to each 400 cm3 of 

 solution, the excess of N/10 thiosulphate should amount to at 

 least 15 em3 , and this excess should be determined by titration 

 with N/10 iodine as soon as the color brought out in the solu- 

 tion before reduction due to the addition of a drop of potassium 

 sulpho-cyanide solution has been discharged. Oudermanns^: 

 has recommended the introduction of cupric sulphate as a cata- 

 lyzer, to hasten the process of reduction, but Norton found this 

 procedure unnecessary. 



This paper is the account of a successful attempt to adapt a 

 similar process of reduction by sodium thiosulphate to the deter- 

 mination of vanadic acid (in the course of work the account of 

 which will be published later) under conditions in which the 

 ordinary modes of determination are impossible or incon- 

 venient. It has been found that the action of reduction may 

 be made to proceed regularly in the sense of the expression 



Vfi 5 + 2Na 2 S 2 3 + 2HC1 = Y 2 4 + Na 2 S 2 6 + 2NaCl + H 2 0. 



The amount of vanadium present may be measured by the 

 difference between the amount of standard thiosulphate intro- 

 duced and the amount of that reagent remaining, as determined 



* Gel. Anzeig. k. Bayrisch, Acad., Aug. 31, 1859. 

 + This Journal, (4), viii, 25, 1899. 

 jZeitschr. anal. Chem., ix, 362. 



