480 ROSA and vinal: silver voltameter 



2. The appearance of the deposit is altered by the presence of 

 impurities in the solutions (such as those resulting from filter 

 paper). Pure solutions give crystalline deposits of very pure 

 silver, but colloids, if present, break up the crystals and produce 

 striated deposits which are too heavy to represent accurately the 

 amount of electricity which passed through the voltameter. 



3. Many forms of voltameter have been compared. The 

 Bureau has found that the most satisfactory are the porous cup 

 voltameter and the new form devised by Mr. F. E. Smith of the 

 National Physical Laboratory. 



4. The Bureau has devised means of preparing pure silver 

 nitrate and suitable tests for it, so that an electrolyte of a uni- 

 formly high state of purity can be prepared. These tests are for 

 acidity and for reducing agents. The Bureau has also found 

 that the agreement between the results obtained from large and 

 from small sizes of voltameters, used simultaneously, is a valu- 

 able test of purity; impure solutions (except for acid) invariably 

 give heavier deposits in the large size voltameters. This phe- 

 nomenon we have called the volume effect. 



5. The temperature coefficient of the voltameter is found to 

 be zero. 



6. Tests of the purity of the silver deposits show that when 

 made from pure electrolyte, the impurities included with the 

 silver crystals represent on the average only 0.004 per cent of 

 the weight of the deposit. 



7. The absolute electrochemical equivalent of silver was found 

 to be 1.11800 mg. per coulomb and the voltage of the Weston 

 normal cell was found to be 1.01827 volts at 20°C. 



8. Comparisons with the iodine voltameter were made and the 

 ratio, the amount of silver deposited to the amount of iodine 

 deposited by the same current, was found to be 0.85017, which, 

 corrected for the inclusions in the silver deposits, gives 0.85013. 

 The electrochemical equivalent of iodine in absolute measure 

 was computed to be 1.31507 mg. per coulomb. 



The value for the faraday on the basis of the absolute electro- 

 chemical equivalent of • silver and of iodine and their atomic 

 weights is as follows: 



