Stephen W. Tobey1 West
Virginia Wesleyon College Buckhonnon
Simplified Moving-Boundary Transference Cell
The transference cell shown in the figure is considerably easier to construct and operate than similar simple cells described earlier2? for determining cation transference numbers by the moving boundary method, and the necessary materials for its construction will usually he readily available in any chemistry laboratory. To prepare the cell for use, the upper (cathode) chamber is filled with the solution to be used in the experiment, a beaker is placed under the anode, and the anode stopper is momentarily loosened. The downward sweep of electrolyte effectively frees the capillary of any impurities or air bubbles present.
-
The predominate anode reaction during operation is Cu
Cut+
+ 2e,
although the deposition of a small amount of a white precipitate on the tip of the anode during opewtiot~ suggests that the reaction sequence Cucu+
Cu+
+ C1--
+ le CuCl
+
may also be occurring simultaneously. The mobilities of Cu++ and Cd++ are almost the same, so that no difficulty is created by substituting Cu for Cd in the anode. The cathode reaction is simply 2H+ + % - H I ?
KNURLED W M B SCREW 9)UXRDD JOINT
COPPER COUPLlND 2 HOLE RUBBER STMPER GLASS TUBlNG VENT
RUBBER STOPPER PI4COPPER WlRE SPIRIL
TEST TUBE
13" 100nm PYREX TEST TUBE WlTH HOLE BLOWN IN SIDE
MIIOENSER JACKET D-,.Om1 PVREX PlPETTE WITH a 0 Z m l DlYlSlDNS
X 14 COPPE WlRE
RUBBER STCPPER
RUBBER STOPPER
CCPPERCDWNG
Since this reaction takes place far removed from the capillary in which the moving boundary is generated, no complications due either to depletion of the catholyte in C1- or mixing of the solution-byevolving H2arise. Table 1 gives the results of a typical experiment performed using the apparatus described. The value of t,. calculated from this data is 0.806, to be compared with the literature value, 0.823.6 Although circulation of constant temperature water around the capillary is not absolutely essential, the results obtained with thermostating were significantly better than those obtained on a prototype model which had only a large volume of water surrounding the capillary to act as a heat sink. A convenient power supply for providing the necessary regulated current for this apparatus is described elsewhere in THIS JOURNAL.^ Table 1.
SOLDERED JOINT
Sample Experimental Results with 0.1022 M HCl at 2 5 0 ° C
KNURLED THUMB SCREW
The cell has been used to determine the transference number of H + ion in dilute HCI solutions. Malachite Green Oxalate4 indicator is more satisfactory than the usually specified Methyl Violet in these experiments because it does not decolorize. If a strip of white paper is taped to the back of the condenser jacket the ascending column of colored solution is much easier to see.
' Present address:
University of Wisconsin, Madison. DANIELS,FARRINGTON, MATREWS,JOSEPHH., ET AL., "EXperimental Physical Chemistry," 5th ed., McGraw-Hill Book Co., Inc., New York, 1956, p. 150. a LONGSWORTH, L. G., J. CHEM.EDUC.,11, 420 (1934). * Eastman Kodak No. 1264. a
516
/
Journal of Chemical Education
SMITE,E. R.,
AND
MACINNES,D. A,, J . Am. Chem. Sac., 47,
1012 (1925).
TOBEY,S. W., J. CHEM.Eoac., 38,517 (1961).
