﻿42 



INDIANA UXIYEESITT 



acid, whicli was first used as an electrolyte, proved nusatisfactory. 

 After prolonged electrolj-sis, basic salts Avere formed upon the 

 anode. This increased the resistance of the cell and made it inac- 

 curate. A doiTble salt of potassium and mercuric iodides^ was 

 found to give satisfactory results. The electrolyte was made by 

 nearly saturating a solution of potassium iodide with mercuric 

 iodide. This solution was placed in a sealed cell containing an 

 iridium cathode near the bottom and a mercury anode near the top. 

 The dense solution, which flowed downward from the anode, mixed 

 A\dtli the dilute solution, whicli arose from the cathode, and thus the 

 electrolyte was automatically stirred. The precipitate of mercury 

 was described as falling in a continuous rain of finely divided par- 

 ticles from the cathode into a graduated tube which was part of the 

 apparatus. This tube was graduated in Board of Trade Units so 

 that the amount of current could be read off directly. Since this 

 voltameter can carry only a small amount of current, on account of 

 the high resistance of the electrolyte, it is set up in a shunt circuit 

 through which only a small fraction of the current passes. After 

 making a reading, the mercury from the cathode can be transferred 

 to the anode chamber by inverting the apparatus for a moment. 

 The mercury is precipitated as divalent merciuw (ic), whose electro- 

 chemical equivalent is 100. A rnercurous mercury bath woidd pro- 

 duce twice as much mercury as this one. 



It was the object of this research to devise a mercury voltameter, 

 and to determine the conditions which are necessary for acctirate 

 results. A solution of merctirotts perchlorate has properties which 

 make it the most satisfactory material for the electrolyte, ^lerctir- 

 ous perchlorate is A^ery solubh;^ in water, is absolutely stable, and 

 may be easily prepared. 



Preparation of Materials. Perchloric acid Avas made by treating 

 barium perchlorate solution Avith an er[uiA alent amotmt of stdphuric 

 acid. The precipitate of barium sulphate Avas removed by filtra- 

 tion. The filtrate, containing the dilute perchloric acid, was heated 

 on the hot plate until the temperature reached about 135°-150°. 

 This treatment volatilized a large part of the Avater and all of the 

 hydrochloric acid which was present as an impurity in the barium 

 perchlorate. This solution Avas then distilled under reduced pres- 

 sure. The low boiling products Avere rejected. AAdiile the distillate, 

 corresponding to 201°-203° C. atmospheric pressure, was saved. 

 This distillate had the approximate composition of HCIO4 .2HoO. 

 The rnercurous perchlorate Avhich was used in the early experiments 



" Wriglit, London Electricdan, LX, 297 and .319. 



