Small Volume, High Performance Cell for Nonaqueous Spectroelectrochemistry Fred M. Hawkridge" Department of Chemistry, Virginia Commonwealth University, Richmond, Virginia 23284
Jeanne E. Pemberton and Henry N. Blount Brown Chemlcal Laboratory, The University of Delaware, Newark, Delaware 1971 1
The growing need to reduce cell volumes while maintaining favorable cell transfer functions for aqueous spectroelectrochemical studies is being met by novel cell designs (1)and modifications thereon (2, 3). The utility of such cells in nonaqueous work is most often limited by solvent attack of cell body material (e.g., Lucite). While cell bodies for nonaqueous work may be fabricated from Kel-F, Teflon, or high density polyethylene, newly available machinable glass-ceramic (Macor, Corning Glass Works, Corning, N.Y.) affords cell bodies for nonaqueous spectroelectrochemistry which have superior thermal and mechanical properties. This report describes the fabrication of such a cell and the comparison of its performance with that of a conventional glass spectroelectrochemical cell.
EXPERIMENTAL Macor machinable glass-ceramic was obtained from Corning Glass Works. Conventional metal machining equipment and techniques (4) were employed in fabrication of a Macor cell similar in design to a previously described small volume Lucite spectroelectrochemical cell ( I ) . The cell described here differed significantly from the previous one in that a circular platinum auxiliary electrode was located adjacent to the quartz rear window and encircled the optical beam. This auxiliary electrode placement afforded a much improved transfer function relative to the previously described cell. The volume of the Macor cell was 0.80 mL and had a working electrode area of 0.38 cmz. RESULTS The electrochemical response of the Macor cell as evidenced by the oxidative cyclic voltammetry of 9,lO-diphenylanthracene (DPA) was found to be identical to that of a conventional spectroelectrochemical cell (5) over a broad range of sweep rates. Identical single potential step spectroelectrochemical (chronoabsorptometry) experiments were conducted using each cell wherein DPA was oxidized to the cation radical at a platinum OTE. The absorbance-time response predicted for such potential step perturbations is that given by Equation 1 (6)
where all terms have their usual meanings (6). Representative absorbance-time responses for the Macor and the conventional cells are shown in Figure 1, A and B, respectively, and the corresponding absorbance-time1/2 behavior is shown in Figure 1,C and D. For eight independent trials using each cell, values of 27.38(*0.14) M-l s-lj2 and 27.09(f0.20) M-' s-1/2 were obtained for cDpA+D1lzDpA in the Macor and conventional cells, respectively. T h e removal of oxygen and its long-term exclusion from the Macor cell which is necessary for reductive work was demonstrated. Oxygen was vacuum outgassed from a pH 7.0 buffer (0.10 M phosphate, 0.10 M NaC1) and its subsequent presence in solution as a function of time was monitored voltammetrically at a tin oxide electrode. No oxygen was in evidence for a period of 3 h. The behavior of the Macor cell 6646
ANALYTICAL CHEMISTRY, VOL. 49,NO. 1 1 , SEPTEMBER 1977
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Figure 1. Spectroelectrochemlcal responses for potentlal step oxldatlon of DPA at R-OTE. Experimental techniques and instrumentatlon (7), electrode preparation (a),and materials (9,10) have been described. (A) absorbance-time response for potential step of 4-0.70to 4-1.35 V vs. SCE in Macor cell. (8)Absorbance-time response for potential step of 4-0.70 to +1.35 V vs. SCE in conventional cell. (C) Fit of absorbance-time data from curve A (Macor cell) to Equation 1. Slope = 1.624(f0.008) X lo-*; Intercept = 6.l(f0.9)X coefficient of correlation = 0.9979. (D) Flt of absorbance-time data from curve B (conventional cell) to Equatlon 1. Slope = 1.601(~0.006) X lo-'; intercept = 6.3(*0.7) X coefficient of correlation = 0.9981
in this regard is superior to that of Lucite (11).
DISCUSSION The use of Macor as a material for the fabrication of small-volume cells for nonaqueous spectroelectrochemical work affords many advantages over other popular materials. The modest cost of this glass-ceramic (ca. $6 for the Macor used in the cell described here), the ease of machining (conventional metal machining techniques, water cooling), and the tolerances routinely obtained (f0.0005in.) (4)permit the inexpensive fabrication of a wide variety of spectroelectrochemical cells. Unlike many ceramic materials, Macor does not require firing to insure nonporous surfaces. The dimensional stability of this material is superior to Kel-F, Teflon, and high density polyethylene such that high-torque screwing into tapped holes and high-stress clamping and similar manipulative operations can be easily executed. Although cell volume minimization was not the thrust of the work reported here, smaller volume cells can be readily constructed which permit the electrochemical and spectroscopic characterization of minute amounts of material. This feature coupled with the extremely low gas permeability of this material gives rise to cells which are ideally suited for the vacuum outgassing necessary in bioelectrochemical work where anaerobic, hydrophobic conditions are required. The temperature stability of Macor and ita small coefficient of thermal expansion ( 4 ) make it an ideal cell body material both for the spectroelectrochemical examination of the temperature dependences of various electrochemically induced kinetic processes and for spectroelectrochemistry in fused salta, especially those of higher fusion temperatures. The inertness
of this glass-ceramic toward acidic and basic materials ( 4 ) permits the use of less common electrochemical solvents such as trifluoroacetic acid or fluorosulfonic acid (12). No cell memory effects (e.g., adsorption, absorption) have been observed with the material. Platinum is reported to adhere to Macor with excellent mechanical stability ( 4 ) . This suggests the vapor deposition of auxiliary electrodes within the cell body in order to obtain electrode configurations necessary for optimal cell transfer functions. Moreover, in the course of this work it has been found that other materials can be readily attached to this glass-ceramic by conventional epoxy cement as well as high-vacuum epoxy-ceramic materials. Like all small-volume cells used in spectroelectrochemical work, the contents of the cell described here are subject to infrared heating if allowed to remain in the spectral beam for extended periods. Use of an infrared filter in the optical path prior to the cell alleviates this problem, as does interruption
of the analyzing beam by means of a shutter when no experiment is in progress.
LITERATURE CITED (1) F. M. Hawkridge and T. Kuwana, Anal. Chem., 45, 1021 (1973). (2) M. Fujlhira and T. Kuwana, private communication, 1975. (3) F. R. Shu and G. S. Wilson, Anal. Chem., 48, 1676 (1976). (4) Corning Glass Works Technical Bulletin 9658, “Machlning Instructions, MACOR Machinable Glass-Ceramic”, Corning, N.Y. 14830. (5) See, for example, T. Kuwana and N. Wlnograd in “Electroanalytical Chemistry”, Vol. 7, A. J. Bard, Ed., Marcel Dekker, New York, N.Y., 1974, Figure 7, p 25. (6) N. Winograd, H. N. Blount, and T. Kuwana, J . Phys. Chern., 73, 3456 (1969). (7) J. F. Evans and H. N. Blount, J . Phys. Chem., 8 0 , 1011 (1976). (8) W. von Benken and T. Kuwana, Anal. Chem., 42, 1114 (1970). (9) J. F. Evans, H. N. Blount, and C. R. Glnnard, J. Necfroanal. Chem., 59, 169 (1975). ( I O ) T. Osa and T. Kuwana, J . Electroanel. Chem., 22, 389 (1969). (1 1) F. M. Hawkridge, unpublished results. (12) H. N. Bbunt et ai., to be published.
RECEIVED for review March 14,1977. Accepted June 20,1977.
CORRECTION Studies on the Mechanism of Atom Formation in Graphite Furnace Atomic Absorption Spectrometry In this article by R. E. Sturgeon, C. L. Chakrabarti, and C. H. Langford, Anal. Chem., 48,1792 (1976), the proposed mechanism of formation of aluminum atoms given on p 1804 is wrong because of a miscalculation of thermodynamic data. However, this does not change the measured E, values. Although no self-consishnt mechanism for Al,, formation that accounts for the observed E, values can be advanced, it seems reasonable to postulate that one or more oxide species (A10, A120, A1202) may be involved, and also that Al(g)may be formed by direct thermal decomposition of A1203(,,.
CORRECTION Gel Permeation Chromatography of Low Molecular Weight Materials with High Efficiency Columns In this article by Anoop Krishen and Ralph G. Tucker (Anal. Chem., 49, 898 (1977)) the equation on page 899, column 2, should read:
Molar Volume (mL/mol at 20 C) = 33.02 + 16.18 (CAU) + 0.0041 (CAU)*
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