ANALYTICAL EDITION
September, 1944
The needle-valve spindle is of 0.3-cm. (0.125-inch) stainless steel rod threaded to fit the valve bearing. A small knurled wheel is threaded to the top of the R indle. A fine point is ground on the end of the valve spindg with a Carborundum wheel and the valve seat produced by grinding this point into the ca illary tubing with a fine grade of emery dust. itrands of absorbent cotton impregnated with a stiff grease are stuffed around the spindle in t,he tiepression of the bearing and compressed to an air-tight seal by the hexagonal nut. A small piece is cut from 0.3-cm. (0.125-inch) Neoprene tubing and slipped over the end of the valve bearing, so that the assembled unit fits tightly to the inside of the Pyrex tube. When properly packed and assembled, such a bearing will easily retain vacuums of 3 to 5 mm. An efficient column for use with this head consists of a Pyrex tube, 8 mm. in inside diameter, providing 30 cm. of packed section. This inner tube is surrounded by a 35-mm. tube wound with Nichrome wire over asbestos spacer cords. The inner
A
tube and heating jacket is covered by a piece of 45-mm. Pyrex tubin which acts a3 as an insulator. The inner tQbewas packed ~ ~ in diameter. The with bilson helices (6) 0.24 cm. ( J /inch) column was operated a t reduced pressures in conjunction with the fraction receiver described by Towne, Eby, and Young (4). This column proved to be very efficient in the purification of small samples (IO to 25 cc.). It had 14.5 theoretical plates at total reflux and an H.E.T.P. of 2.06 cm. LITERATURE CITED
(1) Loveless, IND.ENG.CEEX.,18,826 (1926). (2) Newman. IND.ENG.CEEY.,ANAL.ED.,14, 902 (1912). (3) Peter and Baker, IND.ENQ.CEEM.,18,69 (1926). (4) Towne, Eby, and Young, IND. ENC).CHEM.,ANAL.ED., 13,626 (1941). (5) Whitmore and Lux, J . Am. Chem. Soc., 54,3448 (1932). (6) Wilson, Parker, and Laughlin, Ibid., 55, 2795 (1933).
Circulating Device for Use with a Hydrogen Electrode JAMES CURRY'
AND
Z. Z. H U G U S , JR.,
T
H E device described here has been found useful in connection with a hydrogen electrode in two circumstances: first, when the solution, whose hydrogen-ion concentration is being measured, contains a very soluble gas which would be carried away if the hydrogen were allowed merely to bubble through it. A presaturator may be used, but under certain conditions this is rather impracticnl. The second situation is when a deuterium electrode is deqired. With the device described here macroquantities of the gas may be used, but an excessive amount is not required. These two condition- were present in meawrements made by the authors on the second ionization constant of deutero-carbonic acid. The results of these measurements have been reported elsewhere (I) but the circulating devices has not been adequately described. The construction of most of the apparatus is self-evident from the diagram. V is a Bunsen valve madc from a medicine dropper 1
585
Present address, 16 Brown St., Cambridge, Msss.
Tube A
Williams
College,
Willirmrtown, Mass.
bulb. The proper size of slit can be found with a few trials and i t may be inserted through the end of the wide tubing when the rubber stopper is removed. The mercury in the side arm is raised and lowered about 10 cm. by means of a motor and eccentric, raising and lowering a leveling bulb at a rate of about 15 times per minute. When the mercury rises the pressure in the adjacent part of the apparatus increases. Hence, the hydrogen bubbles through the solution around the platinum electrode until the pressure in the entire apparatus becomes uniform. When the mercury is lowered the hydrogen escapes through the Bunsen value. Thus the hydrogen tends to circulate throu h the a paratus. During this period the stopcock of the cell, shoulf be kept closed. When an e.m.f. measurement is made the circulation may be stopped and the stopcock of the cell opened if desired. The apparatus is filled and flushed out by proper manipulation of clamps CI, CZ,CS,stopcocks SIand SZ, and the mercury column, hydrogen being admitted through tubes A and B. Admittance of dry hydrogen would alter the concentration of the solution slightly. This is ordinarily a very small error but ma be avoided to a large extent by allowing the hydrogen to buible through water, or better yet, through a Sam le of the solution before it is admitted to the apparatus througg A and B. As a precautionary measure all rubber connections should be kept coated with collodion. I n t.he authors' apparatus the volume, including the space in the cell above the solution was 35 cc. For one determination about 100 cC. of hydrogen (deuterium) were used, the excess gas being used for flushing. It is usually most convenient to fill the device with hydrogen, so that when the mercury column is a t its mean position the total pressure in the system is e ual to that of the atmosphere. The corrections to a ply in or2er to obtain the partial pressure of the hydrogen are ogvious. I n the authors' measurements, at equilibrium, the fluctuations in e.m.f. due t o change' in hydrogen pressure were less than 0.1 millivolt. In order to avoid condensation i t is necessary to kee the gasphase portion of the apparatus a t a temperature somewgat higher than that of the solution. This was accomplished by fastening the circulating device to a wooden block and suspending it in a small box in such a manner that the bottom of the box waa slight1 above the surface of the water in the thermostat in which the cef was immersed. When the thermostat was adjusted to 25' C. a current of air warmed to 30' C. waa passed up through the bottom of the box through several holes. The hydrogen which circulates through the solution is also warmed to about 30" C. and this introduces a slight but unavoidable error.
&,
This circulating device is obviously not limited t o the particular type of cell depicted here. Harned and Scholes ( 8 ) have described in an extremely brief manner what is evidently an elaborate device for the circulation of hydrogen. Apparently their device is somewhat similar to the one described here. LITERATURE CITED
(1) Curry and Hugus, J . Am. Chem. Soc., 66,653 (1944). (2) Harned and Scholes, Ibid., 63,1706 (1941).
