AN APPARATUS FOR PLATING HYDROGEN ELECTRODES VERNON A. LAMB University of Maryland, College Park, Maryland
IN A physical chemistry laboratory experiment on the hydrogen electrode, i t is desirable for the students themselves to obtain experience in preparing and plating the electrodes. However, if odds and ends of temporarily assembled apparatus are used for carrying out the several steps of the plating process, the procedure is not well adapted to class use. Therefore, the
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apparatus described herein has been designed for this purpose. The only previously described apparatus for plating hydrogen electrodes of which the author is aware' does not appear to be as simple and convenient as this. A perspective drawing of the apparatus is shown in Figure 1. At A is a 2" X 3" block bored with six holes for supporting the tubes which contsin the plating solutions. These tubes are approximately 1" X 3" and are made by cutting off the tops of 1" X 8" test tubes. The electrode B is soldered permanently to the flexible wire lead shown. As illustrated in the detail drawing, it is made from a strip of platinum foil sealed into 6-mm. glass tubing. The tubing extends to just below the bottom of the foil to protect it from bending or shorting. Two holes in the tube a t opposite sides of the upper end of the foil allow escape of gas bubbles. A copper lead is fused to the platinum and cemented in a t the top. C is an ordinary Hildebrand type hydrogen electrode. Connection to i t is made by means of a small spring clip connected to the other flexible lead. Both leads are long enough to permit use of the electrodes in any cup. D is a reversing switch, E a 500-milliampere ammeter, and F is a knob for adjusting the current. For this purpose an ordinary variable radio resistance of 150 ohms, 50 watts, is suitable. At G are bulldog battery clips, with polarity indicated, 1 CLARK, "The determination of hydrogen ions," 3rd ed., The Williams and WiIkins Co.,Baltimore, 1928, p. 286.
for attaching to a &volt storage battery. At H is shown the wiring diagram. Figure 2 is a photograph of the apparatus. An alternative source of direct current is a small transformer and copper oxide rectifier, capable of supplying 0.5 ampere a t 6 volts, installed in the back of the box. This arrangement, by eliminating the battery, makes the apparatus entirely self-contained and hence still more convenient to use. All electrical Darts required can be obtained from any electrical or radio supply house. In operation the following solutions are used, one in each tube in the order given: a 6 N hydrochloric acid solution for stripping off old black, a 1 N sulfuric acid solution for testing cleanliness of the electrode,' distilled water for rinsing off acid, plating solution, distilled water for rinsing off plating solution, and 1 N sulfuric acid solution for charging the electrode with hydrogen. The electrodes are transferred from one tube to the next. No s u ~ ~ ofor r t the electrodes is necessary, other than that &en by the tubes. One advantage of the small volume of plating solution required is that accidental spoilage by students is not a serious loss. Typed instructions for operation of the apparatus are glued to the rise: behind the tube
