Anal. Chem. 1981, 5 3 , 1952-1954
1952
a sample holder does have an effect on the reflectance determination. If the sample is not near the top of the holder, the sample is shaded by the edge of the holder and a diminished reflectance reading results. The edges of the holders have been beveled around the sample so that the shadowing effect is minimal.
LITERATURE CITED “ReflectanceSpectroscopy”;Springer-Verlag,New York, 1969;p 219. (2) Judd, D. B.; Wyszecki, G. ”Color In Business, Science and Industry”,
(1) Kortum, G.
3rd ed.; Wiley: New York, 1975;p 139.
RECEIVED for review March 16,1981. Accepted June 29,1.981.
Electrochemical Cell for Voltammetry, Coulometry, and Synthesis In Superdry Media Herbert Kiesele Fakunat fur Chemle der Universitat Konstanz, 0-7750 Konstanz, Postfach 5560, West Germany
Elimination of water is of general importance in nonaqueous electrochemistry, humidity giving rise to adsorption, to additional heterogeneous electron transfer, and to homogeneous side reactions. Therefore a lot of work was done to develop cell systems for the rigorous exclusion of water ( I ) . Some years ago it was shown by Hammerich and Parker (2) that the commonly used vacuum technique (3-6) is not capable of getting rid of adsorbed water. They proposed the addition of activated aluminum oxide as “superdrying” agent to the electrolytic solution. However, this procedure is only practicable with stable solvents because in most instances substantial decomposition is taking place as will be shown for acetonitrile (MeCN). Furthermore only qualitative measurements can be done because of uncontrolled adsorption of electroactive species. Coulometry and preparative work are impossible. Later on the direct addition of aluminum oxide was replaced by an external filtration method (7). By means of excess nitrogen pressure and vacuum, the electrolytic solution is repeatedly percolated through an aluminum oxide column, externally attached to the electrochemical cell. We found this method time-consuming, solvent consuming, sensitive to leaks, and, most of all, uncontrollable with respect to solvent decomposition. The essential feature of the electrochemical cell presented in this paper is an internal drying column, which allows for repetitive filtrations at low temperature in a tightly closed and grease-free system. The cell is easy to handle and may be used for voltammetric and coulometric measurements a t any desired temperature. It is also suitable far preparative work on the millimole scale and photochemical investigations. With an overall price of $1750, this rather versatile and highly functional device is moderately inexpensive.
EXPERIMENTAL SECTION Instrumentation. Electrochemical experiments were performed with a PAR Electrochemistry System Model 170, equipped with a 912 Tektronix storage oscilloscope and an additional interface to a Digital PDP 11/40 computer. UV spectra were run on a Cary 118 spectrometer. Scattering of light was measured at 514.5 nm with a Cary 83 Raman spectrometer and a Spectra Physics 164-08 argon ion laser. Sodium was analyzed with a Perkin-Elmer 300 S atom absorption spectrophotometer. The trace water content has been determined with a Mitsubishi Moisture Meter Model CA-02 as described (8). Cell Description. The design of the electrochemical cell is to be seen from detailed drawings in Figure 1. The cell body, built from quartz to prevent thermal strain and to allow for UV irradiation, consists of a single piece, thus avoiding leaking junctions. The Schlenk-type cohstruction offers additional advantage for maintaining anhydrous and anaerobic conditions. Figure 2A shows the combined voltammetric auxiliary and working electrode, held in place by an adjustable electrode adapter (Figure 1,a). The coaxial arrangement of working electrode, auxiliary electrode, and Luggin tip provides uniform current and potential distribution across the surface of the working electrode 0003-2700/81/0353-1952$01.25/0
while the short interelectrode distances result in uncompensated resistances as small as geometrically possible (9). During drying operation (next section) a platinum wire quasi-reference electrode is used to prevent contamination (Figure 2B). Later on it may be replaced by the reference electrode shown in Figure 2C if accurate potentials are needed. The large scale electrodes, consisting of platinum wire gauze, are sealed with the cell to reduce the number of openings to a minimum. Their parallel arrangement allows for uniform potential distribution. (In superdry media the use of mercury offers no advantages.) The cell is attached to the high vacuum line by a stainless steel adapter (Figure 3A); connection to the cryostat and thermostat is established by Teflon slip joints (Figure 3B). Cell Operation. The drying operation is schematically r e p resented in Figure 4, followed by a detailed description. The thoroughly cleaned and predried cell is equipped with the combined auxiliary and working electrode, the quasi-reference electrode, and a quartz-coated stirring bar. Drying tube o is half filled with aluminum oxide (10 g Woelm neutral, Super I) through joint s1 with the aid of a long small funnel. To remove oxygen and most of the adsorbed water we attach the cell to a highvacuum line by the stainless steel adapter, hot air (200 “C) from a heat gun being blown through the coolant jackets for 1 h. Thereafter 60 mL of the electrolytic solution is transferred from a storage vessel through the rubber septum (Figure 3A) and tube p (Figure 1A) into the three electrode compartments under an atmosphere of dry argon by means of a stainless steel needle (Figure 4A). To prevent solvent decomposition and to improve the exothermic water adsorption on the aluminum oxide, supporting electrolyte and aluminum oxide are cooled down. (For routine work cold tap water is sufficient.) The cold solution is then transferred into the right half of the cell top by a simple 90’ rotation of the electrochemical cell, the solution passing through tube d (Figure 4B). (The content of the auxiliary compartment and the content of the reference electrode compartment slowly drop into the main compartment.) Back-rotation of the cell spreads the electrolytic solution over the bottom of the cell top. From the slightly concave bottom of the cell top it flows back into the electrode compartments, passing the drying tube (Figure 4C). The filtration is repeated until small background currents and a sharp rise at the cathodic end of the potential window are obtained. Normally two cycles are sufficient,taking 15 min each. Then the substance under investigation is introduced into the cell top through the stainless steel adapter and finally it, is wmhed into the electrode compartments by rotating the cell twice. (If the electroactive substance should be destroyed or markedly retained on the aluminum oxide the use of break seal ampules is recommended.) For quantitative measirements a small percentage of the solution is removed through tube p until the liquid level just reaches the upper wall of the main compartment. The measured absorbance of the separated solution and the known volume of the solution remaining in the main compartment give mass and concentration of the electroactive substance. Volume calibration of the working electrode compartment is readily obtained by dissolving a known amount of some dyestuff and measuring the absorbance of the resulting solution. Materials. Argon passed through BTS catalyst (BASF), potassium hydroxide, silica gel, and molxular sieve (5-A). MeCN 0 1981 American Chemical Society
ANALYTICAL CHEMISTRY. VOL. 53,NO. 12, OCTOBER 1981
Figure 1. ElectrOchemical cell with integrated drying tube. front view (A) and top view (6): a. adjustable electrode adapter (threaded Pyrex tube. TeRon-silicon compwnd seal, Nybn bushing); b,-b,. gad& seal (quart-Pyrex or quart-soft glass); c, cell top (contains the electrolytic Solutbn during drying operation): d. tube wnnecting cell top and main compartment; e,. platinum lead wire of the large scale working electrode (sealed wim b2):e*. platinum lead wire of the large Scale auxiliary eiectrde (sealed with b3 and b,); I,,*, cooling jacket; g. main compartment (waking and auxiliary e l e m wmpartment in voltammetric experiments, working electrode compartment in large scale experiments); h. window for irradiation of the electrolytic solution and temperature determination: i, Luggin tip (used in voltammetric and large scale experiments): k, "salt bidge"; I,, waking eteectmde for large scale expsriments (platinum wire gauze): 12, auxiliary electrode for large scale experiments (platinum wire gauze); m,. quartz frit of porosity 4 (diaphragm); m2. bored quartz frit of porosity 2 (support for aluminum oxue); n. auxiliary electrode wrnpartment for large scale experiments; 0 , drying tube (becomes half filled with aluminum oxide); p. tube for pressure equalization and cleaning of the two frits (may be anached to a water jet pump); q,-q,, quartz rods stabilizing tube p; r. tube for pressure equalization and mechanical stability: s1. fire-polished NS 14 Pyrex standard joint (connection to the high vacuum line); s2. firepolished NS 14 Pyrex standard joint (supports the reference electrode): t. reference electrode compartment
was purified as described elsewhere (8). The preparation of pure NBu,BF, and NBu,CIO, followed h o r n literature (10). SO, was previously dried with concentrated H,SO,.
SYSTEM EVALUATION M e C N probably i s t h e most frequently used solvent in nonaqueous electrochemistry, and therefore percolation o f M e C N and MeCN/0.2 M NBu,BF4 w i l l be reported in more detail. F o r M e C N low-temperature filtration a t -20 O C waa found to give o p t i m u m results. T w o filtrations a t -20 "C reduce the 60-mL volume of the electrolytic solution to 51 mL (85901,the concentration of the tetrabutylammonium salt being decreased from 0.20 to 0.19 M (95%). A sodium content of 6 X lo* m o l L-' indicates t h a t n o significant ion exchange takes place on the aluminum oxide. To detect colloidal aluminum oxide, eventually present in the working electrode compartment, we monitored scattering
1953
Flgure 2. (A) Combined auxiliary and working electrode for voitammetric measurements: a. BNC connector (cemented to the outer soft glass tube with epoxy resin); b,z. lead wire of working and auxiliary electrode: c, double-walled soft glass tube: d, polished platinum rod (1.0, 1.5. u 2.0 mm diameter) used as working electrode: e. platinum wire spiral us6d as auxiliary electrode. (B) Quasi-reference electrode: a, NS 14 stalnless steel stopper; b, socket: c, O-ring (Vlon A): d. set screw: e. platinum wire spiral. (C) Reference electrode: a. machined BNC socket: b. bwed screw cap: c, Teflon-silicon compound seal: d. threaded tube; e, O-ring (Won A); I. NS 14 joint (inner member): g, 0-rina ITeflonl: h.. sealed maonesia rod loressure eoualizationl. h,. seal2 kgnesla & (diiaphragk): i i, elecboytic solution: k. glass tubif I,silver wire.
A
B
F C n 3. (A) Stainless s t d adapter (connectionto hlghvacuum line): a, rubber septum; b. NS 14 pint (outer memoer): c. flexible metal tube; d. NS 14 pint (mer member): e. O-ring (Vilon A). (B) Tefbn sip joint (connection to cryostat and thermostat): a. bored screw cap: b. Teflon-silicon compound seal: c. Tenon body: d. foam rubber tube: e, silicon tube.
of light w i t h a Raman spectrometer. Using purified M e C N (8)as a standard. we found only 5090 of the i n i t i a l light intensity after two filtrations but ahout 50090 after addition of a small amount o f aluminum oxide to give a 10 M "solution". These data show, that at low temperatures the aluminum oxide (together with dust paHiclesJ is quantitatively retained o n the drying column, contrasting our observations made for room-temperature filtration o f M e C N on acidic aluminum oxide (R). At -20 "C the rather sensitive M e C N is not affected. T h e transmittance remains unchanged up to 200 nm. whereas one filtration at room temperature without any cooling reduces a 97% transmittance at 200 nm t o 7090,probably due to cyclization and polymerization ( 1 1 ) . T h e water content o f the solution is below the detection limit o f m o l L I. Unfiltrated solutions typically contain 10 m o l I,? of water, enough t o destroy the total amount of radical ions produced in a large scale electrolysis.
'
'
1954
Anal. Chem. 1981, 53, 1954-1955
A
NBu4C104decreased below 0.1 mA cm-2 within -1.30 and 2.00 V after two filtrations at -45 “C. Within -1.20 and 1.90 V the current density was even below 0.03 mA cm-2 (cyclic voltammetry, u = 100 mV s-l, T = -45 “C, 1 mm Pt disk electrode, Pt quasi-referenceelectrode). These data compare very favorable with those recently reported by Tinker and Bard (12). They also demonstrate the low-temperature capabilities (9) of our cell system. The cell has been successfully used for the voltammetric analysis of sigmatropic indene dianion rearrangements (13, 14), the photochemical “in situ” conversion of indenes into isoindenes at low temperatures (14),and last but not least for the electrochemical synthesis of heterocycles (e.g., isoquinolines, oxazoles, phenanthridines) by anodically induced nitrile cycloaddition (14).
C
B
ACKNOWLEDGMENT I am indebted to Ewald Daltrozzo for his support and in.... ,
terest in this work and to Georg Kollmannsberger-von Ne11
.- .... measuring stray light intensities. Last but not least the [ ’ ., m . .... lforskilled glass blowing of Erich Harms (Medizinisch-Glastech,
I
,,. .
,
...... . .:
@
eiectrolytic solution
0
aluminum oxide
@jcooiant Flgure 4. Schematic representation of the drying operation: A, the cell is assembled and filled with aluminum oxide and with electrolytic solution; B, the electrolytic solution is intermediately transferred Into the cell top by a 90” rotation; C, after back-roation the electrolytic
solution flows into the electrode comp_artmentspassing the cooled alumlnum oxide drying tube. The potential window of a 0.2 M NBu4BF4/MeCNsolution ranges from 2.70 to -3.30 V (vs. Ag/O.Ol M Ag+),the current density being less than 0.1 mA cm-2 between 2.35 and -3.20 V (8). Before filtration the potential window (I < 0.1 mA cm-? was found essentially smaller, ranging from 1.80 to -2.60 V. Also significantly higher background currents within these limits were noted. This fact seems especially important with respect to double layer investigations and kinetic work. Other electrolytic solutions showed similar results. As an example the background current density of liquid S02/0.2 M
nische Werkstatten, Berlin) is gratefully appreciated.
LITERATURE CITED (1) Sawyer, D. T.; Roberts, J. L., Jr. “Experimental Electrochemlstry for Chemists”; Wlley: New York, London, Sydney, Toronto, 1974, pl17. (2) Hammerich, 0.;Parker, V. D. Electrochlm. Acta 1973, 18, 537. (3) Bard, A. J. Pure Appl. Chem. 1971, 25, 379. (4) Mills, J. L.; Nelson, R.; Shore, S. G.; Anderson, L. 8. Anal. Chem. 1971, 43, 157. (5) Schmulbach, C. D; Oommen, T. V. Anal. Chem. 11973, 45, 820. (6) Holloway, J. D. L; SenRleber, F. C.; Geiger, W. E., Jr. Anal. Chem. 1978, 50, 1010. (7) Lines, R.; Jensen, B. S.;Parker, V. D. Acta Chem. Scand., Ser. 8 1978, 832, 510. (8) Kiesele, H. Anal. Chem. 1980, 52, 2230. (9) Van Duyne, R. P; Rellley, C. N. Anal. Chem. 1972, 44, 142. Feng, E.; Peet, N. P. J. Org. Chem. 1971, 36, 2371. (IO) House, H. 0;. (11) Woehrle, D. Fortschr. HOChpO&m.-FOf6ch. 1972, 10, 35. (12) Tinker, L. A.; Bard, A. J. J. Am. Chem. Soc. 1979, 101, 2316. (13)Kiesele, H. Chemledozenten-Tagung, Tiibingen, 1981. (14) Kiesele, H. Habllltatlonsschrlft,Unlversitiit Konstanz, 1980,and unpub llshed work.
RECEIVED for review July 28, 1980. Resubmitted April 30, 1981. Accepted July 26,1981. This work was supported by the Deutsche Forschungsgemeinschaft.
Ten-Bit Interface to an Eight-Bit Microcomputer in One Clock Pulse Mark M. Doxtader and R. Ken For&* Department of Chemistry, University of Rhode Island, Kingston, Rhode Island 0288 1
Advances in microelectronics in the past few years and the wide commercial availability of fairly sophisticated and inexpensive microcomputers has made it possible for many laboratories to interface laboratory instrumentation to microcomputers. However, many of the microcomputers are eight-bit machines which limits the input resolution to one part in 256. In many cases, the instrumentation yields a higher resolution signal. Also, some high-speed data collection systems, such as transient recorders, have memory units with larger bit words than can be accommodated by the computer on hand. One possible compromise is to use only the most significant bits disregarding the information contained in the balance or change memory size to fit that of the computer. In our laboratory, we have successfully interfaced a 10-bit transient recorder (TR) to an 8-bit Commodore Model 2001-8K P E T microcomputer without loss of the information 0003-2700/81/0353-1954$01.25/0
contained in the two least significant bits of the TR. The TR used in this application is a modified version of the design of Betty and Horlick (1). Interfacing is accomplished by means of three integrated circuits, two hex noninverting tristate buffers (National Semiconductor MM80C95, DM80C97) (2) and an inverter (7408). The outputs of the ten bits from the TR are used as the inputs to the two buffers. The third through eighth bits go to inputs 1-6 on the MM80C95, while the seventh through tenth (LSB) bits go to inputs 1-4 on the DM80C97. The two most significant bits, 1 and 2, are used as inputs 5 and 6 on the DM80C97. The computer clock line (CB2) which controls the movement of the 10-bit words from the TR serial memory is also attached directly to three disable lines 1, 2, and 3 located on the two buffers. An inverted clock is used to control disable 4. Figure 1 shows a schematic of the interface and 0 1981 American Chemlcal Society
