Anal. Chem. 1986, 58,233-236
schutkin reaction is determined to be 1.09 f 0.03 (for d3). The magnitude of the KIE satisfies the Schowen linear fractionation plot (22) for various types of transmethylations as a typical SN2 type of reaction (9). Recently, Harris et al. (IO) reported the a-D KIE of the corresponding forward Menschutkin reaction, kzf(CH3)/ kzf(CD3). Therefore, we can estimate the equilibrium isotope effect (EIE) for the present reverse Menschutkin reaction by their combination (eq 1 and 2). The EIE is estimated to be c1
€P = kzr/kzf
(1)
EIE = Kr(CHJ/€P(CDJ [k,’(CHJ /k,’(CD3)I
/ [kzf(C&) /kzf(CDJI (2)
1.31 (=1.09/0.835). This combined experimental value shows an excellent agreement with a theoretical value (1.31) obtained from Shiner and Schowen’s deuterium fractionation factors as a first-order approximation (22, 23). Further studies by means of 31PNMR quantitative measurements are now in progress (24).
ACKNOWLEDGMENT The authors are grateful to Y. Kyogoku and H. Akutsu for their kind measurements by a Bruker WM360 NMR spectrometer at a preliminary stage of the work. Registry No. 1-Methyl-3-chloropyridinium iodide, 32188-17-3; triphenylphosphine,603-35-0; deuterium, 7782-39-0. LITERATURE CITED (1) White, R. F. M. Annu. Rep. Prog. Chem., Sect. 8 1982, 7 9 , 9-10. (2) Parente, J. E.; Risley, J. M.; Van Etten, R. L. J . Am. Chem. SOC. 1984, 706, 8156-8161. (3) Saunders. M.; Telkowskl, L.; Kates, M. R. J . Am. Chem. SOC.1977, 99,8070-8073. (4) Gorenstein, D. 0.; Rowell, R. J . Am. Chem. SOC. 1980, 702, 6165-6166.
233
(5) Hill, J. G.; Nakashlma, T. T.; Vederas, J. C. J. Am. Chem. SOC.1982,
104, 1745-1748. (6) Jameson, A. K.; Jameson, C. J. J. Magn. Reson. 1978, 32,455-457. (7) Sawada, M.; Ichihara, M.; Ando, T.; Yukawa, Y. Tetrahedron Lett. 1981, 22, 4733-4736. (8) Tsuno, Y.; Fujio, M.; Sawada, M.; Yukawa, Y. Tetrahedron Lett. 1982, 23, 213-216. (9) Arnett, E. M.; Reich, R. J. Am. Chem. SOC.1980, 102, 5892-5902. (10) Harris, J. M.; Paiey, M. S.; Prasthofer, T. W. J. Am. Chem. SOC. 1981, 103, 5915-5916. (11) Kevill, D. N. J. Chem. SOC.,Chem. Commun. 1981, 427-422. (12) Abraham, M. H.; Nasehzadeh, A. J . Chem. SOC.,Chem. Commun. 1981, 905-906. (13) Johnson, C. D. Tetrahedron Lett. 1982 21, 2217-2218. (14) Kurz, J. L.;Seif El-Nasr J . Am. Chem. SOC.1982, 104, 5823-5824. (15) Stanislawski, D. A,; Van Wazer, J. R. Anal. Chem. 1980, 52, 96-101. (18) Martln, M. L.; Martin, G. J.; Deipeuch, J. “Practical NMR Spectroscopy”; Heyden: London, 1980; Chapter 9. (17) Wehrli, F. W.; Giger, W.; Simon, W. Helv. Chim. Acta 1971, 54, 229-243. (18) Berg, U.; Gailo, R.; Metzger, J. J . Org. Chem. 1978, 41, 2621-2624. (19) Deady, L. W.; Korytsky, 0. L. Tetrahedron Lett. 1979, 451-452. (20) Thorstenson, T.; Songstad, J. Acta Chem. Scand., Ser. A . 1976, 30, 724-730. (21) Riddick, J. A.; Bunger, W. B. “Organic Solvents”, 3rd ed.; Wiley: New York, 1970; p 399. (22) Gray, C. H.; Coward, J. K.; Schowen, K. B.; Schowen, R. L. J. Am. Chem. SOC. 1979, 707, 4351-4358. (23) Hartshorn, S . R.; Shiner, V. J., Jr. J . Am. Chem. SOC. 1972, 94, 9002-9012. (24) Sawada, M.; Takai, Y.; Chong, C.; Hanafusa, T.; Misumi, S.; Tsuno, Y. Tetrahedron Lett. 1985, 26, 5065-5068. (25) Katritzky, A. R.; Musumarra, G.; Saikizedeh, K.; M-Vukovlc, M. J . Org. Chem. 1981, 46, 3820-3823.
Masami Sawada* Yoshio Takai Chang Chong Terukiyo Hanafusa Soichi Misumi Material Analysis Center The Institute of Scientific and Industrial Research Osaka University Ibaraki, Osaka, Japan
RECEIVED for review June 3,1985. Accepted August 12,1985. A part of this work was supported by a Grant-in-Aid (No. 59840012) from the Ministry of Education, Science and Culture, Japan.
Method for Evaluation of Thermal Hysteresis Characteristics of Membrane Electrode-Reference Electrode Pairs Sir: Potential drift is a common source of inaccuracy during direct potentiometric measurements with membrane electrodes. Inherent defects or “poisoning”of the membrane by other constituents of the solution and temperature heterogeneity within the electrode body are the main causes of potential drift. Of particular importance is temperature heterogeneity, which appears to depend on constructional characteristics of the electrode; its effect hereafter is referred to as ”thermal drift”. Conditions giving rise to thermal drift inevitably occur, during direct potentiometry, whenever successive measurements take place in unthermostated solutions with temperatures different from the environment and when there is a heat transfer from the magnetic stirrer to the solution ( I ) . The use of thermostated cells is recommended to minimize thermal drift, but thermal gradients may still appear along the electrode body, generating junction potentials due to Seebeck
(thermocouple) effects at junctions between dissimilar conductors ( 2 ) . Rinsing of electrodes with water followed by rubbing with soft tissue may contribute, periodically, to an overall thermal heterogeneity. Electrode assemblies, thermostated over their entire length to eliminate thermal gradients, have been used when the utmost of potential stability is required over long periods ( 3 ) )but such precautions are not practical for routine analytical work. Manufacturers of ionselective electrodes recommend, in certain cases, equilibration times up to 1 h, when measurements have to be taken in solutions with temperatures substantially different from room temperature ( 4 ) . The temperature coefficient, dE/dT, of a particular membrane electrode (in conjuction with an appropriate reference electrode) and its “isopotential point”, defined as the analyte concentration where the temperature coefficient is zero (5), are useful quality characteristics of membrane electrodes,
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 1, JANUARY 1986
EXPERIMENTAL SECTION
to
t,
t2
TIME
Temperature-time variation during testing of the thermal hysteresis of a membrane electrode-reference electrode pair.
Flgure 1.
though they are rarely reported. The contribution of the various temperature-dependent terms on the temperature coefficient has been reported (6). These terms include the prelogarithmic term of the Nernst equation, the potential of the internal and external reference electrodes, and the activity of the detected species. The latter appears to be important particularly when the detected species participates in a chemical equilibrium with other constituents of the solution. Whereas the aforementioned thermal characteristics describe well the expected change of the electromotive4orce of the whole electrochemical cell, hereafter referred to, for simplicity, as potential (E),they do not provide information about how fast the latter acquires its equilibrium value after a stepwise or a gradual change of the temperature of the solution under measurement. We believe that another quality characteristic of practical importance is the potential-temperature (E-?') hysteresis described in terms of the E-T pattern obtained by recording the potential vs. the temperature of the solution, when the latter is varied in a fixed "cyclic" fashion, diagramatically shown in Figure 1. During the time period tl-to the electrode pair is thermally equilibrated, immersed in an analyte solution of temperature T,, whereas during the time period t2-tl the cyclic temperature change takes place. Subsequently, we present an experimental system suitable for studying thermal hysteresis of electrochemical cells and the E-T patterns obtained under a variety of conditions, with two homemade thiocyanate-sensitive electrodes of the liquid and solid (polycrystalline) membrane type, during a thorough assessment of their quality characteristics using a microcomputer-controlled potentiometric system. In addition, some E-T patterns were obtained with a fluoride-selective electrode (monocrystal membrane) and a hydrogen cation, pH (glass membrane) electrode.
Apparatus. The whole experimental system is schematically shown in Figure 2. The microcomputer-controlledpotentiometric analysis system is described elsewhere (7, 8). A double-walled 100-mLbeaker is used as a measurement cell, and all measurements are obtained under constant magneticstirring. A bead-type thermistor hermetically sealed in a glass rod, with a nominal resistance of about 100 kO at 25 O C and a thermal time constant of about 0.3 s (Series P-60 Thermornetrics, Edison, NJ), is used to monitor the temperature of the solution in the cell. The thermistor is configured in a resistance bridge, and the out-ofbalance voltage, properly amplified and digitized, is sampled by the microcomputer (along with potential indications) where it is converted to a degrees Celsius scale by use of the software. A thermostated pump (Type 01-T-623,Heto, Denmark) (pump A) circulates water through the jacket of the measurement cell. A second low-temperature thermostated pump (Ultra-Kryomat, Model TK-30D) (pump B) circulates cold (--lo "C) ethanol through an additional copper tube cooling coil immersed in the water tank of pump A. Some experimentation may be needed in order to obtain a temperature cycle as symmetrical as possible by adjusting the length of the copper coil and the temperature of the circulated ethanol. Solid-membrane (AgSCN + Ag2S) and liquid-membrane (tetraoctylammonium thiocyanate in 2-nitrotoluene) thiocyanate-sensitiveelectrodesare constructed according to standard procedures (9, IO). The solid-membrane electrode is used in conjuction with a double-junction Ag/AgCl reference electrode (Orion, Model 9002-00) with its outer chamber filled with a 10% (w/w) NH4N03 solution, whereas the liquid-membrane electrode is used in conjuction with a single-junctionAg/AgCl reference electrode (Orion, Model 90-01-00). The fluoride (Orion, Model 96-09) and the glass (H' sensitive, Metrohm Herisaw, Model EA-120) electrodes were of the combination type. An X-Y recorder (Hewlett-Packard, Model 7015B),properly interfaced with the microcomputer, is used for recording E-T patterns. Measurements. A typical sequence of actions for approximating the temperature change shown in Figure 1and obtaining the E-T patterns for a particular electrode pair, in a given anal@ solution, is the following: (i) A 50-mL aliquot of the analyte solution is transferred in the measurement cell. (ii) The circulator valve, V, of pump B is kept closed, and the temperature of pump A is adjusted to T , (typically 20 "C). (iii) At time to (Figure 11, the electrode pair under examinationis immersed into the solution in the cell, and it is thermally equilibrated for the time period tl - to (typically 15 min) at temperature T1. (iv) At time tl, the thermostat of pump A is adjusted to temperature T2 (v) At this point, potential-temperature indications start being collected by the microcomputer and stored in memory. (vi) When the temperature of the solution approaches T2,the operator is warned by the computer and switches off (switch S) the heating resistor of pump A and simultaneously opens the circulator valve, V, of
ELECTROMETER
CONTROL UNIT
X-Y
Flgure 2.
Schematic of
RECORDER
t b
THERMOSTATED PUMP
A
experimental system used for obtaining potential-temperature patterns.
THERMOSTATED PUMP
B
ANALYTICAL CHEMISTRY, VOL. 58, NO. 1, JANUARY 1986
!
B
A
SOLID
MEMBRANE
LIQUID MEMBRANE
:I
D
C
235
M KSCN
KSCN
4
r
Tl
T2
Tl
M KSCN
T2
TEMPERATURE
Figure 3. Potential-temperature patterns: (A) ideal, temperature-lndependent behavior; (6) “semi-ideal,” reverslble, temperature-dependent behavior; (C) real, irreversible, temperaturedependent behavior, slight thermal drift expected; (D) as in C, heavy thermal drift expected.
pump B. (vii) When the temperature of the solution returns back the potential-temperature indications acquisition is terto TI, minated, and the operator informs the computer about the required potential and temperature recording scale;the E-T pattern, properly scaled, is recorded by the X-Y recorder.
M KSCN
lom2M
RESULTS AND DISCUSSION Some general cases of E-T patterns are shown in Figure 3. Figure 3A shows the E-T pattern obtained with a membrane electrode-reference electrode pair showing an ideal thermal behavior, denoting temperature-independentpotential indications, ensuring the absence of thermal drift even with unthermostated solutions. Theoretically, such behavior is expected at the isopotential point and in the absence of thermal gradients. Taking into consideration that the potential is always temperature dependent, such a situation is unrealistic unless a thermal compensation has been introduced by purely electronic means. Figure 3B illustrates the E-T pattern obtained with an electrode pair showing a “semi-ideal”thermal behavior, denoting reversible temperature-dependent potential indications without hysteresis, ensuring the absence of thermal drift in thermostated solutions. Figure 3C shows the E-T pattern obtained with an electrode pair showing a real thermal behavior, denoting irreversible temperature-dependent potential indications with a small degree of hysteresis. In this case, a thermal drift is expected after a temperature disturbance, which will quickly subside down. Figure 3D is a qualitatively analogous case, denoting an electrode pair with much worse thermal hysteresis. In this case, an intense thermal drift is expected from sample-tosample measurement, and more time should be allowed before obtaining valid potential indications. Typical E-T patterns obtained (in triplicate) with the solid and liquid membrane thiocyanate-sensitive electrode-reference electrode pairs are shown in Figure 4. It can be seen that relatively reproducibleE-T patterns are obtained under any condition with both electrode pairs. In the case of the solid membrane-reference electrode pair, the E-T patterns depend to some degree on the concentration and ionic strength, and the potential is more temperature sensitive, whereas in the case of the liquid membrane electrode-refer-
20
20
25
TEMPERATURE,
Na2S04
25 OC
Flgure 4. Potential-temperature patterns obtained under fixed temperature cyclic change ( t i - t o = 15 min, t , - t i = 15 min) with a solid and a liquid membrane SCN--sensitive electrode with various anaiyte solutions. F-
ELECTRODE
H
’
1 w
ELECTRODE
pH : 4.0
P
i
7 pH:Q.O
w
10-4M NaF
20
25 TEMPERATURE
25
20
,
‘C
Figure 5. Potential-temperature patterns obtained under fixed temperature cyclic change (as in Flgure 3) with a F--sensitive (monocrystal) membrane electrode and a H+-sensitive (glass) membrane electrode (both of combination type). Acetate (0.10 M) and 0.10 M borate buffers were used as solutions with pHs of 4.0 and 9.0, respectively.
ence electrode pair the same general shape of E-T patterns is obtained in all cases, and the potential appears less tem-
Anal. Chem. 1986, 58,236-239
236
perature sensitive. Both electrode pairs show approximately the same thermal hysteresis. The E-T patterns obtained with the fluoride and the glass (H') electrodes of the combination type, are shown in Figure 5. It can be seen that the fluoride electrode shows a heavy thermal hysteresis, whereas the glass electrode shows a thermd hysteresis dependent on the actual pH of the solution. It should be noted that E-T patterns should always be obtained under fixed temperature cyclic change in order to obtain reproducible patterns appropriate for comparison of the thermal drift performance of each membrane electrodereference electrode pair. Also, meaningless E-T patterns will result if there is a potential drift not attributable to temperature changes (i.e., if the potential has not been stabilized during the to - tl period).
CONCLUSIONS The thermal hysteresis of membrane electrode-reference electrode pairs expressed in terms of the potential-temperature pattern obtained during a fixed cyclic temperature change should be considered as a valuable quality characteristic for the particular potentiometric sensor system. An electrode pair with significant thermal hysteresis (broad E-T pattern), should be considered as a "slow" transducer, and a low sample throughput should be expected with direct potentiometric measurements, because more time for thermal equilibration is needed. In the case of reference electrodes one of the primary constructional targets is always a small temperature coefficient. This fact ensures a small contribution of the reference electrode to the observed thermal hysteresis of the whole electrochemical cell. Therefore, thermal hysteresis is mostly attributed to the membrane electrode and must be considered as a constructional characteristic dependent not only on the
type of the sensing membrane but also on the heat capacity and the thermal conductivity of the various compartments and internal solutions of the electrode body. Probably, miniaturization of the membrane electrodes will greatly improve their thermal hysteresis characteristics.
Registry No. SCN-, 302-04-5;F-,16984-48-8. LITERATURE CITED (1) Durst, R. A. I n "Ion-Selective Electrodes in Analytical Chemistry"; Freiser, H., Ed.; Plenum: New York, 1978; Voi. 1, pp 331-332. (2) Sawyer, D. T.; Roberts, J. L., Jr. "Experimental Electrochemistry for Chemists"; Wiiey: New York, 1974; p 51. (3) Arnold, A. P.; Daignault, S. A,; Rabenstein, D. L. Anal. Chem. 1985, 57, 1112-1116. (4) "Instruction Manual: Fluoride Electrodes, Models 94-09 and 96-09"; Orion Research, Inc.: Cambridge, MA, 1982; p 27. (5) Orion Research, Inc., Newsletter, 1972; Voi. I V (3-4), p 8. (6) Light, T. S. I n "Ion-Seiectlve Electrodes"; Durst, R. A,, Ed.; National Bureau of Standards: Washington, DC, NBS Special Publication314, 1969; pp 354-355. (7) Efstathiou, C. E.; Hadjiioannou, T. P. Talanta 1983, 30, 145-149. (8) Efstathiou, C. E. Anal. Chlm. Acta 1983, 154, 41-49. (9) Papastathopoulos, D. S.; Karayannis, M. I.J. Chem. Educ. 1980, 57, 904-906. (IO) Pentari, J. G.: Efstathlou, C. E. Anal. Chim. Acta 1983, 153, 161-168.
C. E. Efstathiou* J. G. Pentari T. P. Hadjiioannou Laboratory of Chemistry University of Athens 104 Solonos Street Athens 106 80, Greece RECEIVED for review June 17,1985. Accepted August 23,1985. This research was supported in part by a grant from the Greek National Institute of Research.
Observation of Concentration Gradients by the Laser Beam Deflection Sensor Sir: Laser beam deflection and lensing methods have been used extensively to probe refractive index gradient (mirage effects) due to temperature gradients. Recently developed photothermal deflection (PTD) and thermal lensing techniques are based on this principle (1). In these techniques, the temperature gradient is produced in a medium when absorption of photons by molecules in the medium, or on the surface, occurs subsequently followed by thermal relaxation. A corresponding refractive index gradient is formed since the refractive index is temperature dependent. A refractive index gradient can be created by using other conditions affecting this physical property, i.e., a concentration gradient of a solute in a medium (2, 3). The mirage effect produced by this phenomenon can then be observed employing the laser beam deflection sensor. The magnitude of this effect will depend on the chemical system, the medium, and the solute. In many analytical processes, concentration gradients are formed. Having a sensitive tool to measure these gradients could, in some cases, improve sensitivity of quantitative determinations and provide additional information about the chemistry involved (4). For instance, in chromatographic techniques, separation occurs between the concentration gradient signals of various components of an analytical mixture (5,6). In electrochemicalexperiments,concentrationgradients of both reactants and electrogenerated products are formed
at the electrode surface (7). Theoretical analysis has recently been performed on the shape as well as the degree of deflection of the laser beam passing through the diffusion layer due to concentration gradients present above the electrode surface (8) but neither comprehensive descriptions nor any experimental results have been reported. Concentration gradients produced during electrolysis have been studied by optical methods including interferometric techniques (9,10) as well as absorbance measurements (11). In this communication, we report our preliminary results of the observation of the mirage effect produced by concentration gradient. The deflection of an unabsorbed He-Ne laser beam passing parallel to the surface was monitored by using a beam position sensor. Reactants used for this study were p-benzoquinone, which produces a stable anion radical upon electrolysis, and oxygen producing a superoxide anion radical. Both of these radicals are transparent to the probing beam, He-Ne laser.
EXPERIMENTAL SECTION The experimentalsetup consisted of an electrochemical cell, laser and laser beam position sensing device. The electrochemical cell was made of Teflon and was equipped with a rectangular (5 mm X 20 mm) platinum working electrode (platinum was sputtered on a glass slide), a silver wire pseudoreference electrode, and a platinum wire counter electrode. Quartz windows were installed on opposite sides of the cell walls to allow the laser beam
0003-2700/86/0358-0236$01.50/00 1985 American Chemical Society
