However, because of the increased difficulty inherent in making phosphorescence and low-temperature luminescence measurements. fluorescence is favored. Luminescent species provide characteristic excitation spectra. Utilization of the excitation and emission spectra, to detect the presence of compounds such as chalcones and menthyl salicylates which may be added to adultrated expressed oils to bring the absorption of the oil up to levels prescribed by the method of Sale, is a definite possibility. Expressed lemon oil exhibits the same fluorescence characteristics in ethanol as expressed lime oil. This blue emission can be attributed to 5-geranoxy-7-methoxycoumarin,5,7dimethoxycoumarin, and 5-isopenteneoxy-7-methoxycoumarin which are present in lemon oil (5). Expressed grapefruit oil shows maximum emission from ethanol solution of the oil below 400 nm. This can be attributed to the presence of 7-geranoxycoumarin(6) which apparently is present
as the major coumarin derivative. Further examination of the total luminescence from expressed lemon oil and grapefruit oil should result in observations similar to those made for expressed lime oil. The major crystalline components in expressed orange oil are flavones with smaller amounts of coumarin derivatives. Orange oil shows emission and excitation maxima at higher wavelength than other citrus oils which may be due to the flavones. ACKNOWLEDGMENT
Samples of expressed citrus oils were provided by Fritsche Brothers, Inc., Dodge and Olcott, Inc., and the Florida Home Juice Company. RECEIVED for review March 5, 1969. Accepted May 19, 1969. Research supported in part by the Esso Foundation.
Novel Impedance Measurements on Ion-Selective Liquid-Membrane Electrodes M. J. D. Brand and G. A. Rechnitz’ Chemistry Department, State University of New York, Buffalo, N . Y. 14214
-
-
Ion selective, liquid membrane electrodes were studied using a novel impedance measurement technique. Evaluation of several commercially available membrane electrodes was carried out in detail as a function of frequency and of solution variables. From these studies, it was possible to identify and characterize some of the processes-e.g., electromigration and ion exchange-occurring within the membrane and at its interfaces. The data obtained suggest a possible means for the quantitative evaluation of fundamental electrode parameters, not accessible to potentiometric studies, and may lead to an experimental test of the current theories of ion-selective electrode operation.
A CONSIDERABLE NUMBER of ion selective electrodes have now been developed to the stage of analytical utility ( I ) , and it seems certain that many others will be introduced in the future. Among the various membrane types used in these electrodes, liquid ion-exchange resins are now firmly established as offering a wide range of possibilities. While liquid membrane electrodes have been known for many years, it was the introduction of the calcium-selective electrode by Ross (2) which led to recent developments. Subsequently, commercially available electrodes have been evaluated for measurement of the activity of not only calcium ( 3 ) but also cupric (4), nitrate (5), perchlorate (6), fluoroborate (7), and chloride (8) ions. Alfred P. Sloan Fellow; to whom reprint requests should be addressed. ( 1 ) G. A. Rechnitz, Chem. Eng. News, 43,(25), 146 (1967).
(2) J. W. Ross, Jr., Science, 155, 1378 (1967). (3) G. A. Rechnitz and Z . F. Lin, ANAL.CHEM.,40, 696 (1968). (4) G. A. Rechnitz and 2. F. Lin, Anal. Letters, 1, 23 (1967). (5) S. Potterton and W. D. Shults, ibid., p 11. (6) T. M. Hseu and G. A. Rechnitz, ibid., p 629 (1968). (7) R. M. Carlson and J. L. Paul, ANAL.CHEM.,40, 1292 (1968). (8) T. G. Lee, ibid., 41, 391 (1969).
A general theory of liquid membrane electrodes based on ion-exchange properties has been presented by Eisenman et al. (9-11) and by Sandblom (12, 13). Potentiometric response to a given counter ion depends not only on the activity of the ion in solution and in the membrane but also on the equilibrium constant of the ion-exchange process and on the mobility of the ion in the membrane. Ionic migration has been assumed to be the only process responsible for the passage of electricity through the membrane. This is equivalent to treating the membrane as a pure resistance (IO, 13) for which the measured conductivity is independent of the frequency of the applied ac signal. Measurements of the conductivity of solid ion-exchange membranes have indicated only small variations with frequency (14, 15). The mechanism of ion transport through membranes and across the membrane-solution interface is not well understood ; kinetic data on the processes involved are not available although it is thought that the ion-exchange reaction is not rate determining (16). It is apparent that such information is not obtainable from steady state electrode potential measurements and a different experimental approach is required. One possible approach involves the study of the power spectrum of noise generated by passage of a relatively high density current through a solid ion-exchange membrane ( I 7). Buck has discussed the impedance of glass electrodes (18) and (9) J. Sandblom, G. Eisenman, and J. L. Walker, J. Phys. Chem., 71, 3862 (1967). (IO) Ibid., p 3871. (11) G. Eisenman, ANAL.CHEM.,40, 310 (1968). (12) J. Sandblom, J. Phys. Chem., 73,249 (1969). (13) Zbid., p 257. (14) J. H. B. George and R. A. Courant, ibid., 71, 246 (1967). ( 1 5 ) V. Subrahmanyan and N. Lakshminarayanaiah, ibid., 72, 4314 (1968). (16) F. Helfferich in “Ion Exchange,” J. Marinsky, Ed., Marcel Dekker, New York, Volume 1, 1966, p 65. (17) M. E. Green and M. Yafuso, J. Phys. Chem., 72, 4072 (1968). (18) R. P. Buck,J. Electroanal. Chem., 18,381 (1968). VOL. 41,NO. 10,AUGUST 1969
1185
b-
Figure 1. Schematic of measuring circuit
5-
2
has made experimental measurements on them (19). Although the impedance method has been widely applied to the study of charge transfer reactions at electrodes (20), little is known about the impedance behavior of ion-exchange membranes. In the present communication, the results of measurements of the impedance of some liquid ion-exchange resin membranes are described. EXPERIMENTAL The liquid membrane electrodes were obtained from Orion Research Inc. (Cambridge, Mass.) and were prepared according to the manufacturer’s directions. It will be convenient to refer to each electrode by the manufacturer’s model number: the electrodes examined were selective to calcium (98.20 and 92.20), copper (92.29), water hardness (92.32) and chloride (92.17). The counter electrode used to complete the cell for impedance measurements was a silver billet electrode, Part No. 39261 (Beckman Instruments Inc., Fullerton, Calif.). All measurements were made on a cell without liquid junction. Measurements of cell impedance and phase angle were made using a specially designed apparatus with oscilloscopic readout. The oscilloscope used was a Type 564 (Tektronix, Beaverton, Ore.) with a Type 3A3 dual trace differential vertical amplifier and a Type 3A75 horizontal amplifier. Ac voltages were obtained from a EUW 27 signal generator (Heath Co., Benton Harbor, Mich.) the frequency calibration of which was checked against an Audio Oscillator Model 201 C (Hewlett Packard, Palo Alto, Calif.). The measuring circuit, shown in Figure 1, was based on two FET input Model 142 A operational amplifiers (Analog Devices, Cambridge, Mass.). Amplifier 1 was connected in an inverting configuration with a 10-MO feedback resistor and the membrane cell served as its input impedance. Application of an ac voltage from the signal generator, of peak to peak amplitude e,, resulted in an output e, from amplifier 1 which was measured on the oscilloscope vertical display. The cell impedance was calculated from
Use of this equation requires that ei and e, should represent true sine waves and assumes a linear membrane cell process. The oscilloscope horizontal amplifier was driven by the output of amplifier 2 which was connected as a follower with gain. The input to this amplifier was derived directly from the signal generator and the amplifier output was equal to 10.09 times the signal generator output. This gain of 10 was necessary because the oscilloscope horizontal amplifier had a maximum full scale sensitivity of 500 mV. In general the oscilloscope display was an ellipse, indicating a phase (19) R. P. Buck and I. Krull, ibid., p 387. (20) B. B. Damaskin, “The Principles of Current Methods for the Study of Electrochemical Reactions,” McGraw-Hill, New York, 1967, Chap. 3. 1186
ANALYTICAL CHEMISTRY
4-
a-
a10
1.0
1.0
3.0
LOG
4.0
F
Figure 2. Frequency dependence of impedance and phase angle of a Ca2+ electrode (98-20) in 10-*M Ca2+ at p = 1.0 difference between the outputs of amplifiers 1 and 2. The phase angle was calculated using the formula (22) 8 = arc sin
():
where e’ and e, are the voltages shown in Figure 1. The oscilloscope display was inverted vertically because of the configuration of amplifier 1. Impedances and phase angles measured in this way are meaningful only if the instrumental gain is frequency independent and the instrumental phase shift is zero over the frequency range of interest. This point was checked by replacing the cell with fixed resistors having values of 0.1 Ma to 10 MO with negligible reactance and measuring the impedance and phase angles over a frequency range of 20 Hz to 100 KHz. The instrument gain was found to be independent of frequency up to 100 KHz for all values of resistance and showed zero phase shift up to 30 KHz for a 10-MQ resistor. For lower resistor values, however, a small phase shift was observed at frequencies below 30 KHz and calibration curves were plotted of phase angle us. frequency. These graphs were used to correct the measured phase shift of a membrane cell for the instrumental phase shift. RESULTS In general the measured impedance of each electrode was almost independent of frequency at low frequencies. At higher frequencies the impedance decreased with increasing frequency, with a corresponding increase in the measured phase angle. At the highest frequencies attainable, the phase angle again decreased. The data contained in these Z-f and 8-fgraphs, shown in Figure 2, was also plotted in the complex impedance plane (22) where
Z
=
ZR
+ jZ,
(3)
ZRis the real (resistive) impedance, 2,the imaginary (reactive) impedance and j = The observed values of Z , were always negative (capacitative reactance) and the normal electrochemical practice of plotting Z R , -2,in the first
4-1.
(21) H. V. Malmstadt, C. G. Enke, and E. C. Toren, “Electronics for Scientists,” W. A. Benjamin, Inc., New York, 1963, p 216. (22) J. H. Siuyters, Rec. Truu. Chim. Pays-Bus, 79,1092 (1960).
7*2t
1.0
''
Mn
cQ
I
I
I
I
I
I
1
1
. 3
4
5
6
2,
7
MA
Figure 4. Effect of ac voltage amplitude on impedance of a Ca2+electrode (98-20) in 1 0 - 2 M C a 2 f a t,u = 0.05 0
1V, O500mV, A100mV, ASOmV
phase angle measurements at ac voltages as low as 50 mV. The small current (1 Mfl) cell on application of a 1-V signal corresponded to a very small perturbation of the membraneelectrode system. Figure 4 shows the complex plane plot of impedance measurements made on a calcium electrode (98-20) at applied ac voltages in the range 50 mV to 1 V; no significant trend in impedance as a function of ac voltage was observed. Variations among the Electrodes. With the exception of the water hardness electrode (92-32), the general shape of the complex impedance plane plot was identical for each of the cation selective electrodes. Figure 5 shows the impedances of each electrode plotted in the complex plane in the form of a reduced variable. Differences in impedance behavior shown by these electrodes can be described completely in terms of the resistive impedance obtained by extrapolating the impedance plot to very low frequencies. The anomalous behavior of the water hardness electrode is not understood at the present time, and no further measurements were made with this electrode. The impedance of a freshly prepared copper selective electrode, conditioned for 24 hours by soaking in 0.1M copper nitrate solution, proved to be too large to measure at
0.15
0 Ca2+(98-20) electrode, ZR, 0 Ca2+(92-20) electrode, Z,, A Cu2+(92-29)electrode, ZR,A Ca2+(92-32) electrode, ZR,-
\
0
quadrant of the complex plane was adopted. In general, the complex impedance plane plots showed a distorted arc of a circle, with center below the real axis and showed some similarity to the plots obtained by Buck and Krull (19) for glass electrodes. The complex impedance plane plot corresponding to the data of Figure 2 is shown in Figure 3. The results obtained for a given electrode were found to depend upon pretreatment of the electrode. All electrodes were conditioned by soaking for 12 hours or more in 0.1M solutions of the ion toward which they were selective; under these conditions, reproducible results were obtained. Variations were observed between individual electrodes of the same type and were presumably due to variations in membrane thickness. To obtain impedance measurements over a wide enough frequency range (20 Hz to 20 KHz) to define the complex impedance plane plots completely, it was necessary to use large amplitude ac voltages of up to 1 V peak-to-peak. In impedance measurements, it is important to use very small amplitude ac signals corresponding to a rectilinear portion of the electrode current-voltage curve and thus minimizing the effects of nonlinearities in the cell processes. With the apparatus used, the minimum amplitude is limited theoretically by the necessity that the input current to the operational amplifiers should be negligibly small (1 %) compared to the current through the cell. In practice, however, it was necessary to use ac voltages much larger than the theoretical minimum to overcome the effect of noise pick-up in the cell. The interference from noise, despite screening, was particularly noticeable at low frequencies. At high frequencies, it was possible to obtain impedance and
Figure 5. Complex impedance plane plots for cation selective electrodes in 10-2M cation solutions at p = 0.05
\*
I
"I 7"f
Figure 3. Complex impedance plane plot for Caz+electrode (98-20) in 10- 2MCa2+at ,u = 1.0
b
1.5
1
i
o'lcl
A
/
0.05
= 6.9 Ma = 11.6 Mn = 33.3 M a = 3.7 M a
VOL. 41, NO. 10,AUGUST 1969
1187
1.6
2,
I
0
I
KII
Y
1
4
2,
Figure 6. Complex impedance plane plot for C1- electrode (92-17) in 10-2MNaCl solution
5
6
Figure 8. Effect of ionic strength on calcium electrode (98-20) impedance at constant [Caz+] = 10-*M p,
low frequencies with the apparatus used. At a frequency of 1 KHz an impedance of approximately 100 M a was observed, which decreased with increasing frequency as expected. However, on allowing the electrode to age for several days in copper nitrate solution, the impedance decreased to a value of 33.3 Ma at low frequencies. The membrane was observed to become black during this period with the probable formation of solid material within the liquid resin. This did not apparently affect the potentiometric response to copper ions. The chloride selective electrode showed impedances some two orders of magnitude lower than those of the cation selective electrodes. The membrane impedance was not much greater than the series impedances associated with the cell solutions, reference electrodes, etc., and in this case the measured impedances may reflect the influence of processes other than those associated with the membrane-solution system. However, the general shape of the complex impedance plot for the chloride electrode, shown in Figure 6, was not dissimilar to those of the cation selective electrodes. Effect of Solution Composition. Variations in the impedance as a function of the solution composition were studied for the calcium electrode (98-20). This electrode has a higher selectivity for calcium over sodium than does the 92-20 electrode (23), and it was established that the 98-20 electrode also showed a negligible selectivity to tetra-alkylammonium
e 0.05; A 0.1; o 0.5
ions. Decreasing the calcium ion concentration at constant ionic strength produced an increase in the low frequency impedance while at higher frequencies the impedance was independent of calcium ion concentration as shown in Figure 7. Increasing the ionic strength by addition of an indifferent electrolyte while maintaining the calcium ion concentration constant resulted in a decrease in low frequency impedance, the impedance at high frequencies again being unaffected, Figure 8 shows the variations in impedance observed on changing the ionic strength over a ten-fold range. Under identical conditions of calcium ion concentration and ionic strength, the impedance was found to depend on the cation of the indifferent electrolyte used to adjust the solution ionic strength. Figure 9 shows 0-f and 2-fcurves measured in the presence of Et4N+,MerN+,and Na+; the corresponding complex impedance plane plots are shown in Figure 10. At low frequencies, the phase angle was approximately independent of the cation, but the impedance showed a clear
25 20 15 10
(23) “Guide to Specific Ion Electrodes and Instrumentation,”
Orion Research Inc., Cambridge, Mass., 1969, p 6.
5 0
1.6
c-
1.4
-
1.2
-
1.0
-
1.8
ZC Mn
0.4
-
0.1
-
0.8
0.6
7 -
6 -
5-
:i / 0
o
u
1
2
l
3
4
2,
9
I
0
0
I
I
I
,
5
6
7
8
Mn
Figure 7. Effect of calcium ion concentration on calcium electrode (98-20) impedance at constant p = 0.05 [Ca2+],0 10-ZM; A 10-aM; e lO-*M
1188
ANALYTICAL CHEMISTRY
7
M n
I
I
LO
I
I
I
I
3.0
2.0
log
I
I
I
4.0
f
Figure 9. Effect of indifferent cation on phase angle and impedance of calcium electrode (98-20) at constant [CaZ+] = 10-*M and p =
0.05
0 Na+, 0 Me4N+,A Et4N-I
16 14 -
18
0 1 12
ZC
M*
10
1
2
4
3
2,
5
b
7
MA
Figure 10. Complex impedance plane plots for calcium electrode (98-20) Points correspond to data of Figure 9
Figure 11. Equivalent circuits of liquid membrane cell
dependence. At higher frequencies, the phase angles for the two tetra-alkylammonium ions were approximately identical, and significantly different from the phase angle measured in the presence of sodium ions. The impedance at high frequencies was independent of the nature of the cation.
Table I. Equivalent Circuit Constants as a Function of Frequency for a Calcium Electrode (92-20) in 10-2M Ca2+ at p = 0.05
DISCUSSION
The impedance or its reciprocal, the admittance, of an electrochemical cell provides an index of the sum of all processes by which electricity flows through the cell between the electrodes. Processes which contribute to the impedance may be capacitative or Faradaic-Le., those processes related to charge transfer reactions at the metal electrode-solution interfaces. Faradaic processes may be rate limited by mass transport of ionic species in solution. The rather large impedances shown by cells containing a liquid ion-exchange membrane are due to transport through the cell being membrane limited. Measurements of cell impedance are therefore equivalent to measurements of the impedance of the membrane and solutions in contact with it. This conclusion is not necessarily valid for any membrane as it has been demonstrated that some membrane impedances-e.g., that of the chloride electrode-may actually be quite low. The use of such membranes for mechanistic studies of the type described here is experimentally attractive as the impedances fall well within the range of most commercial impedance bridges. However,. in this case the measured impedance will reflect the influence of charge transfer and other processes taking place at the reference electrodes. This discussion will be limited to a consideration of results obtained for the cation selective electrodes (with the exception of the water hardness electrode) as these are self-consistant, formally similar to data obtained for glass electrodes (19) and essentially indicative of processes occurring across the membrane. The shape of the complex impedance plane plots indicates that the membrane cell is electrically equivalent to an RC network which reduces to a pure resistance at both high and low frequencies. At a given frequency, this network is mathematically equivalent to a series resistor R1and capacitor C1 (Figure l l a ) and plotted values of Z , and 2, represent the impedances of R t and Cl. Table I shows values of R1 and C1 as a function of frequency for a calcium electrode (92-20). The cell capacitance showed a continuous decrease with increasing frequency until at the highest frequencies used a value was obtained comparable to the stray capacitance expected in any electrical circuit.
F, Hz
RI, MQ
Ci, PF
11.36 11.62 11.61 11.22 11.31 11.03
30,800 9,290 4,190 2,520 865 371 150 69
20.0 31.6 56.2 100
178 316 562 loo0 1780 3160 5620
10.71
9.94 9.04 7.17 5.29 3.50 1.96
10000
17800
C ‘ , PF
30
15 9 6 6
13.9 11.4 7.48 4.99 3.68 3.42 2.22 2.05 1.84 1.67 1.62 2.22
At very high frequencies the impedance of CI tends to zero but the complex impedance plane plots show that R1 reaches a finite limiting value Rz, 1 M a for the 92-20 electrode in 0.01M Ca2+ at p = 0.05. Rz may be interpreted as a frequency independent resistance, representing the sum of the resistances due to solutions, reference electrodes, etc., in series with a complex impedancez’, (Figure 1lb), where
and the real term of 2’ is given by
The simplest possible network equivalent to 2‘ is a parallel resistor and capacitor, but such a circuit would have a complex impedance plane plot of a true semicircle with center on the real axis. The complex impedance Z ‘ may be transformed into a complex admittance Y (24) which may be plotted in the complex admittance plane, where Y
= YR
fjY,
(6)
Figure 12 shows the complex admittance plane plot for the (24) E. Brenner and M. Javid, “Analysis of Electric Circuits,” McGraw-Hill Book Co., New York, 1959, p 405. VOL. 41, NO. 10,AUGUST 1969
1189
calculated and are shown in Tabie I. The frequency dependence of C’suggested that this was an inadequate model for a liquid membrane electrode. Furthermore, the real admittance Y E - Y ~ z p owas found not to be linearly dependent on w+l12, as would be required of a Warburg impedance in parallel with a capacitor. In fact, the real and imaginary admittances were found to be dependent on w , although the point corresponding to the highest frequency deviated significantly from the straight line fit. This may reflect the influence of stray capacitance, however. From the experimental results, it would appear that the complex impedance Z” can be represented by
0
0.1
Y,
I
I
0.2
0.3
10-’2
Figure 12. Complex admittance plane plot of the impedance 2’ for a calcium electrode (92-20) in 10-2M Ca2+at p
=
0.05
calcium electrode 92-20. The intercept on the real axis YRz=ocorresponds to the limiting admittance of 2’ at low frequencies which may be represented as a frequency independent resistance R3 shunting a complex impedance 2” as shown in Figure l l c . The value of Rg is given by the reciprocal of YE,-,, and was equal to 10.87 MQ for the calcium electrode 92-20 in 10-2M CaZ+ at p = 0.05. The physical interpretation of R3 would be the transport of ions across the solution-membrane interface by an electromigration process. At the same time an additional transport mechanism operates, represented by the complex impedance 2”. The equivalent circuit of this impedance is obscure, and its physical interpretation more so. Buck (18) has proposed an identical equivalent circuit for the impedance of a glass electrode in the absence of a hydrolyzed surface film. It was shown in this case that the impedance Z” was equivalent to a fixed capacitance C’ in parallel with a transmission line representing a Warburg diffusional impedance (25). This Warburg impedance can be represented by Z , = g o - ~ i Z- jgw-112 (7) where w is the angular frequency and u a constant, and its complex admittance plane plot is a straight line with a phase angle of 45’-i.e., a slope of 1. The observed complex admittance plot for the impedance 2” shown in Figure 12 was indeed a straight line but with a slope of almost exactly 2. Values of capacitance C’ necessary to shunt a 45’ transmission line to duplicate the observed admittance plot were (25) D. C . Graharne, J. Electrockem. Soc., 99, 37OC (1952).
1190
ANALYTICAL CHEMISTRY
where a is a constant. However, at the present time it is not known whether 2” is the electrical analog of a single transport mechanism or the resultant of a number of processes. It may also be noted that if, in fact, 2” does represent a fixed capacitor in parallel with another complex impedance, then its value will be extremely small. The presence of such a capacitor could be interpreted as representing a doublelayer at the membrane-solution interface. However, such a capacity would be of the order of lo5 times smaller than that commonly observed at metal-solution interfaces. Although the separation of ionic species by a phase boundary must exhibit a double layer across the interface, it must be concluded that the structure of the membrane-solution double layer is rather different from that observed at a metal solution interface. When the circuit of Figure l l c is used, it seems probable that the resistance R 3can be interpreted as representing a contribution from electromigration of ions across the membrane. Figure 8, in which the concentration of Me4N+ion was increased at constant calcium ion concentration, shows that R 3 decreased with increasing concentration of Me4N+. This apparently indicates that the tetra-methyl-ammonium ion can take part in ion exchange across the membrane, as there is no reason to assume that co-ions are not excluded under these conditions. The 98-20 calcium electrode is known to show no potentiometric response to tetra-alkyl ammonium ions, which is usually assumed to mean that ion-exchange does not occur. However, there are factors other than a low ionexchange equilibrium constant which would result in a low potentiometric selectivity for monovalent cations over calcium ion (9). It is not possible to distinguish between these possibilities here, but the data of Figure 9 suggest that the rate of migration of ions within the membrane is in the order Et4N+ > Me4N+> Na+. The results of this investigation may be summarized as follows. Transport of ions through a liquid membrane and across the membrane solution interface occurs by (I) electromigration and (11) a process, for which there is no simple physical analogy. This latter process may be represented by a complex impedance, the real and imaginary parts of which are both proportional to o-1. Counter ions to which the electrode shows no potentiometric response can and do enter into ion exchange reactions at the membrane solution interface. There is evidence to suggest that the structure of the double layer at the membrane solution interface is rather different from that observed at metal solution interfaces. In the authors opinion, it is no more appropriate to discuss the ‘resistance’ of a membrane electrode than it is to define the resistance of a polarographic cell as the cell potential-current ratio. With the impedance measurement technique employed here,
the qualitative description of the electrode processes involved in the operation of liquid membrane electrodes cannot yet be made quantitative. Accurate measurements of a high impedance over a wide frequency band-width are extremely difficult to carry out, but it seems likely that refinements and improvements of the present technique can lead to a possible means for the quantitative evaluation of fundamental electrode parameters.
ACKNOWLEDGMENT Special experimental facilities were provided by the University Program for Scientific Measurement and Instrumentation. RECEIVED for review February 26, 1969. Accepted May 23, 1969. Work supported by a grant from the National Science Foundation.
Optimizing Concentration Determinations in the Presence of Adsorption Phenomena Using the Vibrating Dropping Mercury Electrode James G . Connery and Richard E. Cover Department of Chemistry, S t . John’s University, Jamaica, New York 11432
The vibrating dropping mercury electrode (VDME) is demonstrated to be significantly superior to the DME for most analytical purposes where adsorption phenomena can inhibit electrode response. Four examples of inhibitory phenomena associated with the adsorption of electroinactive substances are examined and the advantages of the VDME demonstrated. Two systems which give adsorption waves at the DME with consequent nonlinear response to concentrations are shown to give linear response at the VDME. The fundamental characteristics of the VDME which cause the improved responses are the short drop time and the large rate of area formation as compared to the DME. Both of these factors operate to minimize the extent of surface coverage by adsorbate during detector life.
THEVIBRATING DROPPING mercury electrode (VDME) consists essentially of a dropping mercury electrode (DME) in which the drop rate is controlled by periodic mechanical shock or vibration of the electrode. Such premature drop detachment has been used over the past 20 years for various purposes. In a previous paper ( I ) , we demonstrated that, for many analytical purposes, the VDME is significantly superior to the DME. When the vibrational frequency is sufficiently high, maxima of the first and second kinds can be eliminated at the VDME without the addition of surfactants. In addition, at high frequencies, catalytic and kinetic currents can be minimized or eliminated from VDME response. Waves of these types are rarely useful analytically and can obscure desired data. Furthermore, the VDME permits the analysis of agitated solutions. The work reported here further illustrates the general superiority as an analytical tool of the VDME over the DME. In this work, various systems are examined where the DME response is inhibited because of the adsorption of substances on the electrode surface. Such inhibition can affect electrode response adversely in several ways. Detector sensitivity may be decreased. The polarographic waves may be grossly distorted preventing meaningful current measurements or the electrode response to concentrations may become nonlinear.
At the VDME under the proper conditions, these adsorption effects can often be minimized or eliminated and analytical data obtained. The prime experimental variable which permits this improved response with a given capillary is the frequency of vibration of the electrode. The relationships between currents observed for various phenomena and the more fundamental parameters such as rate of mercury flow and drop time are under investigation and will be reported in a separate paper. These relationships are complex-e.g., the number of drops per cycle varies from about 0.25 at 42 Hz to 1.0 at 210 Hz. In the experiments reported here, the drop time reached a lower limit of about 5 msec at a vibrational frequency of 210 Hz. The fundamental characteristics of the VDME which cause the improved response are the decreased drop time and the increased rate of area formation (as much as 20 times greater than at the DME). Both of these factors operate to minimize the extent of surface coverage during detector life. Although only inhibitory adsorption processes are considered here, there are cases where adsorbed substances can accelerate the electrode process, One of these, the catalytic hydrogen wave due to quinine, was considered in our previous paper ( I ) . The effects of various adsorption phenomena have been well reviewed by Delahay (2), Mairanovskii (3),and Heyrovsky and Kuta ( 4 ) . EXPERIMENTAL Apparatus. All apparatus used were previously described
(0.
Reagents. The methylene blue chloride was Allied Chemical Biological Stain Grade and was recrystallized from ethanol. The tribenzylamine was Eastman Grade 1015. All other reagents were Baker, Mallinckrodt, or Alfa Inorganics reagent grade. (2) P. Delahay, “Double Layer and Electrode Kinetics,” Interscience Publishers, New York, N. Y., 1965. (3) S. G. Mairanovskii, “Catalytic and Kinetic Waves in Polar-
ography,” Plenum Press, New York, N. Y., 1968. (1) R. E. Cover and J. G. Connery, ANAL.CHEM., 41,918 (1969).
(4) J. Heyrovsky and J. Kuta, “Principles of Polarography,”
Academic Press, New York, N. Y., 1966, pp 287-337. VOL. 41, NO. 10,AUGUST 1969
1191
