Goocli and Beyer — Electrolytic Estimation of Lead. 63 



simplest. Fifty-centimeter portions of a solution of pure 

 manganese sulphate, standardized by evaporation of measured 

 portions and gentle ignition of the residue over a radiator,* 

 were treated, in each case, with six drops (0'17 cm3 ) of concen- 

 trated sulphuric acid and electrolyzed in the filtering cell 

 with a current of 2 amperes (JSTD. 100 = 5 amp.) and 20-10 

 volts, the voltage decreasing as the solution became heated. 

 In one set of experiments the process of continuous filtration 

 during electrolysis, for which the adjustment of apparatus is 

 shown in figure 1, was employed : in a second set of experi- 

 ments the closed cell, shown in figure 2, was used during the 

 electrolysis and the adjustment for filtration made subsequently 

 as previously described, f The time required for the deposition 

 of 0*1860 grm. of the dioxide was one hour and three-quarters 

 in the former process : a period varying from two hours and 

 ten minutes to two hours and fifty minutes is required in the 

 latter process. Tests with hydrogen dioxide and ammonia 

 showed that the deposition was complete in the process of 

 continuous filtration and practically so in the closed-cell pro- 

 cess. The closed-cell process naturally requires less attention 

 during the electrolysis, and so it is advantageous to run the 

 process for a period, perhaps two hours, with the closed cell 

 and then to adjust the apparatus for filtration during further 

 electrolytic action in order that floating particles of the dioxide 

 may be drawn to the felt and completeness of precipitation 

 may be assured. In this way the advantage of the circulating 

 process may be obtained with less attention to manipulation 

 than is required when the filtration is continuous from the 

 start. The deposit was washed with water after interruption 

 of the current, first dried at 200° for ten or fifteen minutes 

 and weighed, and thereafter ignited to low redness in the 

 spreading flame of a large burner 4 Results of experiments 

 with the cell arranged for continuous filtration, and of experi- 

 ments in which the closed cell was used until the electrolysis 

 was nearly over, are given in the accompaning table. 



The results are evidently as good as could be expected of 

 any process which involves the weighing of manganese 

 dioxide brought to condition by heating. The degree of 

 oxidation of the oxide thrown down under the conditions 

 which obtain apparently approximates closely to that of the 

 ideal oxide represented by the symbol Mn0 2 .H 2 0, formerly 

 assigned by Eiidorff§ to the electrolytically formed oxide, 

 and differing in that respect from the electrolytically deposited 

 oxide which was studied by Groeger.] 



*This Journal, v, 209, 1898. f This Journal, xxv, 249, 1908. 



{This Journal, v, 214, 1898. gZeitschr. angew. Cheni., 1892, 6. 



|| Zeitschr. angew. Chem., 1895, 253. 



