A Dropping Mercury Electrode GEORGE J. KA" University of Minnesota, Minneapolis, Minn.
A
k and the cell should have
NUMBER of suggestions have been made for constructing the dropping mercury electrode (1-6). The trend is t o avoid t h e use of rubber tubing, since i t reacts with the mercury. The rubber connections can be eliminated by a very simple siphon system. The mercury storage vessel has the form shown in a, Figure 1, the capillary itself is b, and the glass tubing, c, dips into the storam vessel. The height of the mercury can be vaned by moviig a up or down. A' mark on the side arm, d, makes it easy to reproduce the mercury pressure for eve polsro ram. The capzary itselfis connected to the siphon with an interchan eable ground oint lOft30, held in place by a couple of springs. This makes it easy to clean the capilla separately. The caprhry should be sealed to the glass tubing of the ground joint. To blow out a capillary as used in polarography it is necessary to use the pressure of a gas cylinder. To make such a seal without a cylinder the capillary tubing is. hlled with mercury usin an ordinary leveling bul%with mercury connected to the capillary with rubber tubing. The open end of the capillary is closed with one finger and the capillary disconnected from the rubber m tubing. One end of the capillary is brought into a very hot flame to close the capillary on this side the other end being closed with the finger; then the other end is also sealed. The capillary should now be nearly full of mercury. If the glass is heated with a moderately hot flame, the pressure of the merwill blow it out to a 1arge enough to be sealed to ordinary glass tubing. The sealed end of the capillary is now broken off. A round joint 12/18 is seafed to the electrode (Figure 2). The cell to 'be used with this electrode has a corresponding ground joint, and g and h are inlet and outlet for the gas. Contact to the mercury pool in the bottom is made through k. The capillary tubing between
B 12/18
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h
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K
a diameter of approximate1 1.5 mm. If an outsiL anode or reference electrode is to be used, a two- or three-necked cell should be made. Such a capillary should be cleaned before being put into service. Concentrated nitric acid, which is mostly used for this Eurpose, is not very suitale, as salts form in the reaction of the acid with the mercury left in the capillary and clog it. A solution of iodine in potassium iodide proved to be very effective in cleanin . To fill the cap.illary wit% mercury, c is dipped into the storage vessel filled with mercury, and using suction cautiously at A , some mercury is brought into m. The tapping of m causes the mercury to fall and fill out the tubing up to point B, where the side arm starts out. Now using pressure a t A , ali mercury over B is driven back into the storage vessel so that the siphon will be free from air bubbles in the end. Then the mercury is brought over by using full suction f o r a m o m e n t . I t is necessary to use a safety bottle to save any mercury carried over by the suction.
This construction allows the side arm with t h e capillary t o be pivoted a b o u t t h e storage vessel aa center, FIQURE2 and swung away from the polarographic cell. When not in use. i t is dipped into a test tube, which is provided with t h e same ground joint as t h e polarographic cell. This test tube should be filled with pure mercury. Tbe pressure in the test tube, which is closed tightly by the ground joint of the capillary, increases as the mercury flows into the test tube, and this back pressure will soon stop the flow of mercury completely. An electrode and cell of this design have been in use for more than six months, and they have worked very satisfactorily.
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Literature Cited (1) Heller, B. A.,Bet. Inst. physik. Chem., Akad. Wiss. U.9. 9. R., 11, 131 (1938). (2) Kolthoff, I. M., and Lingane, J. J., Chem. Reus.,24,1 (1939). (3) Langer, A.,IND.ENQ.CHEM., ANAL.ED.,13, 794 (1941). (4) Lingane, J. J., and Laitinen H. A,, Ibid., 11, 604 (1939). (6) Mueller, E. F., Ibid., 12, 171 (1940). (6) Stackelberg, M. v., Klinger, P., Koch, W., and Krath, E., Tech. Mi#.Krupp Faech. Bar., 2,69 (1939)
FIGURE 1
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