The ratio of values obtained for the two membrane samples was 1.01 + 5.0z. This agreement is fairly good considering that the cross sectional area of exposed membrane was only 0.79 cm2 and any inhomogenity in membrane structure could cause appreciable differences in jss. The values of jss/C, for the compounds listed in Table I varied considerably. It appears that dialysis tubing does not behave as a passive membrane for all compounds studied and elucidation of the interactions taking place a t the membrane will be the subject of further investigation. Measurement of mass transport for a series of compounds across Millipore filter is reported in Table 11. For the reported data, the value of j,,/C,D varies by about 2.1 and indicates that the membrane behaves either inertly or similarly with respect to these compounds a t the specified concentration. These studies indicate that the rate of mass transport across membranes can be determined in a few minutes with good
precision. Since most of the measurement is processed by flowing streams, the measurement is convenient experimentally. The relative rapid response should permit the observation of transient phenomena that might be missed with the much slower conventional sampling methodology. ACKNOWLEDGMENT
The Dialysis Tubing was supplied through the courtesy of Union Carbide Corp., Chicago, Ill., and the Millipore Filter through the courtesy of the Millipore Corporation, Bedford, Mass. RECEIVED for review September 26, 1968. Accepted December 18, 1968. These studies were supported by Grant G M 12998 from the National Institute of Health, U S . Public Health Service, Bethesda, Md., and by the Sterling Winthrop Research Institute, Rensselaer, N.Y.
A Controlled Atmosphere Cell for Rotating Disk Electrode Voltammetry in Fused Salts Pier Giorgio Zamboninl
Department of Chemistry, The Pennsylcania State Unicersity, Unicersity Park, Pa. 16802
A ROTATING DISK ELECTRODE (RDE) has unique advantages in the field of solid electrode voltammetry. It is the only solid electrode (including both convection and diffusion controlled devices) for which a rigorous mathematical theory is available and experimentally demonstrated usually within 1 %. Nevertheless it has found only limited applications in fused salts studies ( I ) . One of the major experimental drawbacks of the R D E is the difficulty of maintaining, under rotation, a perfectly controlled atmosphere over the melt. In many instances a small inadvertent influx of air (COz, 02,H20) can be of critical importance in these systems-e.g. some controversial results in literature concerning the chemistry of oxide species in fused salts can be ascribed ( 2 , 3) to such experimental accidents. For this reason other techniques employing solid, immobile electrode configurations-e.g. chronopotentiometry and rapid scan voltammetry-are widely utilized. Another distinct problem with fused salt studies is the corrosive nature of either the melt or one of its components. Often studies performed in glass containers have produced inconsistent experimental data-e.g. some ambiguities in the chemistry of oxide species can be ascribed ( 2 , 3) to side reactions of the type Si02
+ 02- = S i O P
(1)
The electrolysis cell presented here was built according t o the following criteria : the mechanical system should permit ‘Present address, Istituto diChimica Analitica, via Amendola 173, Universita’ di Bari, Bari, Italy (1) R. A. Bailey and G. 3. Janz in “The Chemistry of Non-Aqueous Solvents,” J. J. Lagowski, Ed., Academic Press, New York, N.Y., 1966, Chapter 7. (2) P. G. Zambonin, J. Jordan, J . Amer. Chem. SOC.,89, 6365, (1967). (3) J. Jordan, W. B. McCarthy, and P. G. Zambonin in “Characterization and Analysis in Molten Salts,” G. Mamantov, Ed., M. Dekker, New York, N.Y., in press. 868
ANALYTICAL CHEMISTRY
the R D E to follow the Levich equation; the supernatant atmosphere should remain uncontaminated for a long period (up to 24 hours); the system including the cell, electrodes, temperature monitor, etc. should be protected from attack by corrosive species ; the system should permit sequential experiments -e.g. massive electrolysis followed by polarographic recording-without exposing the melt to the ambient atmosphere. EXPERIMENTAL Apparatus. CELL. The external mantel of the electrolysis cell was a long, thermostated, borosilicate glass cylinder divided in two separate compartments. The divider served as a mount for the rotating electrode. The cylinder was covered with a borosilicate glass head ( 4 ) to which seven ground joints were sealed. Only three of them are illustrated in Figure 1. The joints allowed the insertion of a n indicator, reference and counter electrode, a thermistor (or a thermocouple), a device for addition of solids, and two gas inlets, one immersed in the melt and the other above the divider (Figure 1). A 400-ml platinum beaker contained the melt and served as a working electrode for massive electrolysis. ROTATORS.The R D E was rotated by coupling the electrode shaft t o a synchronous motor and a gear box. Two distinct coupling mechanisms were used. The first, a magnetic system illustrated in Figure 1, was designed to work under reduced pressure or vacuum. The second, a much simpler direct coupler, shown in Figure 2,I, was employed for work under gas flow (one atmosphere). Whenever necessary, the O-ring seal (Ace Glass, Vineland, N.J.) allowed the R D E to be raised above the melt when not in use. The indicator electrode was obtained by coaxial sealing of a platinum wire (1.5 mm in diameter) in a perfectly straight soft glass tube (-40 cm long). The finished electrode, after polishing, showed no protuberances under 20 fold magnification. As shown in Figure 1, the cylindrical electrode was fitted in a platinum tube in order t o diminish the area in direct contact with the melt. Obviously, it was impossible to protect (4)T. E. Geckle; Thesis, Pennsylvania State University, State College, Pa., 1964.
1“l
Asbestos W i r e 1
I
m
II
Figure 2. Accessories for the cell in Figure 1 I. alternative mount for the RDE, 11. reference electrode, 111. counter electrode
i Glass Brass Cork
Teflon 0-R’ng Thermostating Jacket
5-10 W
f
r
--NO*
/NO;
Figure 1. Vacuum tight electrolysis cell with RDE the bottom of the RDE. Consequently, in particularly corrosive melts, only the direct coupling system of Figure 2 was employed. The reference electrode (asbestos wick type) was covered with a platinum protector, as shown in Figure 2,II. Upon opening a hole in a or 6, it could be utilized under atmospheric pressure or vacuum. The counter electrode, a large platinum cylinder (-4 cmz), was placed in a fritted glass compartment externally protected by a partially perforated platinum foil (Figure 2,111). The introduction of a second electrode of this type (in place of the device for introduction of solid samples) permitted controlled potential massive electrolysis and simultaneous voltammetry. Procedure. The prepurified melt was completely dried by applying a vacuum or by bubbling an inert gas through the fused salt. After introduction of the solute (by addition of a solid or by electrogeneration in situ) a vacuum or an inert gas atmosphere was maintained over the melt. In the latter case purified Argon was directed into the topmost compartment of the cell uia the gas inlet of Figure 1. RESULTS AND DISCUSSION
Both the rotating systems proved suitable for the maintenance of an inert atmosphere over the melt. With the apparatus of Figure 1, it was possible to keep the cell under a good vacuum almost indefinitely. The device in Figure 2,I, could work over periods of 20 hours (while recording many
-20
I/
I
0
+05
E (VOLT)
+e
I
I
I 1
I
-05
s, A g / A g f
-IO
-15
(007m)
Figure 3. RDE voltammogram recorded in (Na-K) nitrate 1.98 X lO-3m in NO2under argon atmosphere (T = 510 O K , w = 63 rad/sec, A = 0.0176 cm*) polarograms) in the presence of ions which are extremely reactive toward dioxide, without significant variations The well known (for a review see process : NO2- = NO2
such as 02-and 0 2 2 moisture and carbon in the system. reference 2 ) reversible
+e
(2)
in fused alkali nitrates was used to test the experimental concordance of the R D E with Levich’s (5) equation: illm= 0.62 nF A D*/3 v-1/6
w1/2
C
(3)
where Y is the kinematic viscosity of the solution (cmzisec) and w the angular velocity of the disk (radisec); all other symbols have their usual electrochemical significance. Figure 3 presents a typical R D E voltammogram of NO2- in dry ( 5 ) V. G. Levich, “Phisiochemical Hydrodynamics,” Prentice-Hall, Inc., Englewood Cliffs, N.J., 1962. VOL. 41, NO. 6,MAY 1969
869
15 < w < 200 (radisec) for the direct coupler. The lower limit common to both devices was probably caused by the thermal convection whose relative effect increases when the velocity of rotation decreases. The upper limit was mainly because of inevitable vibrations which increase with the velocity of rotation and are amplified by the unusual length of the electrode and elastic properties of the glass. The velocity interval in which the electrodes satisfactorily followed Levich’s equation was sufficiently great to permit useful applications of the device for the elucidation of electrode processes. Under optimum conditions ( w = 40-60 radisec), a linearity within 1 for current /concentration curves was readily attained. This certainly qualifies the rotating disk electrode as one of the best quantitative tools for fused salts studies. Under the reported optimum conditions the diffusion coefficient and the temperature coefficient for the NO*- were calculated : [NO;]
; MCLE/1030g.MELT
D N O~ (510 OK) = 5.25
(RAD/SEC)
Figure 4. Graphical verification of the Levich’s equation
o Points obtained with the mount of Figure 1 e Points obtained with the mount of Figure 2,1 sodium-potassium nitrate eutectic melt under argon at 510 OK. The limiting currents obtained from a series of such curves ( w = 63 rad/sec) are given in Figure 4,A as a function of the nitrite concentration. A plot of the limiting currents obtained for a given concentration ([NOz-] = 2.45 lO-3m) as a function of w1/2 is shown in Figure 4,B. The plots of Figure 4,A and 4,B, representative of a series of experiments, demonstrate the good agreement of our experimental results with Levich’s theory. The deviation from linearity was estimated to be better t h a n 2 % with both designs described whenever 15 < w < 90 (radisec) for the magnetic coupler and whenever
E (500
- 560
O K )
=
+ 0.1 X
10-6 cmZ/sec
-5.4 Kcal/mole
(4) (5)
Because of the characteristics of the material used, the device described herein cannot be used above 650 OK. However, if a classical, water cooled brass head is substituted for the glass cover, the apparatus can then be used up to 1000 OK-Le. the softening temperature of the glass. ACKNOWLEDGMENT
The author expresses his appreciation to Joseph Jordan and his research group for helpful discussion. RECEIVED for review September 3, 1968. Accepted December 31, 1968. Investigation supported by U S . Atomic Energy Commission, Report No. NYO-2133-46.
Gas Chromatographic Separation of Chlorosilanes, Methylchlorosilanes, and Associated Siloxanes K. Ray Burson Texas Instruments Inc., Mail Station 913, P . 0. Box 5012, Dallas, Texas 75222
Charles T . Kenner Department of Chemistry, Southern Methodist University, Dallas, Texas 75222 SEVERAL methods have been reported for gas chromatographic separation of chlorosilicon compounds. Turkel’taub et al. ( I ) studied the effect of the nature and amount of the stationary phase, the solid support, the flow rate of the carrier gas and the temperature of the column during the investigation of the separation of hydrogen chloride, silicon tetrachloride, and the methylchlorosilanes. They used a column containing nitrobenzene preceded by another identical column to offset bleeding. Oiwa ( 2 ) separated the methylchlorosilanes by using two columns in series, one containing tritolylphosphate and the other dioctylphthalate as the liquid
(1) N. M. Turkel’taub, et al., Ref. Zhurl. Khim., 23, Abs. No. 23D135 (1961); Anal. Abst., 3294 (1962). (2) T. J. Oiwa, Chem. Soc. Japan, Pure Chem. Sect., 84, 409 (1963); Anal. Abst., 5230 (1965). 870
ANALYTICAL CHEMISTRY
phases. Bersadschi et al. (3) separated trichlorosilane and silicon tetrachloride mixtures in carbon tetrachloride using transformer oil activated by glycerol as the liquid phase. Lengyel et al. ( 4 ) and Palamarchuk et al. ( 5 ) investigated the separation of methylchlorosilanes using different liquid stationary phases and found those with the highest dipole moment to be the most effective. Popov et al. (6) studied the separation of chlorosilanes, methyl chlorosilanes, and phosphorus (3) D. Bersadschi, V. Stefan, and Petroianu, Rev. Chem. (Bucharest), 15, 224 (1964); Ana/.Abst., 3808 (1965). (4) B. Lengyel, G. Garzo, and T. Szekely, Acta. Chim. Acad. Sci. Hung., 37,37 (1963); C. A , , 5 9 : 4541c (1963). ( 5 ) N. A. Palamarchuk, et al., Inst. Geochim. i Analit. Khim. 13, 277 (1963); C. A., 5 9 : 6994 (1963). (6) A. N. Popov, V. M. Gorbacher, and E. I. Torgova, Ser. Khim. Nauk, 3, 17 (1966); C . A , , 67: 28950s (1967).
