A N ILLUSTRATION OF THE ABSORPTION OF HYDROGEN BY METALS F. A. LEWIS The Queen's University of Belfast, North Ireland
THEabsorption of hydrogen by palladium and other transition metals has been widely studied.' Palladium will absorb hydrogen in an atmosphere of the gas a t temperatures above 200°C. However, hydrogen can also be introduced into palladium at room temperature if the metal is made the cathode of an electrolytic cell. Once introduced, hydrogen call be removed by heating to a temperature of about 200°C. An interesting feature of these metal hydrides is the volume expansion of the metal which occurs when the hydrogen is absorbed. This expansion is usually revealed by X-ray examination. The absorption and desorption of hydrogen accompanied by expansion of the metal can he illustrated by the simple experiment described here. An 8-cm. length of palladium wire, approximately .27 mm. in diameter (B & S gauge 29 or 30), is held in one side of an insulated twin connector. A similar length of platinum wire of about the same gauge is connected in the other side. The wires are thus separated by about 1 cm. These wires, which constitute the electrodes, are connected t o a d.-c. supply with a milliammeter, a switch and a variable resistance in the circuit. Forty-eight volts and a variable resistance of about 10,000 ohms are convenient. The connector is then clamped above a breaker containing 2y0 sodium fluoride solution so that the electrodes are immersed to a depth of about 5 cm., Figure 1. A manometer is made from 3-mm. i.d. tubing and filled with mercury to a level of about 15 cm., Figure 2. A is a small trap filled with calcium chloride, held in position by plugs of glass wool, and B is a 6-mm. i.d. tube. Connections are made by rubber or plastic tubing. A micrometer screw gauge which reads accurately to .O1 mm. and a Bunsen burner are also required. The palladium is connected as anode and a current of 5 milliamps. passed for 5 minutes. This preliminary anodizing increases the efficiency of absorption of hydrogen. Moreover, it leaves a slightly blackened surface on the metal which indicates the level of immersion. The polarity is then reversed and with the palladium wire now connected as cathode a current of 5 milliamps. is passed for about 50 minutes. The palladium wire is then disconnected, quickly washed with water and dried with filter paper. Examination by the micrometer screw gauge shows that the diameter of the wire below the level of immersion has increased by about 5%. Little or no increase 'SMITH,D. P., "Hydrogen in Metals," Chicago University Press, 1948.
'LEWIS,F. A.,
AND
A. R. UBBELOHDE, J. Chern. Sm.,1954,
1710.
VOLUME 34, NO. 12, DECEMBER, 1957
A
30 cm.
in the diameter of the wire above the immersion level can he observed. The wire is then introduced into the tube B which is connected to the manometer. The mercury levels of the manometer are read and about 5 minutes are allowed to elapse during which no further change of the levels should occur. Gentle heating of the wire by a Bunsen flame results in release of hydrogen gas as indicated by the final manometer levels when the tuhe has been allowed to recool. A palladium wire of the dimensions indicated, when saturated with hydrogen, contains 0.6 g. atoms H/Pd. This corresponds to a release of about 2 ml. (STP) of gaseous Hz. On higher heating of the wire hydrogen-oxygen combination takes place and often occurs suddenly. The water vapor formed can be seen in the tuhe containing the specimen, and the reaction is indicated by a further change of the mercury levels of the manometer. When the wire is removed from the tube B, reexamination by the micrometer gauge shows that i t has not completely returned to its initial diameter hut to a diameter less than that when charged with hydrogen. Hydrogen can also be removed from the wire by means of reversed electrolysis. Other methods of oxidation a t room temperature can easily be added to the demons t r a t i ~ n . ~Using these latter methods the wire is found to return to its initial diameter. The same length of palladium wire can be used for several demonstrations although, after the wire has been heated to high temperatures, it may be necessary to abrade the surface gently with emery cloth t o reactivate the surface for reproduction of the results. 601
