Table I. Aqueous Solution Detection Limits (2 g ) Element
I
Ag As Cd co Cr cu Fe Hg Mg Mn
I
I
Pb Se Zn
/I
Figure 3. Spectrum of aqueous solution containing 100 k g ml''
Detection limit,
urj
ml-1
0.0001
0.0005 0.001 0.00004 0.00008 0.0008 0.00006 0.14 0.001 0.0009 0.00005 0.02 0.008
suggest that this technique of Plasma Sampling Mass Analysis (3)has a useful place in multielement analysis.
of
mercury
LITERATURE CITED By setting the mass analyzer to the peak of the element of interest, counts may be integrated for the sample and for background on a blank. In this way, a series of values may be obtained for detection limits. Limits obtained for 30second integrations for a number of elements, expressed as the level equivalent to 2 cr background, are shown in Table
I. Although much remains to be done in optimizing the system, these detection limits, the ease with which the sample may be introduced with little preparation, and the simplicity of programming element selection with a quadrupole
Knewstubb, "Mass Spectrometry of Organic tons," Academic Press, New York, N. Y., 1963, Chapter 6, p 255. (2) J. L. Jones, R. L. Dahlquist, and R. E. Hoyt, Appl. Spectrosc. 25, 628 (1) P. F.
(1971). (3)
Patents Applied for.
Alan L. G r a y Applied Research Laboratories Ltd. Wingate Road Luton, England
RECEIVEDfor review July 29, 1974. Accepted October 21, 1974.
I AIDS FOR ANALYTICAL CHEMISTS Separator Tubes for Electrolysis Cells Made from Fluoropolymer Ion-Exchange Membranes J. E. Harrar' and R. J. Sherry2 Lawrence Livermore Laboratory, University of California, Livermore, Calif. 94550
One of the critical components in most cells for controlled-potential electrolysis and coulometry is a semipermeable separator or diaphragm for isolating the counterelectrode compartment from the working-electrode compartment. Various kinds of materials and assembly designs have been used (1-3) to obtain the ideal properties of low electrical resistance, negligible flow of the solvent and the species of interest, chemical inertness, and good mechanical stability under diverse conditions. Ion-exchange membranes have performed satisfactorily in many electrolysis cells ( I , 2 ) ;however, they have not always possessed the desirable long-term chemical inertness, and it has not always been possible to fabricate these membranes in the configurations required for optimizing the cells. This Aid describes the fabrication and use of counterGeneral Chemistry Division. Research Engineering Division.
electrode separator tubes incorporating a relatively new type of ion-exchange membrane, the base polymer of which has properties similar to those of Teflon. The membrane is in the form of tubing, which is unique because it permits the fabrication of a cylindrical separator that is optimum for metal-gauze working-electrode cells. Two techniques for fabricating the counter-electrode separator tubes are described: one using a Kel-F holder for the membrane, and the other employing a new fluoropolymer tubing that is heat-shrinkable at a relatively low temperature that does not damage the membrane. EXPERIMENTAL The material tested for use as a separator is the Nafion-brand perfluorosulfonic-acid membrane manufactured by E. I. du Pont de Nemours and Company. The method of manufacture of this membrane and some of its uses are described in Reference 4; and its physical, mechanical, and general electrochemical characteris-
ANALYTICAL CHEMISTRY, VOL. 47, NO. 3, MARCH 1975
601
6.4-mm heatshrinkable Tefzel tubing
0 . 4 - 0.8 wall
6-m-o.d.
borosilicate j
4
4
5
5
-i
As required for friction fit in cell cap
rod
6-mm-0.d. 4-mm-i.d. borosilicateglass tubing
Length as required
I
o . d . o f Nafion0.12 (-7.0) 6.20
4,011 0 . 5 1 (d)
(e)
Figure 1. Fabrication of Nafion/glass/Tefzel tube
tics are described in Reference 5. The primary form used was the type XR-72, 6-mm o.d., 4.3-mm i.d. tubing with an equivalent weight of 1200; it is also available as flat sheet. It is available from du Pont only as a cation permselective membrane. RAI Research Corporation manufactures Teflon-based ion-exchange membrane sheet of both catonic and anionic forms, but does not provide tubing as a standard item. The heat-shrinkable tubing is Penntube IX 6.4-mm diam (before shrinking) Tefzel-plastic (6) tubing, available from the Penntube Plastics Division of Dixon Industries. This plastic is chemically similar to Teflon, but has a much higher,tensile strength (6). In the heat-shrinkable form, the plastic will contract to approximately half its original diameter at temperatures below 150 "C. Nafion/Glass/Tefzel Tubes. The technique for fabricating the separator tubes using the heat-shrinkable tubing is shown in Figure 1. A short length of the Tefzel tubing is preshrunk over a flame, or with a heat gun, and then cut as shown in Figure l b to form two sections that fit snugly over the glass end plug, and the glass tubing as shown in Figure IC. The desired length of Nafion tubing is then cut and inserted as shown in Figure I d . This assembly can be held vertically by inserting the end of the glass tubing into a hole in a metal block. The assembly is then placed in a drying oven a t 140 f 10 "C for 0.5 hr for the final shrinking. After shrinking, the Nafion will have darkened slightly, and there may be a slight looseness in the union between the Tefzel and the Nafion. This bond will become very tight when the tube is placed in the electrolyte and the Nafion expands. Nafion/Kel-F Tubes. The production of a satisfactory separator tube, in which the Nafion tubing is held in a machined assembly, requires that the Nafion be dimensionally improved and stabilized. This is done by boiling in water the Nafion tubing to be used, and then drying it on a Teflon mandrel. As received, the Nafion tubing is flattened in cross section, and has considerable curl. The boiling-heating pretreatment produces straight pieces with a tolerable amount of out-of-roundness. To determine the size of the mandrel, which consists of a Teflon rod to be inserted into the Nafion tubing, a sample of the Nafion is boiled in water for 30 min. The diameter of the Teflon rod is then adjusted to within 0.02 mm of the mean inside diameter of the boiled Nafion. The procedure to pretreat the Nafion is to boil the selected piece for 30 min in water, place the piece on the mandrel, heat a t 110 "C in a drying oven for 10 min, cool, remove the Nafion from the mandrel, and place it in the supporting electrolyte to be used in the electrolysis for 24 hr. The Nafion then assumes the dimensions that it will have when functioning as the separator in the electrolysis cell. The Kel-F holder and end plug are then fabricated, as shown in Figure 2, and adapted to the dimensions of the soaked Nafion tubing. The inside diameter of the lower section of the holder is machined so that a good crush fit is obtained with the soaked, wet, Nafion-tube section. The diameter of the end plug is similarly se602
ANALYTICAL CHEMISTRY, VOL. 47,
NO. 3,
MARCH 1975
refmahC-
i.d. o f Nafion + 0.12 (-5.2) 15-30' x 1.0
I -I W 6 . 3 Figure 2. Kel-F holder and plug for Nafion separator tube lected to be greater than the inside diameter of the Nafion. The upper section of the Kel-F tube can be adapted to the other parts of the electrolysis cell; however, the wall should remain thin so that the tube retains good transparency, and the inside diameter should not be so small that gas bubbles evolving from the wire electrode become trapped inside the tube. Leak-Checking and Maintaining the Tubes. The integrity of the finished tube is checked by filling the tube with solvent or supporting electrolyte, placing the membrane end in a beaker of the solvent or supporting electrolyte, soaking the membrane for several hours, and then noting whether the level of solvent in the tube decreases when left overnight. There should be no visible loss other than that caused by evaporation. If the membrane becomes dry, it will contract to its original size. Thus, to maintain a good seal between the Nafion membrane and Tefzel (or Kel-F), the separator tube, when not in use, should be kept soaking in supporting electrolyte or solvent. A single Nafion/ glass/Tefzel tube can be used with several different supporting electrolytes in the same solvent, provided that several hours are allowed to exchange the old electrolyte for the new, and the two electrolytes together do not form a precipitate. Changing from a lowionic-strength solution to one of much higher ionic strength, or changing solvents, may cause loosening of the seals of the Nafion/ glass/Tefzel tube, thus necessitating fabrication of a new tube. In the case of the Nafion/Kel-F tube, which is much less tolerant to dimensional changes in the membrane, it is recommended that a new tube always be assembled if the solvent is changed, or if the ionic strength of the electrolyte solution is increased by more than a factor of two. Apparatus and Instrumentation. Alternating-current resistances were measured by means of a Hewlett-Packard Model 4800A vector impedance-meter. The dc power supplies were Harrison Laboratories Models 855B and 6207A. The coulometric instrumentation and electrolysis cells have been described previously (7, 8). Performance and resistance tests of the tubes, apart from the electrolysis cells, were conducted with a two-electrode arrangement: one electrode was a 13-mm-long straight platinum wire placed inside the tube nearly filled with test solution; the other electrode was a platinum gauze electrode of about 28 cm2 planar area, placed coaxially outside the membrane portion of the separator tube. Both the separator tube and the gauze electrode were then placed in a beaker of the test solution.
RESULTS AND DISCUSSION General Characteristics of the Tubes. The Nafion membrane is based on a perfluorocarbon-sulfonyl-fluoride copolymer which is fabricated into the shape desired, saponified to give the sodium salt, and then converted to an acid form ( 4 ) that functions as a strong-acid cation-perme-
able material ( 5 ) . It has performed satisfactorily as the separator material in several medium- and large-scale industrial electrolysis systems such as the chlorine-caustic process ( 4 ) .Its promise of chemical resistance, especially to such solutions as strong alkali and hydrofluoric acid, which attack the conventional glass separators of laboratory electrolysis cells, makes it very attractive for use in controlledpotential electrolysis and coulometry cells. However, Nafion has the property of absorbing a significant amount of solvent, thus changing its volume and linear dimensions considerably. This solvent swelling is a function of the nature of the solvent and solute, concentration of solute, past history of the membrane, and temperature ( 5 ) .For example, in the case of the 6-mm 0.d. tubing used here, there is about a 10% increase in the diameter of the tubing on soaking in pure water or acetonitrile. Ionic species in the solvent decrease the degree of solvent absorption in direct proportion to their concentrations and the nature of the cations. The Nafion/glass/Tefzel tubes are preferred, when they can be used, because of their wider adaptability to change of supporting electrolytes. Fabrication of these tubes in the manner described is very simple, and it owes its success to the fact that the Tefzel can be shrunk a t temperatures well below the -200 "C degradation temperature of the Nafion. Until recently, the only heat-shrinkable fluorocarbon-type tubing that was available required temperatures greater than -230 "C. It is probable that the new low-temperatureheat-shrinkable, FEP-Teflon could be used in place of the Tefzel, although the latter is much stronger mechanically. At 140 "C, the temperature recommended for shrinking, there is a slight browning of the membrane, but its performance does not appear to be affected. Excessive browning indicates that the membrane has been exposed to too high a temperature or too long a heating time. The Nafion/glass/Tefzel tubes have been found satisfactory for a variety of' solvent/solute systems, with the following exceptions. I n 9M H2S04(aq),there is apparently less water absorption than in even the 60% relative-humidity laboratory air; hence the material does not expand after assembly and the tube does not seal. In N,N- dimethylformamide (DMF), in which the Nafion expands to more than 20%)of its "dry" diameter, an excessive length of Tefzel is required to hold it. Several other organic solvents also cause a pronounced swelling of the membrane ( 5 ) . In hydrofluoric acid solutions, the glass tubing and plug would corrode excessively. Thus, for the above-mentioned media, and there are surely others, the Kel-F tube arrangement is recommended instead of the Nafion/glass/Tefzel tube. The swelling of the Nafion membrane does not limit its use for the type of separator tube that is ideal for mercurypool electrolysis cells ( I ) because sheet, rather than tubing, can be used. Nafion has been used a t this laboratory, without such problems, in a configuration of the type described by Carson, Michelson, and Koyama ( I ) , Figure 3a The disadvantage of the Kel-F tube assembly is that the dimensions of the Kel-F tube and plug must be rather carefully matched to the final, wet dimensions of the Nafion tube. (Plastics other than Kel-F can be used, but they should be transparent so that the inside of the tube can be observed.) The boiling-water pretreatment for the membrane, which is recommended by the manufacturer ( 5 ) ,followed by baking on the mandrel at 110 "C, functions as a standard pretreatment procedure for all media, and facilitates fitting of the Nafion to the Kel-F for a particular electrolyte solution. Within the limitations outlined above, the same assembled tube can be used in different supporting electrolytes,
and this is of importance in routine analytical work. However, the low cost of the Nafion membrane tubing does make it economically disposable, or the sections interchangeable, for different types of experiments. Electrochemical Characteristics of the Tubes. The total resistance of the tube assembly (from the connection a t the inner wire electrode, through the membrane, to the outer solution) was strongly dependent upon the position of the inner wire electrode inside the tube. That is, when the wire tip rested at the bottom of the tube so that a portion of the wire was directly inside the membrane, the total resistance of the assembly was 5 to 10 times lower than when the tip of the wire was above the membrane. This fact, arising from the difference in current distribution inside the tube, is important because this resistance is the principal component of the total cell resistance in a controlled-potential electrolysis cell, which, in general, should be minimized. The minimum-resistance configuration was used to test the operating characteristics of the separator tubes. The ac resistance, at 10 kHz, of separator tubes having a 10-mm length of exposed membrane is 3 Q for 1M HzS04, 13 Q for 0.5M NaOH, 20 Q for 1M HF, 100 Q for a 0.1M solution of tetrabutylammonium perchlorate (TBAP) in DMF, and 140 Q for 0.1M TBAP in acetonitrile. Typical total cell voltages during direct-current flow are 2.5 and 8 V, respectively, for the HzS04 and H F solutions at 200-mA current levels, and 15 V a t 100 mA for the acetonitrile electrolyte. No degradation of the membranes has been observed at current densities below 200 mA/cm2 of exposed membrane. Passage of these high direct currents for several hours results in electroosmotic transport (9) of a detectable quantity of solvent toward the cathode, a phenomenon also exhibited by porous glass separators. Assuming that the separator material exhibits negligible solvent flow, and is of low resistance, the possible loss of electroactive species by diffusion into and through the separator is then the most critical factor to be considered in high-accuracy coulometry. Sample cross-contamination errors also may arise if the diffusion effect is significant. Errors resulting from loss of analyte by diffusion into porous glass separators have been reported by a number of investigators ( 3 ) .In the case of the Nafion membrane, of principal concern is the loss of positively charged species, since the membrane is a cation exchanger. No direct studies of this factor were done, but the Nafion separators can be visually observed to absorb highly colored cationic species, and there is evidence that Nafion excludes anionic species better than does the porous glass. The Nafion separator tubes have been used successfully for the accurate, coulometric determination of several substances in cells where porous glass was previously used (7, 8). The fluorocarbon membrane in its various forms should also be of value in voltammetric work, where the requirements for low solvent- and solute-flow are usually less severe.
LITERATURE CITED (1) W. N. Carson, Jr., C. E. Michelson, and K. Koyama, Anal. Cbem., 27, 472 (1955). (2) H. Lund and P. E. Iversen, in "Organic Electrochemistry," M. M. Baizer, Ed., Marcel Dekker. New York, N.Y., 1973, Chap. IV. (3) J. E. Harrar, in "Electroanalytical Chemistry," Vol. 8. A. J. Bard. Ed., Marcel Dekker, New York, N.Y., 1974, pp 75-77. (4) D. J. Vaughan, Du font Innovation, 4 (3). 10 (1973). (5) W. F. Grot, G. E. Munn. and P. N. Walmsley. paper presented at The Electrochemical Society Meeting, Houston, Texas, May 1972. (6) flast. Techno/., 16 (6), 19 (1970). (7) J. E. Harrar and C. L. Pomernacki, Anal. Chem., 45, 57 (1973). (8)L. P. Rigdon and J. E. Harrar, Anal. Cbem., 46, 696 (1974). (9) P. E . Lorenz, in "Encyclopedia of Electrochemistry," C. A. Hampei. Ed., Reinhold, New York, N.Y., 1964, pp 536-540.
A N A L Y T I C A L C H E M I S T R Y , VOL. 47, NO. 3 , M A R C H 1975
603
RECEIVEDfor review September 16, 1974. Accepted November 4, 1974. This work was performed under the auspices of the U S . Atomic Energy Commission. Reference to a company or product name does not imply approval or
recommendation of the product by the University of California or the U S . Atomic Energy Commission to the exclusion of others that may be suitable.
An Improved Glassy Carbon Electrode Samuel C. Levy Exploratory Batteries Division, Sandia Laboratories, Albuquerque, N.M. 87 1 15
Patrick R. Farina' Scientific Glass Laboratory, Sandia Laboratories, Albuquerque, N.M. 87 1 15
A standard method of preparing working electrodes for voltammetric studies is to seal a rod of the desired electrode material in an inert sheath, cut the end to expose a flat circular cross-section of the electrode material, and then polish to reduce the surface roughness. This technique has been reported for the fabrication of glassy carbon electrodes in which the glassy carbon rod is sealed in Pyrex glass (1) and boron nitride ( 2 ) .When preparing electrodes in this manner, using Pyrex, we found that a good seal could not be obtained between the glass and the glassy carbon since the glass will not wet carbon. This resulted in a thin void between the glass and carbon into which glassy carbon dust, formed during the final polishing step, may accumulate and which fills with electrolyte during use, giving rise to anomalously high residual currents and making analysis of the data extremely difficult. To solve this problem we developed a technique for fabricating glassy carbon electrodes having a leak-tight seal. These electrodes are ideally suited for voltammetric studies in fused salts, having a small residual current and the capability to operate over a wide voltage range. Carbon-to-Glass Seal. To obtain a good seal between glassy carbon'and glass, it is necessary first to coat the carbon with a thin layer of silicon (0.0005 to 0.005 in.) ( 3 ) .Before coating, the glassy carbon rod is centerless ground to obtain the desired diameter and to remove ridges, formed during the molding of the rod, which run the length of the rod. The glassy carbon is next placed inside a Pyrex vessel and connected across the output of an auto transformer (Figure 1).A stream of silane (SiHJ, diluted with hydrogen and argon, is passed through the vessel. The variable auto transformer is turned on for approximately 2-3 seconds (42 volts, 60-cycle ac), heating the rod. Silane, upon contacting the hot carbon, decomposes and deposits a thin film of silicon on the rod. Initially we attempted to seal a silicon-coated glassy carbon rod in Pyrex. A good electrode could not be prepared because of the mismatch in expansion between the glassy carbon and Pyrex below the set point of the glass. This resulted in spontaneous cracking of the Pyrex upon cooling. A survey of the literature indicated that GSC-4, a borosilicate glass manufactured by General Electric Co., matches the glassy carbon in expansion more closely in the critical temperature range than does Pyrex (Table I). The GSC-4 glass we obtained had many bubbles in it. The presence of bubbles resulted in a poor electrode, since some bubbles invariably were located a t the glass-carbon interface. To eliminate this problem, we remelted the glass Present address, Chemistry Department, Syracuse University, Syracuse, N.Y. 604
ANALYTICAL CHEMISTRY, VOL. 47, NO. 3, MARCH 1975
-Kulgrid Leads UJ.@Oin.)
Kovar Foil Cap over C a m n Ro
4
,Flexible lead coiled to form spring for eiectrital contact pressure. Argon -100 cciminute from flowmeter 3% SiH4 in hydrogen u p t o
0.25 in. Diam. Carhon Rod
100 cciminute from flowmeter
'15-mm
OD Pyrex, 15-in. overall length
-Indent at 120' to center carbon rod (2 places on tube) into hole in urbon rod
- Nickel or Kovar 0.25 in.
diam. dish spotwelded to lead wire for electrical contact.
light burner. Join top and bottom leads to 30-amp auto transformer: 42 volts required.
Figure 1. Apparatus for deposition of silicon on glassy carbon
at 1700 "C in a platinum crucible for 40-60 hours. This produced a glass free of crystalline inclusion and bubbles. The molten glass was cast into a rectangular block and allowed to cool. The block was core-drilled to form a rod of GSC-4 glass which was then heated and pulled down to a diameter of 4 mm. This was then made into tubing by coiling on the end of a 0.5-inch diameter Pyrex tube, remelting, and smoothing. Another Pyrex tube was attached to the end of the coiled GSC-4 glass, so it could be drawn down to a 1-mm wall thickness having an inner diameter very slightly larger than the silicon coated glassy carbon rod (Figure 2). This tubing was then slipped over the glassy carbon and sealed to it by evenly heating to the softening point in a glass lathe (Figure 3). To avoid the arduous task of remelting and forming tubes with the GSC-4 glass, one should examine the glass under a 30 power microscope and select only those portions ~~
~
Table I. Coefficients of Thermal Expansion i v e r a q * r a l u e of a ,
Matenal
renip. ranqr, 'C
Glassy carbon
0-1 00
Pyrex GSC -4
100-1000 Ck300 0-300
i mf
Crn,
O C
2.2 x 10-6 3.2 x IOm6 3.2 x 2.4 x
