46 



Prof. J. Dewar and A. Scott. 



[Feb. 15 r 



Table I gives the results of the direct determination of the 

 equivalent of the permanganate of silver by reduction in hydrogen. 



The silver permanganate was heated in a bulb of hard glass, first 

 in a current of pure air and then in hydrogen, at a red heat, until the 

 resulting mixture of silver and oxide of manganese had a constant 

 weight. The residue was allowed to cool in hydrogen, which was 

 finally displaced by nitrogen before weighing. The results obtained 

 by this method show great variation, the errors being probably 

 due to the occlusion of hydrogen and the suspension of some 

 oxide of manganese in the oxygen evolved. The method finally 

 employed was to dissolve the permanganate of silver in dilute nitric 

 acid in presence of various reducing agents, such as sulphurous acid, 

 sodium formate, and potassium nitrite. The silver was then deter- 

 mined by adding very nearly an equivalent quantity of pure 

 potassium bromide, and titrating the small amount of silver remain- 

 ing in solution, by means of very dilute potassium bromide, contain- 

 ing about 1*19 mgrms. of the pure salt per gramme of solution. 

 The solutions were in all cases weighed, thus avoiding errors due to 

 fluid expansion, faulty graduation of burettes, &c. The titrations 

 were performed in yellow light in an apparatus similar to that used 

 by Stas, and with all the precautions insisted on by him as essential 

 to the accuracy of such determinations. 



The permanganate of silver crystallises readily from warm water, 

 and is a very stable salt. It is also quite anhydrous and not in the 

 slightest degree hygroscopic. From its small solubility it is easily 

 freed from adhering impurities by recrystallisation. The purity of 

 the salt was tested by reducing about 5 grms. by means of alcohol and 

 filtering, when the total residue only weighed T9 mgrms. This 

 residue when tested with the spectroscope was found to consist almost 

 entirely of calcium salts from an accidental impurity in the distilled 

 water, only the faintest trace of potassium being detected. The sample 

 which was tested in this way had only been recrystallised once after 

 precipitation. The salt was usually prepared by the precipitation of 

 silver nitrate by means of an equivalent quantity of potassium per- 

 manganate, the solutions being warm, and the silver permanganate thus 

 obtained in fine needles, was easily drained, washed, and recrystallised. 

 A quantity of the salt was also prepared from crystallised barium 

 permanganate, which was made from barium chloride and silver per- 

 manganate, the barium salt being afterwards decomposed with pure 

 silver sulphate. This method of preparing the permanganate of silver 

 ensures the absence of any trace of silver nitrate, which as Stas has 

 shown adheres most persistently to many silver salts. 



Permanganate of silver has several very important advantages over 

 the other bodies previously used for the determination of the atomic 

 weight of manganese. Its freedom from hygroscopic properties and 



