94 Iodine Voltameter for the Measurement of Small Currents. 



sponds to a solution containing 12*845 grms. of pure sodium 

 thiosulphate per litre ; but it is necessary to standardize it 

 from time to time with a standard solution of iodine, as its 

 value is liable to change somewhat on keeping. 



As pure zinc iodide is not always readily obtainable, though 

 it can be easily prepared, the author has found that a solution 

 of 1 5 per cent, zinc chloride, to which about 5 per cent, 

 potassic iodide has been added, may be substituted for pure 

 zinc iodide. Any chlorine, even if liberated, which is im- 

 probable with small current-densities, would at once yield its 

 equivalent in iodine without escaping, and should any caustic 

 potash form at the kathode it would merely produce zinc 

 hydrate and potassic chloride. 



A single example of the accuracy and rapidity of the 

 method, selected from laboratory notes, may here be given : — 



" Dec. 19, 1894. — Current through calibrated galvanometer 

 = 20°. 



Time current was passing = 2 hours. 



Weight of silver deposited = 0*2122 grm. 



Current (from above deposit) = 0*0264 ampere. 



The same deflexion (20°) obtained using iodine voltameter. 



Time = 30 minutes. 



Iodine liberated required 9*6 cub. centim. of standard thiosul- 



48 

 phate sol. 9*6 x 5 = 48 coulombs. Currents ^~ = 0*0266 



amp." lb0 ° 



The latter result is seen to be in fairly close agreement with 



that obtained with the silver voltameter, but slightly higher, 



as was to be expected from the reasons stated above. It will 



be noticed that the duration of the experiment was only a 



quarter of that with the silver voltameter. 



In conclusion the points which recommend the iodine 



voltameter in the author's opinion are as follows : — 



1. Its freedom from the effect of dissolved oxygen which 

 gives " low " results with kationic voltameters. 



2. The relatively short duration of the current necessary to 

 obtain results of fair accuracy. 



3. The rapidity of the volumetric estimation of the liberated 

 iodine. 



4. The ease and rapidity in calculating the results. 



