456 



Van Name and Broion — Ionization and 



tion, and could quickly be put into commission again by 

 emptying the siphon arms and renewing the intermediate 

 liquid. The stop-cocks in the siphon arms were generally left 

 open during the actual measurement. In the earlier experi- 

 ments, contact between dissimilar solutions was brought about 

 in the U tube in a layer of sea-sand, as recommended by 

 Bjerrum,^ but the use of sand was later abandoned as trouble- 

 some and unnecessary for the present purpose. 



The measurements were made by the Poggendorf compensa- 

 tion method with the aid of a galvanometer sensitive to 



4 X 10'^ amperes. Owing, however, to the high resistance of 

 the cells measured, the accuracy of a single bridge-reading did 

 not much exceed one millivolt. 



Prejyaration of the Reference Electrodes. — The complete 

 saturation of the iodide solutions with iodine was accomplished 

 by sealing the solution with powdered iodine in a large glass 

 tabe, which was attached to the stirring axle of the thermostat 

 and rotated for a period of at least 24 hours. With solutions 

 so prepared the two reference electrodes, wlien measured 

 against one another, usually gave a constant potential difference 

 within 12 hours, which retained its value practically unchanged 

 for many days. Occasional shaking of the half-cells (which 

 contained a little solid iodine) was found to favor constant 

 results. The liquid in the siphon arms was emptied periodi- 

 cally, and the half-cell when necessary could be relilled with a 

 portion of the original (iodine saturated) solution without alter- 

 ing the measured electromotive force. All measurements 

 were at 25° C. 



Ex][>erimental Procedure. — A cell composed of the two 

 reference electrodes was first set up and its electromotive force 

 *Zeitschr. f. Elektrochem., xvii, 58 and 389, 1911. 



