Walker and Gillespie — Application of Iodine, etc. 455 



Art. XLYI. — The Application of Iodine in the Analysis of 

 Alkalies and Acids ; by Claude F. Walker and David 

 H. M. Gillespie. 



[Contributions from the Kent Chemical Laboratory of Yale University — LXXYI.J 



It is well known that when a free mineral acid is added to a 

 neutral mixture of metallic iodate and iodide, the iodate is 

 reduced and iodine is liberated according to the equation : 



RI0 3 + 5RI + 3H 2 S0 4 = 3I 2 + 3R a S0 4 + 3H 2 0. 



This reaction is complete and non-reversible under the condi- 

 tions of analysis, and it may therefore be applied to the esti- 

 mation of amounts of iodate, iodide or mineral acid present 

 in an unknown solution. A solution of iodate to be analyzed 

 is mixed with an excess of iodide and mineral acid, the result- 

 ing free iodine estimated by directly titrating with sodium 

 thiosulphate or arsenious acid, and one-sixth of the amount 

 found taken as equivalent to the iodate originally present.* 

 Similarly, a solution of iodide to be analyzed is mixed with an 

 excess of iodate and mineral acid, the resulting free iodine 

 estimated by directly titrating in alkaline solution with arseni- 

 ous acid, and five-sixths of its amount taken as equivalent to 

 the iodide originally present. f A solution of mineral acid to be 

 analyzed is mixed with an excess of iodate and iodide, the 

 resulting free iodine estimated by directly titrating with 

 sodium thiosulphate, and its entire amount taken as equivalent 

 to the amount of mineral acid originally present.^ Groger 

 has applied the last mentioned method to the direct analysis of 

 various mineral acids, and has obtained results manifestly 

 better than those afforded by the use of vegetable indicators. 

 Groger has also indirectly analyzed solutions of alkali hydrox- 

 ides and carbonates by adding the solution to be analyzed to a 

 measured volume of mineral acid, previously standardized by 

 the above method, and estimating the small excess of free 

 mineral acid that finally remains by the same method. The 

 only difficulty with the Groger process lies in the fact that 

 under the conditions present the end-point of the final reaction 

 between iodine and sodium thiosulphate is somewhat obscured 

 by a peculiar back-play of color due to a continuous slow liber- 

 ation of iodine in the system. 



When a solution of a metallic hydroxide is acted on by 

 iodine at a temperature high enough to decompose the small 



* Rammelsberg, Pogg. Ann., cxxxv, 493 : Walker, this Journal, iv, 235. 

 f Gooeh and Walker, this Journal, iii, 293. 



\ Kjeldahl, Zeitschr. fur Analyt. Chem., xxii, 366; Furry, Am. Chem. Jour., vi, 

 341 ; Groger, Zeitschr. fur Angw. Chem., 1894, 52. 



Am. Jour. Sci. — Fourth Series, Vol. TI, No. 36.— December, 1898. 

 32 



