with the Mercury Cathode. 373 



It is evident that the weight of the anode decreases until 

 the temperature reaches 400-500°. If we assume that this 

 loss in weight is due to the decomposition of silver oxide it is 

 evident that heating to'300° for a short time cannot always be 

 depended upon to break up all the silver oxide. Goldbaum 

 and Smith* used a temperature of 300° and Goldbaumf 

 later exposed his anode to a temperature of 330-400°. The 

 necessity for heating at the higher temperature will seem 

 more reasonable if the point of view is taken that the anode 

 after electrolysis does not consist of successive distinct layers 

 of platinum, silver, silver chloride and silver oxide, but rather, 

 on some occasions, of the chloride and oxide intermixed, these 

 being deposited simultaneously. One would expect silver 

 oxide covered with silver chloride, particularly fused silver 

 chloride, to decompose with more difficulty than silver oxide 

 by itself. 



Richards and "VVellsJ state that the hardened silver chloride 

 must be fused to rid it of the last traces of water and further 

 that the vapor pressure of the silver chloride at 500° is ana- 

 lytically inappreciable. § These statements furnish an added 

 argument for the use of a high temperature, over 487°, in pre- 

 paring the silver coated anode for weighing and tend to show 

 that there would be no loss by volatilization by so doing. 



In two instances heating in the crucible at the highest tem- 

 perature — over 600°— caused a slow but decided loss in weight 

 of the silver chloride coated anode. This was evidently due 

 to the volatilization of the silver chloride, as the glaze of the 

 crucible in the immediate vicinity of the anode was attacked 

 and turned yellow. This fact would set 550° as a limit to the 

 temperature to which the anode safely may be heated. 



Weighing Silver Chloride. — In view of the facts stated in the 

 last seven paragraphs : the danger of increase of weight upon 

 long heating ; the danger of insufficient heating at about 300°, 

 the following method was adopted for treatment of the anode 

 covered with silver chloride and oxide before weighing. The 

 anode was introduced into the heating device at a temperature 

 below the fusion point of silver chloride, 400-450°, and the 

 temperature of the oven increased until the silver chloride 

 fused. When the silver chloride had assumed the "yellow 

 hot " condition or had acquired the still hotter " brown red " 

 color the heating was considered sufficient. The process 

 required from 5 to 15 minutes, varying with the manipulation 

 of the oven. 



*Jour. Am. Chem. Soc, xxxii, 1469. 

 •(•Jour. Am. Chem. Soc, xxxiii, 42. 

 X Jour. Am. Chem. Soc., xxvii, 496. 

 § Jour. Am. Chem. Soc, xxvii, 518. 



