J. H. Reedy — Anodic Potentials of Silver. 401 



since some silver always went into solution. He therefore 

 recommended estimating chlorine volumetrically after the 

 iodine and bromine had been precipitated. No attention seems 

 to have been given by Specketer to the possibility that, for 

 certain concentrations, halogens might be deposited simultane- 

 ously. 



E. F. Smith and his collaborators* have studied extensively 

 the electronic precipitation of the halogens on silver anodes. 

 In the same laboratory the so-called " double-cup " was 

 developed by Hildebrand, by means of which both metal and 

 halogen are determined in one electrolysis. Very accurate 

 results were reported. 



G-ooch and E-ead,f and Peters, J however, found the estimation 

 of chlorine by deposition on silver anodes to give results that 

 were " irregular and always low," owing to a tendency of the 

 silver to be dissolved from the anode, and to be transported to 

 the cathode, or precipitated as silver chloride in the solution. 



Very recently Boettger and Kelly § have proposed a 

 modification of the Hildebrand " double-cup " method. It 

 consists in adding an excess of a salt whose cation is more 

 readily deposited than the ions of the alkali metals, such as 

 "cadmium sulphate. The mercury dissolves the deposited 

 metal, and owing to the high over-voltage of the hydrogen on 

 the amalgam cathode, no hydrogen is evolved, and the solution 

 remains neutral. No report was made of estimating chlorine 

 in this way. Presumably the authors found that it was not 

 practicable. The determination of the decomposition potentials 

 of solutions of potassium iodide, potassium bromide and potas- 

 sium chloride of arbitrary concentration led Boettger and Kelly 

 to the conclusion that the iodine could be separated from bro- 

 mine and chlorine, but not bromine from chlorine. Satisfactory 

 results were obtained. The methods used were more or less 

 empirical, and no attention was given to the effect of varying 

 the concentration of the halide solutions. 



Experimental Part. 



Preliminary Experiments. — Solutions of potassium chloride 

 were dissolved in various dilute acids (0 - 5 molar sulphuric, 

 - 33 molar phosphoric, 0*5 molar oxalic, and I/O molar formic) and 

 electrolyzed between a platinum cathode and a rotating silver 

 anode, using very low tensions, — 50 to 200 millivolts. In all 

 cases it was found that shortly before the precipitation was 



*Withrow, Jour. Amer. Chem. Soc, xxviii, 1356; Hildebrand, ibid., 

 xxix, 447 ; McCuteheon, ibid., xxix, 1445, 1460 ; Lvtkens, ibid., xxix, 1455 ; 

 Goldbaum and Smith, ibid., xxx, 1705 ; xxxii, 1468. 



\ This Journal, xxviii, 435. % Ibid., xxxii, 365. 



§ Verh. Ges. deutsch. Aertze, 1913 (1914), II, 361. 



