44 ROSA, VINAL AND McDANIEL: THE SILVER VOLTAMETER 



For purifying the silver nitrate for use in the voltameter it is 

 convenient to start with the best c. p. salt obtainable from the 

 manufacturing chemists. If this salt is repeatedly recrystallized 

 from neutral solution the resulting product shows a tendency to 

 become basic which under certain conditions gives rise to the 

 volume effect in the voltameter. To avoid this condition it is 

 necessary that the last mother liquor should be acid to the extent 

 of from 0.1 to 1 per cent of the water present depending on the 

 efficiency of the draining. In the present work a centrifuge has 

 been employed and the crystallization carried out in porcelain, 

 quartz and platinum vessels without any significant differences 

 in the final product. It is best that the preliminary recrystalli- 

 zations should be made from strongly acid solutions as this 

 increases the yield of crystals owing to .the less solubility of the 

 AgNOs and decreases the number of crystallizations required to 

 satisfy the permanganate test. 



Salt yielding the same result in the voltameter has also been 

 prepared by recrystallization from strongly acid solution and 

 subsequent fusion. The fusion appears to break up the more 

 resistant impurities contained and in some cases these come to 

 the surface and may be readily removed by washing the surface 

 of the fused cake as suggested by Mr. F. E. Smith. The fusion 

 also furnishes a ready means of controlling the amount of acid 

 in the final product, but especial precautions must be taken. 

 Whether the silver nitrate is decomposed by heat when fused 

 seems to depend less on the temperature than upon the amount 

 of acid retained by the melted salt. As soon as the last trace of 

 acid is expelled the salt begins to decompose into silver oxide and 

 colloidal metallic silver as shown by the tests with iod-eosine and 

 potassium permanganate respectively. By removing the salt 

 from the furnace as soon as the last trace of solid material has 

 melted it is found that a small amount of acid (1 or 2 parts in 

 100,000 of the solid salt) is retained and that this is sufficient 

 to prevent decomposition, and in a 10 per cent solution is negli- 

 gible in the voltameter. 



Salt prepared in this way which is satisfactory for the voltame- 

 ter is invariably a beautiful pearl white and never darkened as 



