Glassy Carbon Rotating Ring-Disc Electrode for Molten Salt Studies J. Phillips, R. J. Gale,' R. G. Wier, and R. A. Osteryoung* Department of Chemistry, Colorado State University, Fort Collins, Colo. 80523
Electrochemical investigations on solid electrodes in molten salt media have been predominantly carried out, up t o t h e present time, using stationary electrode techniques such as chronopotentiometry, chronoamperometry, cyclic voltammetry, pulse polarography, and coulometry. As t h e rotating disc electrode finds increasing application in this field (1, Z), t h e highly sensitive rotating ring-disc electrode is notably absent. This presumably reflects t h e difficulty in fabrication of these particular electrodes, since the problem of solution leakage, often encountered at ambient temperatures in aqueous solution, is exacerbated by elevation of t h e temperature. T h i s problem precludes t h e use of Teflon in t h e design because of t h e large coefficient of expansion. We have been successful in fabricating a glassy carbon rotating ring-disc electrode, sealed i n Pyrex glass, suitable for work in a variety of molten salts. Originally intended for use in t h e sodium tetrachloroaluminate melt at 175 "C, with certain modifications, it may be satisfactorily employed in a LiCl/KCl eutectic at 450 O C .
EXPERIMENTAL Electrochemical Instrumentation. The independent control of the disc and ring potentials was achieved using a bipotentiostat based on the circuitry of Bruckenstein et al. ( 3 ) .An A.S.R. servo controlled motor-tachometer (as supplied by the Pine Instrument Co.) served to rotate the electrodes. Current-voltage traces were recorded on a Houston Model 2000 X-Y recorder. Reagents. The chemicals employed were of reagent grade; no further purification was attempted. Melt Preparation and Purification. The experimental work was carried out in a Vacuum Atmosphere Co. dry box. Constant recirculation of the argon atmosphere through a column of activated copper and molecular sieves ensured minimal contamination of the melts from both water and oxygen. As an additional precaution, PzO5 was exposed t o the dry box atmosphere. The criteria used to assess the purity of the atmosphere in the dry box was the duration of lifetime of a 25-W light bulb filament run from the line voltage. A seven-day period was associated with an atmosphere of acceptable purity ( 4 ) . The basic sodium tetrachloroaluminate melt was prepared by the addition of aluminum chloride (Fluka A.G.) to a molar excess of sodium chloride (Fisher certified A.C.S. grade) and their subsequent fusion at 175 "C. Purification of the melt was accomplished by a pre-electrolysis using an aluminum rod anode (Johnson-Matthey Specpure grade) and a coiled aluminum wire cathode (Alfa Inorganicsm5N) (5).After ten days pre-electrolysis at a current density of 1mA cmd2,the resultant melt was clear and colorless. The melt was then filtered through a course glass sinter to remove suspended aluminum particles prior to introduction into the electrolysis cell. Background voltammograms on a rotating glassy carbon disc electrode revealed an almost featureless current-voltage trace (Figure 1). The preparation of the LiCl/KCl eutectic was conducted outside of the dry box. Lithium chloride and potassium chloride were fused in the proportions 58.8:41.2 mol oh. The purification procedure adopted was to treat the melt with chlorine ( 6 )and to subsequently degas with nitrogen. Reference Electrodes. In all of the work, the reference electrodes were isolated from the bulk of the melt by a fine glass frit. The reference electrode employed in the sodium chloride saturated tetrachloroaluminate melt was merely an aluminum wire. The electrode system using an LiCliKCl melt was the Ag I Ag+ electrode, an arbitrary amount of AgCl being added t o the reference electrode compartment, containing the melt and a silver wire. Fabrication of t h e RRDE. The electrodes were constructed of glassy carbon G.C. 30 pipe (approximately 6-mm internal diameter) Present address, Department of Chemistry, University of Southampton, Southampton, England. 1266
ANALYTICAL CHEMISTRY, VOL. 48, NO. 8, JULY 1976
and glassy carbon G.C. 30 rod (diameter approximately 3 mm) as supplied by the Tokai Electrode Mfg. Co. Ltd. This grade has been found superior to G.C. 10 and G.C. 20 in that it is subjected to higher temperatures during manufacture, and this results in less degassing during electrode fabrication. The process involved the encapsulation of a 2-cm length of 3-mm diameter rod in a Pyrex glass sheath (7). A Pyrex glass tube of slightly larger diameter than the G.C. rod was sealed at one end and the rod inserted. The rod was suspended vertically in a nichrome wire wound furnace and a vacuum was drawn on the tube as the temperature was raised to about 900 "C. This facilitated the collapse of the Pyrex onto the G.C. rod. The coated rod was then rotated at 500 rpm for several hours in -8 M HF with additional surfactant (Alconox).The exact time varied in accordance with the thickness of the glass coating and the desired final thickness. Difficulties associated with differences in the expansion of Pyrex at 300 "C) and glassy glass (coefficient of expansion = 3.25 X at 100 "C) renders it imcarbon (coefficient of expansion 2.2 X possible to form a good seal on the inner surface of the ring by heating the encapsulated rod within a short length of glassy carbon pipe. It was necessary t o coat the inside of the pipe with Pyrex glass by drawing the glass into the pipe under vacuum. The operations involved are shown schematicallyin Figure 2. The initial stage involves the vertical suspension of a sealed pyrex tube, containing a 1-cm length of the G.C. cylinder, in the furnace. Upon application of heat and vacuum, the glass is drawn up within the G.C. pipe until stage B is achieved. Further heating until the inner glass bursts is not detrimental to the process. Rotation of this coated cylinder in dilute hydrogen fluoride results in the removal of the high spots of glass and
-0.050h
a 0.05-
5
f
O"-
' 0.15-
-
0.2
I
I
I
20
15
-_L-.---L--+ 10
05
E vs AI (Volt)
Figure 1. Background current-voltage curve in a sodium chloride saturated sodium tetrachloroaluminate melt; sweep rate 100 mV s-'; rotation speed 105 rad s-'
cut cut
Figure 2. Stages in electrode fabrication
c
1.0
il 3-0.05
s Y
-0.1
j-0.1
I 10
15
L’
U
-
E
0
E vs. Agl Ag’
(Volt)
I
I
- 0.5
I
- 1.0
Figure 5. Background current-voltage curves for LiCI/CI obtained independently on the disc (curve a) and ring (curve b) electrodes; sweep rate 200 mV s-‘; rotation rate 105 rad s-‘
1
0.2
I 0.5
0 -0.2 -0.4 -0.6 vs. S.C.E. (Volt)
Figure 3. Current-voltage curve obtained on the ring-disc electrode in aqueous solution of CuC12; rotation speed 160 rpm; disc current (curve a);ring current (curve b); ring potential 0.4 V
7----I .
05
--
0
05
Disc potential vs AglAg (Volt)
-101
,
,
,
15
10
05
Flgure 6. Reduction of In(l1l) in LiCVKCI at 450 OC on the ring-disc electrode: rotation speed 52 rad s-’; ring potential 0.5 V; disc current (curve a); ring current (curve b) 1-01
Disc potential vs. A I (Volt)
lo-* M FeC13 in basic sodium tetrachloroaluminate at 175 OC; ring potential 2.0 V; rotation rate 314 rad s-‘; disc current (curve a); ring current (curve 6 ) Figure 4. Reduction of
a symmetrical coating of glass on the inner wall of the carbon cylinder. The outer wall of glass is usually thin enough to be stripped quite easily or fortuitously shatters during the cutting process on the silicon carbide wheel. As a result of this sequence of operations, the inner coating of glass is usually tapered. This is advantageous in that the encapsulated rod may be “wedged” inside the cylinder and the two married under an open oxy-hydrogen flame. If the ring and disc were not perfectly concentric, minor adjustments were possible in the flame. Finally the unit was sealed into a length of medium wall Pyrex glass tubing in a manner similar to the 3-mm rod. The electrode was revealed by amputating the glass with a silicon carbide cutting wheel. A planar surface was then applied to the surface of the electrode with emery paper. A mirror finish was obtained by lapping with 0.3 and 0.05 micron alumina. Internal electric connections to the ring disc were made by pressure contacts, the insulation within the electrode being provided by glass impregnated Teflon. The external contacts were via carbon brushes spring-loaded onto brass rings, concentric with the electrode shaft.
RESULTS The electrodes were tested in aqueous solution. Figure 3 illustrates a typical current-voltage curve obtained in a M solution of CuClz ( 3 ) .The collection efficiency (8) of the ring electrode was 35%.This was in good agreement with the
theoretical value (8) of 36% (r1 = 5.3 X in., r2 = 1.14 X 10-1 in., r3 1.59 X 10-1in.). The behavior of the electrode was examined in a basic sodium tetrachloroaluminate melt at 175 O C . A collection efficiency of 35% was observed. The depoM FeC13 (5).Figure 4 illustrates the relariser used was sult of scanning the disc potential range while maintaining the ring potential at 2.0 V, where reoxidation of iron(I1) to iron(111)occurs. In the LiCl/KCl eutectic, the electrode was immersed in the melt immediately upon melting and the temperature was raised to 450 “C. A background sweep was run on both the ring and the disc (Figure 5). Indium trichloride was then added to the melt and the collection efficiency measured for this particular electrode. A value of 38%was obtained in good agreement with the theoretical value of 37% ( r l = 5.4 X in., r2 = 1.14 X 10-1in., r3 = 1.59 X 10-1in.) (Figure 6). The limiting current on the disc was found to be diffusion controlled and a Tafel plot, assuming the reaction to be first order, yielded an an value of 1.85 indicating the reversible nature of the two-electron transfer. Although the electrode was shown to be serviceable at 450 “C,removal of the electrode from the melt revealed that the Teflon insulation within the electrode was damaged. T o operate successfully for longer durations a t this temperature, other insulation materials must be utilized. These electrodes were found to behave ideally between the rotation range 0-6000 rpm. Above this range of rotation speeds, the collection efficiency fell, probably as a result of ANALYTICAL CHEMISTRY, VOL. 48, NO. 8, JULY 1976
0
1267
some eccentricity as well as a lack of accurate machining of the electrode surface to yield a perfect plane perpendicular to the axis of rotation. Above 6000 rpm, it was possible, however, to construct a working curve. DISCUSSION
The inert annulus so formed is significantly wider (approximately 1mm) than that normally encountered in commercial ring-disc electrodes and, hence, calculations based upon thin ring, thin gap electrodes were invalid. The wider annulus will, in addition, increase the average transit time (9) for the transport of a species from the disc to the ring. The transit time is further increased, relative to aqueous solution, as a result of working in molten salt media where greater viscosities and smaller diffusion coefficients usually prevail. In conclusion, despite the large annulus, this rotating ring-disc electrode is capable of definitive investigations of
electrochemical phenomena. It has proved particularly useful in this laboratory in the interpretation of molten salt transition metal chemistry, where complex adsorption phenomena are prevalent (IO). LITERATURE CITED (1) D. E. Bartak and R . A. Osteryoung, J. Nectroanal. Chem., in press. (2) G. L. Holleck. J. Electrochem. Soc., 119, 1159 (1972). (3) D. T. Napp, D. C. Johnson, and S. Bruckenstein, Anal. Chem., 39, 481 (1967). (4) I. D. Eubanks and F. J. Abbott, Anal. Chem., 41, 18 (1969). (5) L. G. Boxall, H. L. Jones, and R. A. Osteryoung,J. Electrochem. Soc., 121, 212 (19). (6) D. L. Maricle and D. N. Hume, J. Electrochem. Soc., 107, 354 (1960). (7) N. K. Gupta, Rev. Sci. Instrum., 42, 1368 (1967). (8) W. J. Albery and S. Bruckenstein, Trans. Faraday SOC.,62, 1920 (1966). (9) S.Bruckenstein and G. A. Feldman, J. Electroanai. Chem., 9, 395 (1967). (IO) J. Phillips and R. A. Osteryoung, forthcoming publication.
RECEIVEDfor review January 5 , 1976. Accepted March 15, 1976. This work was supported by the Air Force Office of Scientific Research.
Apparatus for in Situ Solvent Extraction of Nonpolar Organic Compounds in Sea and River Water Martin Ahnoff * and Bjorn Josefsson Department of Analytical Chemistry, University of Gothenburg, Fack, 5-402 20 Goteborg 5, Sweden
Sampling and enriching techniques are of primary importance in the analysis of trace organics in water. Often large sample volumes are required to obtain the desired sensitivity. This makes it inconvenient to transport the original sample to the laboratory. Also, because of the risk of sample loss and sample contamination, it is desirable to carry out the enrichment as close as possible to the sampling point. In on site techniques, the enrichment process is performed in close vicinity to the sampling point, e.g., on a ship or in a land-based station. The water is then transported from the sampling point, typically by means of a tubing and a pump. However, deposition of particles may occur in the tubing. The tubing walls and the inner surfaces of the pump may also contaminate the sample. The influence of the sampling process on the constituents of water may be further decreased by utilizing an in situ enrichment technique. Few examples of such procedures have been reported in the literature. At Kiel, the “Institut fur Meereskunde” has used a permanent marine station, “Perkeo buoy”, situated in the Kiel Bay, to enrich organics from sea water on Amberlite XAD-2 macroreticular resin filters ( I , 2). The same adsorbent was used by Roger Dawson ( 3 ) , who studied polychlorinated biphenyls and pesticides in sea water around the British Isles. A column with the resin was placed in a towed “fish” so that an integrated sample was taken while the ship was under way. The water, up to 30 l., was pumped by a peristaltic pump placed on board. For sampling in lakes, a diver-operated equipment was used. A 50-1. drum was placed a t 10-m depth and filled with air. When the air escaped it was continuously replaced by the same volume of water, which first had to pass through an Amberlite XAD-2 filter. Gether and Lunde ( 4 ) used an in situ apparatus for investigation of halogenated organic compounds in Norwegian coastal waters. Up to 200 1. of water was pumped downwards through a vertical glass cylinder, where a nonpolar solvent, n-hexane, was trapped by gravity. The cylinder was packed with small glass beads to increase the contact area between the solvent and the water passing through. A pump was connected to the outlet and was served by a car battery. The apparatus was used down to 10-m depth. 1268
ANALYTICAL CHEMISTRY, VOL. 48, NO. 8, JULY 1976
The in situ apparatus described here is designed to perform solvent extraction of large amounts of water. This is done continuously while the apparatus is situated a t the sampling point a t a desired depth. EXPERIMENTAL The in situ apparatus consists of a water-tight, nonmagnetic, stainless steel container equipped with one pump and two magnetic stirring motors. On the top of and outside the container, in direct contact with the surrounding water, are attached two extractors which are connected with the pump inlet as can be seen in Figure 1. The extractors are mixer-settlers, previously described in detail ( 5 )and tested in field studies ( 6 ) .The extractor is a separate plug-in unit, constructed with glass and PTFE as the only contacting materials. The water is drawn a t constant rate, typically 3 l h , directly into the first extractor. There, a rotating magnetic stirring bar produces a vortex mixing of the water with a stationary, nonpolar solvent. Phase separation occurs in two consecutive circular chambers located outside and below the mixing chamber. Droplets of the solvent, which is lighter than water, rise back into the mixing chamber. The water leaves the first extractor and passes through the second extractor, where the same extraction process is repeated. Finally, the water enters the pump and is expelled back into the surrounding water. Before an extraction is started, the pump and stirring motor speeds are set, as well as the timer, when used. The container is closed by means of band clamps (Figure 1).The extractors are filled with solvent, cyclohexane or cyclohexane-hexane, and distilled water in the laboratory, and are coupled to the container before it is lowered into the sea. The apparatus weighs about 50 kg and is held at the desired depth (0-50 m) by a 60-1. buoy. The depth is restricted by the type of pump arrangement. If ca. 150 1. of water is to be extracted, the apparatus will operate for 48 h. When the extraction is completed, the extraction units are brought to the laboratory, where the extracts are taken out and analyzed. In a prototype version, schematically shown in Figure 2, the in situ apparatus is served by an external 220-V ac power supply. The stirring motors are standard laboratory magnetic stirrers. The pump is a membrane pump which gives a constant flow within wide pressure limits. Another version of the in situ apparatus, also shown in Figure 2, has an internal 12-V dc storage battery source. The stirrers are driven by miniature direct current motors (Philips). The peristaltic pump (Struers, Denmark) consists of a pump head coupled to a standard windscreen wiper motor. The flexible tubing is of silicone rubber. The motors are fed with a pulsed current (200 Hz, variable
