A Semi-Automatic Moving Boundary Apparatus
Billy J. Yager
and P a t r i c i a Y. Smith Southwest Texas State University San Marcos, Texas 78666
Several papers in recent years have described modifications in the apparatus for the moving boundary method for determination of transference numbers and ionic mobilities (1-6). Three involve apparatus which eliminate the need for optical detection of the boundary (5-6). These modifications were of great interest to us a t the outset of a problem involving the determination of transference numbers in a variety of solvent systems in which optical boundary detection was quite difficult. However, the published methods were rather tedious in operation or required electronic equipment which was not available in our laboratory. We have used ideas from the above references to develop a moving boundary apparatus which records the movement of the boundary past pairs of microelectrodes by the change in conductance. (Since the leading ion must have a higher conductance than the trailing ion, a marked change in conductance results.) Gordon, et al. (5) used a similar cell with a different ac detection circuit and recorded an amplified signal with a recording ammeter. The apparatus described here uses a commercially available conductance meter
(Industrial Instruments "Solumeter") in conjunction with a standard millivolt recorder. (A block diagram of the ac and dc components is shown in Fig. 1.) A 10,000-ohm decade resistan& box (R2)was used to adjust the resistance of the ac circuit to the range of the conductance meter. Details of the cell construction are shown in Figure 2. The probes were made from platinum wire sealed in an 8-mm Pyrex tube. The pairs of probes were spaced approximately 3 cm apart. It is essential that each set of probes be exactly parallel to prevent a do potential from the electrolysis reaction being superimposed on the ac detecting circuit. Satisfactory results were obtained with a cell with only 2 sets of probes but another cell was constru~tedwith 3 sets for a consistency check within each run. The probes were sealed flush with the sides of the tube and were coated with platinum black in the usual ~ ~ b b . ,stopps. ( p ~ ~ t i d lcut y away) 1 8 x 7 0 - m m Test Tubs
H O IIn~S i d e of T e s t Tubs ~ 0 x 1 0 0 - m m TmR Tubs
Mrcury Ap/ApCI Electrode Glass Indmtotions UPPER PROBES
p~atinum Probe Electrodes
4
LOWER PROBES
I 7
AgCI ELECTROOE
> Cd
ELECTRODE
Figwe 1. Block diagrom of ac detecting circuit and the ds electmlysi* circuit. Labeled port, ore: A, milliommeter; 8, millivolt recorder; C, conduc tome meter; PS, Heathkit IP 17 power supply; R, megohm variable resistor; Rn, I 0 Kohm variable resistor.
L(4 Figwe 2.
Detoil of cell design.
Volume 49, Number 5, May 1972
/ 363
Comparison of Experimental and Literature Transference Numbers
Solution
conc (eqn. (1))
KC1
0.09772 0.09930 0.09896 0.10232
KC1 HCI HCI a
n
+ (exp)
0.492 0.487 0.831 0.830
n
+ (lit)" 0.490 0.490 0.831 0.831
Reference (6').
manner prior to cell use. Leads of light flexible wire were soldered to the platinum probes. The leads were firmly bound to the cell to prevent mechanical stress and subsequent leakage around the probes. (Kay, et al., (4) recommend uranium glass as superior to sodium glass for platinum seals.) Saureisen "AcidProof" cement was used to insulate the soldered joint and to give added strength. The leads were run to a switch which connected the probes, in turn, to the conductance meter. A cadmium anode, cast to the size of the 8-mm tubing, was inserted in the lower end of the cell and sealed in with Apiezon wax. Details of the cathode compartment ard shown in Figure 2. Satisfactory silver-silver chloride electrodes were made by dipping a platinum wire electrode in molten silver chloride. A high-voltage power supply (Heathkit IP-17) was used to give a potential of 400 V across the cell at 3, 4, or 5 ma. A variable megohm resistor (RI) was used to adjust and maintain the current at the desired amperage as indicated by a sensitive rnilliammeter (A) in the circuit. During operation of the apparatus, the only necessary activities were shifting the switch position to the next set of probes and occasional changes of R1 as the cell resistance changed. The cell and solutions were thermostated in a constant-temperature air bath for 1 br prior to use. The apparatus was rinsed several times with the solution. The rinse solution was removed by a vacuum catheter tubc so the cell was not moved from its upright position. The apparatus was then filled and any bubbles in the solution were carefully eliminated with the catheter. In operation, the conductance of the solutions between the lowest set of probes was recorded until the boundary ~ a s s e dthese probes as indicated by a sigmoid curve on the recorder. The switch was then set to record the conductance at the next set of electrodes. The recorder was left running a t a constant chart speed during the operation so that the time interval between boundary
364 / Journal of Chemical Education
passage a t the electrodes was obtained from the time axis of the recorder trace. Since boundary passage took several seconds, the inflection points of the sigmoid curves were taken as reference points for time interval determinations. A solid glass rod aided in the location of the inflection points. The volume between the sets of electrodes must be accurately known. Approximate volumes were obtained by adding solution from a buret. More accurate volumes were obtained by use of 0.10 N KC1 solutiom as a standard of known transference number and calculation of the volume, V, from the experimental data with the equat,ion CFV It
n+ = -
The literature value for the transference number of the cation, n+,for 0.10 N KC1 is 0.490 at 25%. Concentration, C, is expressed in equivalents per liter, F, is the Faraday constant, and I is the current in amperes used for time, t, in seconds. The validity of the method was demonstrated by using the volume thus determined in several experiments in the calculation of transference numbers for other KC1 solutions and for several HC1 solutions. Representative data, shown in the table, illustrate the precision and accuracy of the apparatus. The apparatus was used to determine transference numbers of KC1 and HC1 in an aqueous-ethanol and aqueous-DMSO solutions. Data was reproducible within 1% for the HC1 solutions and within 0.5y0 for the KC1 solutions. The apparatus can be operated manually by substituting a conductance bridge for the conductivity meter and recorder and taking conductance readings at intervals as needed to indicate boundary passage at each set of electrodes. There is more uncertainty in the time interval in this operation, however. In summary, the apparatus described herein has been found to give accurate, reproducible results with a minimum of effort. Literature Cited (1) Toemr. S. W.. I. CHEX.EDOC.. 38,510 (19011. (2) L O N E R ~ AG N..A.. AND PEPPSR, D, C., I, CWEI.EDUC., 42.82 (19051. (3) BACA,G. A N D HILL,R. D.. J. CREY. EDYC.,47,235 (1910). (4) KAY. R. L.. v~ou~rcn. G. A.. A N D FRATIZLLO, A.. Chcm. Inslr., 1. 301 ,.-en>
,.V"S,.
(5) L o s r r ~ nI. , W., G n ~ m l r J. . R.. A N D Gonoow, A. R.. . I . Amcr. Cham. Soc., 79, 2347 (1957). (0) Lorrcswon~a.L. G., J. Amer. Chem. Soc., 57, 1185 (1935): 60, 3070 (1938).
