96 Van Name and Huff- — Estimation of 



possible source of error even in the direct titration of an 

 arsenite by iodine in the presence of a bicarbonate, and 

 states that for accuracy it is absolutely necessary that 

 the solution be kept saturated with carbon dioxide. 12 It 

 is obvious that the danger is far greater in a process in 

 which an excess of iodine is left in prolonged contact 

 with the bicarbonate, as in the present case. 



Since all the iodine which enters into this reaction is 

 set free again when the solution is acidified, it shouldj be 

 possible to improve upon Sieverts' method by carrying 

 out the final titration of the iodine in acid solution by 

 means of thiosulphate. In practice, however, this is 

 inconvenient because of the effervescence produced when 

 the solution is acidified, and the special precautions 

 necessary to prevent consequent loss of iodine. 



For this reason we have adopted disodium hydrogen 

 phosphate as a substitute for the bicarbonate. As solu- 

 tions of this salt have a higher alkalinity than alkali 

 bicarbonate solutions which are saturated with carbon 

 dioxide, the amount of iodate formed during the first 

 stage of the process may be much greater, but this does 

 not affect the results because the solution is acidified 

 before the final titration of the iodine. The absence of 

 effervescence is a great advantage. The amount of the 

 sodium phosphate used must, of course, be great enough 

 to neutralize the hydriodic acid formed, and to keep the 

 solution alkaline during the oxidation of the phosphorous 

 acid. In an initially neutral solution, 11 grams of 

 Na 2 rIP0 4 .12H 2 for every 100 cm 3 of N/10 iodine 

 reduced, will just suffice to leave the solution neutral at 

 the end. 13 This quantity represents the absolute mini- 

 mum which must not be overstepped. In practice it is 

 a safe rule to add twice as much, or at least 11 grams 

 for each 50 cm 3 of N/10 iodine to be introduced. It is 

 immaterial whether the amount used exceeds this or not, 

 but it is desirable that the total be known as an aid in 

 estimating the amount of acid needed for the acidifica- 

 tion. A rough estimate is quite sufficient, for it is well 

 known that the degree of acidity may vary over quite a 

 wide range without affecting the accuracy of the thiosul- 

 phate titration. 



That this procedure eliminates the iodate error is 



12 J. Am. Chem. Soc, 30, 43, 1908. 



13 Washburn, loc. eit., p. 44. 



