C. F. Walker — Titration of Sodium Thiosulphate, etc. 235 



Art. XXV". — The Titration of Sodium Thiosulphate with 

 Iodic Acid ; by Claude F. Walker. 



[Contributions from the Kent Chemical Laboratory of Yale University — LXV.] 



This investigation was undertaken to determine the nature 

 and limitations of the reaction between iodic acid and thiosul- 

 phuric acid, and to show the expediency of employing iodic 

 acid in standard solution for the direct titration of sodium 

 thiosulphate. Biegler* states that iodic acid is readily obtained 

 in the pure state, that it may be accurately weighed out, and 

 that a solution of it may be exactly made up to a desired 

 strength and kept for a long time unaltered. He further states 

 that when a solution of sodium thiosulphate is titrated with 

 iodic acid the reaction takes place according to the equation, 



6Na 2 S 2 3 + 6HI0 3 = 3Na 2 S 4 6 + 5NaI0 3 + Nal + 3H 2 0, 



under which circumstances no free iodine will be evolved until 

 all the sodium thiosulphate has been oxidized to tetrathionate ; 

 the first drop of iodic acid in excess, however, will react with 

 the sodium iodide that has been formed, and separate iodine, 

 as shown by the equation, 



5NaI + 6HI0 3 = 5NaI0 3 + 3H 2 + 3l 2 , 



thus furnishing an accurate means for determining the end 

 point. 



A careful repetition of the work of Riegler has shown that 

 his conclusions are in a large measure erroneous. Thus, it has 

 been found that the ordinary " chemically pure" iodic acid, 

 purchased from reliable manufacturers, is likely to contain 

 more than the theoretical amount of iodine, due probably to 

 the presence of the anhydride, although iodic acid can be 

 safely employed for standardizing when it is made in the 

 laboratory by dissolving the purified anhydride, crystallizing 

 out the acid, and drying over sulphuric acid. Such a carefully 

 prepared product, if used immediately, will be found to con- 

 tain the theoretical amount of iodine. Riegler's proposed 

 method of titration depends on two different reactions, and to 

 insure the accuracy of the process these must be definite, com- 

 plete and non-reversible under the conditions of analysis. 

 Thus one molecule out of every six of iodic acid should be 

 reduced by six molecules of thiosulphate, with the formation 

 of a neutral mixture of iodide and iodate, free from other 

 -oxidizing or reducing substances. Under these circumstances 

 it might be expected that iodine will be liberated by the first 

 trace of iodic acid in excess. It has been found by investiga- 



*Riegler, Zeit. fur Analyt. Chem., xxxv, 308. 



