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.
(1) R. E. Cover and J. G. Connery, ANAL.CHEM.,41,918 (1969).
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 Polarography,” Plenum Press, New York, N. Y., 1968. (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
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Figure 1. Elimination of adsorption inhibition at the VDME, 4.00 rnM Cd(II), 0.20M HCI.DME, mercury pressure 60 cm. VDME, mercury pressure 160 cm, 210 Hz. DME, no TBA (b) VDME, no TBA (c) VDME, 5.0 mM TBA ( d ) DME, 5.0 mM TEA (a)
RESULTS AND DISCUSSION Adsorption of Electroinactive Species. One type of electrode process inhibition that has been observed is that in which the presence of an electroinactive substance causes a decrease in the limiting current of a polarographic wave with no significant shift in the half-wave potential. A theory has been derived for this phenomenon when the rate of adsorption is diffusion-controlled (5). Figure 1 contains polarographic and voltammetric data for such a system, Cd(I1) in dilute HC1, where the surfactant is tribenzylamine (TBA). As can be seen from comparison of Curves (a) and (4, the addition of 5.0 m M TBA effectively obliterates all DME response to the presence of Cd. Actually, the net current in the presence of TBA is about 2% of that observed when it is absent. The VDME response, on the other hand, is virtually unaffected by the presence of TBA. The analytical implications of these data are clear. For this system, and probably for related ones as well, the VDME is considerably less sensitive than the DME to the presence of surface-active materials such as TBA. Where such surfactants are present in fixed concentration levels, the VDME offers higher sensitivity. Where the surface-active materials may be adventitiously present at varying but unknown concentrations, the VDME offers more dependable response. Another common type of inhibition which does not distort polarographic waves from their normal sigmoid shape is that in which the presence of a surfactant causes both a decrease in the limiting current and a shift of the cathodic wave to more negative potentials. A theory has been developed for these phenomena where the rate of adsorption is diffusion-controlled (6). Although this type of inhibition can drastically reduce DME response to electroactive species (7), this is not always true. For example, the Cr(II1) wave in 0.100M (5) J. Heyrovsky and J. Kuta, “Principles of Polarography,”
Academic Press, New York, N. Y., 1966, pp 326-329. (6) Ibid., pp 304-322. ( 7 ) Ibid., p 300.
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I
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VI.
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Figure 2. Effect of Triton X-100 on the polarography of CU-EDTA 1.0 mM Cu(IItEDTA, 9.0 m M EDTA, 0.1M tartrate buffer, pH 3.5. Mercury pressure 60 cm. Triton X-100 (a) 0 % (b) 0.009%
NaC104 at pH 3.10 can be inhibited by the addition of Triton X-100 but the effects are much less severe. The DME and VDME responses to this system were studied with different concentrations of the surfactant throughout the range from 0% to 0.2%. The shifts in half-wave potentials are virtually the same at the two electrodes. At 0% and 0.2% surfactant, the respective half-wave potentials are: DME, -0.84 V and - 1.04 V; VDME, -0.96 V and - 1.20 V. The decreases in limiting currents going from 0 % to 0.2% Triton are: DME, -8.3%; VDME, -5.0%. To put these data in context, Triton X-100 is frequently effective as a maximum suppressor at a concentration of 0.001 % and, at the 0.009% level, it can severely distort DME response to some systems. In the light of the data summarized in Figure 2, for example, severe inhibition of the Cr(II1) wave might well be expected at the 0.2 % Triton level. Although the observed effects of such high surfactant concentrations on the Cr(1II) wave are relatively mild, the above data do demonstrate that the VDME is less sensitive to the inhibitory process than the DME. This insensitivity is further illustrated by the fact that the VDME response at 210 Hz is constant to +2.5% for 4.00mM Cr(II1) over the range of 0-0.2 % surfactant. Table I contains analytical data for Cr(1II) in these media. A third kind of inhibitory phenomenon which is observed in the presence of electroinactive adsorbates is wave splitting (8-10). This phenomenon has been observed with many systems and can apparently be due to a variety of causes (8, 11). An example of this phenomenon is provided by the electrochemical behavior of Cu(I1)-EDTA in tartrate buffer, Figure 2 and Figure 3. As can be seen, the addition of Triton X-100 in relatively large amounts not only suppresses the maximum, it also causes the single wave to split into two distinct waves neither of which is amenable to analytical use. The VDME response, on the other hand, is insensitive to the ( 8 ) Ibid., pp 299-302. (9) L. Meites and T. Meites, J. Amer. Chem. Soc., 73, 177 (1951). (10) R. W. Schmid and C. N. Reilley, ibid., 80, 2087 (1958). (11) S. G. Mairanovskii, “Catalytic and Kinetic Waves in Polarography,” Plenum Press, New York, N. Y . , 1968, pp 101, 102, 105.
30
,,I
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n
.-2E i
z 3
U
I t0.3
E vs. S
Voltammetry of Cu-EDTA at the VDME
4.00 mM Cu(II), 0.100M HBO4, O.lmM TBA (a) VDME, 160 cm, 210 Hz (b) DME, 60 cm
1.0 mMCu(I1)-EDTA,9.0 mMEDTA 0.1M tartrate buffer, pH 3.5, VDME, Mercury pressure 160 cm, 210 Hz Triton X-100 (a) 0 (b)0.009
presence of the surfactant and permits straightforward estimates of Cu(I1) concentrations. The last type of inhibitory phenomenon due to electroinactive species to be considered here is that in which the adsorbate causes the appearance of a minimum in the limiting current of the polarographic wave (8). This effect has also been observed by Folliard (12) on the oxygen wave in 0 . 1 M KCl in the presence of potassium oleate. Figure 4 illustrates the phenomenon with data obtained on Cu(I1) in dilute HzS04 with added tribenzylamine (TBA). Clearly, the DME response to this system is useless analytically. The VDME response, on the other hand, permits analytical measurements to be made. Adsorption of Electroactive Species. A common phenomenon which can prevent obtaining useful concentrationdependent response in electroanalysis is observed when the products of an electrode reaction are adsorbed on the electrode. For these adsorption waves, when the amount of reactant in solution is sufficiently small so that complete coverage of the electrode does not occur, the electrode response to concentration can be linear. Above this range, however, a plot of limiting current cs. concentration drops (12) J. T. Folliard, St. John’s University, Jamaica, New York, unpublished Work, 1968.
Table I.
Determinable Cr(II1)
Concentration range 0.5 -1O.OmM
a
Methylene blue
0.02-1. OmM
a
As(II1)
0.02-4. OmM
a
-1.5
CE,V
Figure 4. Elimination of adsorption inhibition at the VDME
Evr.SCE,V
Figure 3.
-0.9
-0.3
offto zero slope. Two systems which exhibit such behavior are examined here. The polarography of methylene blue (MB) offers a classic case of adsorption-limited current. This system exhibits two waves (Figure 5) which were interpreted by Brdicka (4) in this way. The first wave which occurs at -0.13 V is a socalled adsorption prewave. It increases with increasing MB concentration up to the 0.06 mMlevel. In this concentration range, the second “normal” wave is not observed. When the second wave does appear, its net wave height is not proportional to concentration but the sum of prewave and normal waves is linearly related to concentration. The prewave is said to be caused by the reduction of the dye to the adsorbed leuco form while the second wave is ascribed to a “normal” reduction process. Our polarographic data are in agreement with those of Brdicka. We also found that the total height of the two waves was proportional to concentration. Polarographic determinations using this total height are possible with a relative standard deviation of *2.1% over the range from 0.02mM to 1.OmM. The VDME response, shown in Figure 5, differs significantly from that of the DME. Here, the prewave is not resolved from the “normal” process at any concentration level. Instead, there is a sharp increase in current directly from the residual current level (which happens to be anodic in this case) with a rather gradual increase to the limiting current at about -0.25 V.
Analyses Possible with the VDME
Medium 0 . 1 M NaClOa, pH 3.1, 0.2z Triton
x-loo
0.05M KHzP04 0.05M Na2HPOa 1. OM HC1
Potential of measurement ... -0.20 v -0.50 v -0.80 V -1.3OV
Relative standard deviation *l.O%
*2,2z &2.2% &0.4% *1.1%
Data were obtained amperometrically.
VOL. 41, NO. 10, AUGUST 1969
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Figure 5. Elimination of adsorption inhibition at the VDME Methylene blue, 0.05M KH2P04,0.05M NazHPOl DME, 60 cm., O.lmM MB (b) VDME, 160 cm., 210 Hz, 1.OmM MB
0.0
(13) L. Meites,J. Amer. Chem. SOC.,76,5927 (1954). (14) E.G.Vasileva, S. I. Zhdanov, and T. A. Kryukova, Elektrokhimiya, 4,24 (1968). (15) W. M. Latimer, “Oxidation Potentials,” 2nd ed., PrenticeHall, Inc., Englewood Cliffs, N. J., 1952,p 114.
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Figure 6. Elimination of adsorption inhibition and plateau irregularities at the VDME 1.01 mM As(III), 1.OM HCl (a) DME, 60 cm (b) VDME, 160 cm, 210 Hz
(a)
At the VDME, concentration determinations are possible at potentials in both the prewave and the normal wave regions (See Table I). Essentially linear response is obtained in both regions up to at least 1.O m M MB. At the DME, on the other hand, the prewave current is a linear function of concentration only up to the 0.06mMlevel. Here again, the causes for the differences in response between the two electrodes seem to be the short drop life and the high rate of surface formation of the VDME. Because of these properties, the concentration of the dye at which total coverage of the surface (and thus inhibition of further reduction) can occur is much higher at the VDME. The gradual approach to the VDME limiting current as well as slight curvature in the limiting current us. concentration plots indicates the presence of some as yet unexplained complications but the views presented above seem to be essentially correct. Another example of an adsorption wave is observed in the polarography of As(II1) in HC1. This wave can be seen at about -0.4 V on the polarogram in Figure 6. Meites (13), who first reported this wave, found that above As concentrations of 0.9 mM, the height of this wave becomes constant. In view of the large maxima observed, surely useful data would not be obtainable elsewhere on the polarogram. These maxima are reported (14) to be catalytic in character; no tangential motions at the surface of the mercury are associated with them. The suppression of these catalytic effects at the VDME are consistent with the observations of Cover and Connery (1).
-0.4
Our experimental data obtained at the DME are in complete agreement with those of Meites. The recent results of Kryukova et al. (14), in which they could find no such adsorption limitation in acid solutions containing 0.1M LiCl, reflect the difference in double layer structure caused by the presence of lithium in the medium. The nature of the VDME response to this system is summarized by Curve (6) in Figure 6. Three waves are obtained. The first and last waves are particularly distinct and are usable for analytical determinations. The total limiting currents on the plateaus of the first and third waves are linearly related to As concentration up to at least the 4.0 mMlevel (Table I). Comparison of VDME with DME response to this system is revealing. No wave is observed at the VDME at the potential where the adsorption wave is detected by the DME. Furthermore, the VDME response showed no evidence of adsorption-controlled behavior within the limits of our experiments. This indicates that the overall process of the first DME wave probably has kinetic character as well as the adsorption complication postulated by Meites. There is apparently a slow chemical step, possibly protolysis of arsenious acid to AsO+ (1.9, preceding electron transfer which does not occur to any detectable extent during the short lifetime of the VDME drop. Similar behavior has been reported for the well-known kinetic wave of formaldehyde
(0. RECEIVED for review March 31, 1969. Accepted June 5 , 1969. Presented in part before the Division of Analytical Chemistry, 157th National Meeting, ACS, Minneapolis, Minn., April 16, 1969. The following support made this work possible: a 1968 Summer Fellowship Award from the ACS Division of Analytical Chemistry for J. G. Connery and a faculty summer research stipend from St. John’s University for R. E. Cover.
