DECOMPOSITION OF HYDRIODIC ACID. — CREIGH'l'ON. 5 



and it was found that when the water had a conductivity of 

 2.0 X 10* or less, at 18°C., expressed in Kohlrausch's unit 

 (ohm\ cm'^)^, the agreement between the amounts of decomposi- 

 tion of several similar solutions was within the limits of experi- 

 mental error. 



The water used in the following experiments was prepared 

 according to the method of Jones and Mackay^ The ordinary 

 distilled water was doubly distilled. The steam from the first 

 flask, which contained th ; water mixed with an alkaline solution 

 of potassium permanganate, was bubbled through an acid 

 solution of potassium bichromate in a second flask. Into the 

 neck of the latter flask was thrust a block-tin condenser, and 

 held there by means of a cork made of a mixture of plaster of 

 Paris and asbestos. The water thus obtained has a mean con- 

 ductivity of 1.6 X 10"^ at 18°C. It was kept in bottles which 

 had been used several years for that purpose. 



It was found that the purity of the water, as determined by 

 the conductivity, played an important part in the rate of decom- 

 position of the solution. The table below shows the results 

 obtained when using water of two difl'erent grades of purity in 

 the preparation of the solutions. 



In this table, and all those that follow, the numbers given 

 denote the amount of jgg^ normal sodium thiosulphate solution 

 required to titrate the free iodine content in the hydriodic acid 

 solution at the specified times after the instant of mixing. The 

 mixing was done in the dark room. In all cases the amount of 

 hydriodic acid solution experimented upon was 50 cc. 



The nunibers in the following table are for the case where 

 the mixture was left to stand in the dark room, and was not 

 subjected to the action of radium or any other external 

 action. The temperature was 15 + 0°.5C. 



1. Kohlrausch und Holborn : Leitvermogen der Elektrolyte, 1898, p. 1. 



2. Zeit. phyrt. Chem., 1897, 22, 237. 



