teresting that a t XK = 0.0 M - l sec-l, I h y is simulated to be a linear function of XII. No experimental confirmation of this correspondence has as yet been made. Simulation of &, us. CBULKA. Simulated values of Ih, are shown plotted in Figure 7 for XK = 0.0 and 3.7 x 105 M - l sec-'. Also shown are values obtained experimentally. The agreement between experimental and simulated data is good. Light Intensity Profile. Uniformity of light intensity was assumed across the surface of the quartz disk for development of the digital program. The assumption permitted simplification of the calculations of mass transport and photochemical reaction in the zone of the quartz disk (r 5 R1) in that dCH/dr = 0. Two intensity profiles taken on perpendicular diameters across the surface of the quartz disk are shown in Figure 8. These profiles demonstrate that the assumption of uniform intensity is not strictly valid. The effect of the incorrect assumption is that experimental values of XK are too large, particularly a t low values of w . This follows since the rate of photochemical generation of electroactive intermediate near the outer edge of the zone of the quartz disk is actually less than the rate simulated. At low values of w , the main source of a chemically reactive electroactive intermediate reaching the surface of the ring electrode is the outer edge
of the zone of the quartz disk. Lack of funds prevented digital simulation based on measured intensity profile and the magnitude of error produced by the assumption has not been evaluated a t this time. CONCLUSIONS The experimental result of the determination of the rate constant for the photopinacolization of benzophenone in alkaline media is in good agreement with the literature values. The experimentally observed dependence of photocurrent on angular velocity of electrode rotation, light intensity, and bulk concentration of benzophenone are in excellent agreement with the simulated results. These findings Confirm the accuracy and applicability of the simulation program for interpretation of the results obtained using a rotating photoelectrode. ACKNOWLEDGMENT The authors are grateful to Val Peacock for assistance in experimentation. Received for review August 27, 1973. Accepted December 21, 1973. The authors are grateful for financial support of this research by Research Corporation and National Science Foundation (GP-18575).
Study of the Behavior of Copper Ion-Selective Electrodes at Submicromolar Concentration Levels W. J. Blaedel and D. E. Dinwiddie Department of Chemistry, University of Wisconsin, Madison, Wis. 53706
The behavior of a copper ion-selective electrode is investigated in the range 10-6-10-9M of copper ion. After cleaning by immersion in dilute H2S04, the potential response changes toward its equilibrium potential upon immersion in dilute Cu(ll) solution. The rate of change of potential is dependent upon the copper ion concentration. EDTA interferes, but 1 0 - 5 M concentrations of H+, Ca2+, Zn2+, AI3+, and Fe3+ interfere only slightly at the 10-7M level of copper ion. Possible analytical use for quantitation purposes is described.
The limit of detection of cupric ion activity in noncomplexing solutions using CuS-Ag2S cupric ion-selective electrodes is given in the literature as 10-8M ( I ) . This limit of detection is far too high to be explained by simple mechanisms involving the solubility of CuS and Ag2S alone. The same type of discrepancy is observed regarding the high limit of detection of silver ion or sulfide ion using the AgzS ion-selective electrode (J. W. Ross, p 57, Ref. 2 ) . Even when the hydrogen sulfide equilibria are taken into account a t pH 6, the solubility of CuS from the sensing electrode is calculated t o be around 10- 14M, far below the observed limit of 10--sM. In solutions which contain rela(1) Orion Research Inc.. Instruction Manual, Cupric ion Electrode Model 94-29 (1968) (2) R A . Durst. E d . , "Ion-Selective Electrodes," N a t . Bur. Stand. ( U S ) , Spec. Pub/. 314, 1969
tively large concentrations of complexing agents, potentials corresponding to free cupric ion activities as low as W20M are observed ( I ) . Such low activities are a t variance with the value of 10-sM observed in uncomplexed systems, and no satisfactory explanation of these discrepancies seems to have been given in the literature to date. In this paper, exploratory experiments and tests are presented on some factors which affect the response of a copper ion-selective electrode to submicromolar concentration levels of copper ion. Based on this work, an explanation is proposed to account for the electrode response in systems containing low concentrations of copper ion. The possible analytical usefulness of the ion-selective electrode for the measurement of submicromolar concentrations of copper ion is explored.
EXPERIMENTAL Apparatus. A commercial cupric ion-selective electrode and a single junction silver-silver chloride reference electrode (Model Nos. 94-29 and 90-01, Orion Research, Cambridge, Mass.) were used in all experiments. The electrode leads were connected to a Leeds and Korthrup specific ion millivolt meter (Model 7410). A Sargent Model SR recorder was used to record the meter output. All potential readings were made on solutions thermostated at 25 f 0.1 "C in a water bath (Model MR-3220A-1, Blue M Electric Company, Blue Island, Ill. ). Reproducible stirring was achieved by mounting the electrodes on a motor-driven holder arm which moved the electrodes back and forth through the solution over a l%inch path length at about 90 cycles per minute. ANALYTICAL CHEMISTRY, VOL. 46, NO. 7, JUNE 1974
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eters other than concentration. The rate of change of early potential was determined by drawing a tangent to the steepest part of the response curve, and calculating the slope in mV/min. A coarsely polished electrode surface was produced by wet-polishing by hand progressively with 23-pm, 8-pm, and 3-pm silicon carbide Ultralap (Pfizer, Minerals, Pigments and Metals Division, New York, N.Y.). A finely polished electrode surface was produced by wet-polishing progressively with 0.3-pm and 0.1-fim polishing alumina (Fisher Scientific Company, Fair Lawn, N.J.).
RESULTS AND DISCUSSION Figure 1. Steady-state response of the copper ion-selective electrode
Reagents. All solutions were made up with triply distilled water, prepared by redistilling tap distilled water, first from alkaline KMn04, and then again from dilute HzS04. AR grade chemicals were used without further purification for all solutions. Copper solutions were prepared by successive tenfold dilutions, beginning with a stock 0.1OM copper nitrate solution, obtained by dissolving 24.16 grams of Cu(N0a)z 3Hz0 and making up to 1 liter in a glass volumetric flask. Solutions from 0.010 to 10-5M were prepared by successive, stepwise 10-fold dilutions, starting with the 0.1OM stock solution in glass volumetric ware. Solutions of nominal concentration from 10- to 10- g M were also prepared by successive, stepwise 10-fold dilutions, starting with the 10- 5M copper solution, but in new Nalgene plastic volumetric flasks (50 and 500 ml). Each copper solution in the micromolar to nanomolar range was prepared in a separate volumetric flask that was used and reserved for that concentration level alone. It is important to point out that all copper ion concentrations given in this paper are nominal ones, representing concentrations added to the distilled water or electrolyte solutions. They do not include blank or background copper ion concentrations in the water or electrolyte solutions, which were too low to determine by simple atomic absorption (less than 10- T M ) . A check was made on the extent of loss of cupric ion through adsorption on the walls of the vessels used to prepare and store the most dilute solutions. New 500-ml plastic bottles which had been used to store 10-?M Cu(N03)z were rinsed for 10 minutes with 5 ml of 0.025M HzS04, and then the rinse was run through a Perkin-Elmer atomic absorption spectrophotometer. The level of copper present in the rinsings was not detectable. Calculations on the lower limit of detection of the AA unit showed that 1% adsorption would have been detectable in the 10- 'M solution bottle. For measurements in various supporting electrolytes, the copper nitrate stock solutions were spiked during preparations with concd HzS04 or solid KC1 to give electrolyte concentrations of 0.025M HzS04, 0.10M HzS04, or 0.10M KCI. Procedure. For the measurement of potentials, 100-ml portions of the solutions were placed in disposable polypropylene beakers (Tri-Pour, T. M. Sherwood Medical Industries, Inc.), which were segregated for use a t each concentration level. The beakers were suspended in the water bath by a rotatable platform support, which permitted easy immersion and removal of the electrodes. Between measurements on copper solutions, the electrodes were cleaned by rinsing with a stream of water from a wash bottle, followed by immersion in 0.025M HzS04, and then stirred until the electrode response reached -15.0 mV. (Selection of this cleaning potential is described later.) The electrodes were then removed from the acid, rinsed with water, blotted dry with a tissue, and then immersed in the solution to be measured. When not in use, the electrodes were stored dry in air. All potentials were measured after the calibration of the electrodes with 10-5M copper nitrate, the meter reading being adjusted to midscale for this solution, with midscale being taken as zero mV. When a series of solutions was measured, calibration was performed a t the beginning of the series and checked occasionally thereafter. When changes of the calibration point exceeded 0.5 mV, the measurements were rejected and repeated. Frequent calibration (3) or normalization ( 4 ) of ion-selective electrodes is used in order to improve their reproducibility and to minimize the effect of variations in potential-determining param(3) W . J. Blaedel, D. B. Easty, L. Anderson, and T. R. Farrell, Anal. Chem., 43,890 (1971). ( 4 ) J. Bagg and G. Rechnitz, Anal. Chem., 45, 271 (1973).
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Steady-State Response of Electrode to Supermicromolar Concentrations of Copper Ion. As shown in Figure 1, the behavior of the cupric ion selective electrode a t supermicromolar concentrations compares favorably with that reported in the literature ( I ) , giving a slope of 30.4 mV per decade for concentrations above 10-6M. Steady-State Potentials in Distilled Water Systems. Cleaning Procedure. It was observed over the course of nine separate experiments that the steady state potential of the electrode when immersed in triply distilled water was always within 5 mV of the asymptotic value of about -75 mV shown in Figure 1. Further experiments showed that both the potential obtained soon after immersion in water ( i e . , the early potential) and the rate of approach to steady-state depended greatly upon the mode of cleaning of the electrodes. For example, when an unpolished electrode was exposed to 10-5M Cu(N03)2, and then cleaned by immersion in distilled water, the early potential in water was -50 mV, and the -75-mV limit was approached within 1 hour. On the other hand, when cleaned for a few minutes by immersion and stirring in 0.025M HzSO4 after exposure to 10-5M Cu(NO3)2, the early potential in water was -160 mV, and the -75-mV limit was approached within 4 hours. The early potential and rate of approach to steady-state in water were found to be more reproducible when the electrodes were cleaned to a certain potential in 0.025M H2S04, rather than for a certain length of time, so this was adopted as standard cleaning procedure. For all data taken with the unpolished electrodes, standard cleaning procedure was first to rinse the electrodes in the holder arm with triply distilled water from the wash bottle, and then to immerse and stir in 100 ml of 0.025M HzS04 until a potential of -15.0 mV, measured in the sulfuric acid medium, was reached. With this procedure, the electrodes were cleaned sufficiently so that a distinguishable difference in response was obtained between 10-QMcopper nitrate and water. For data taken with the coarsely polished electrode (uide infra), -25.0 mV was selected for the cleaning limit. The time required for such cleaning was around 1-4 minutes, depending on what concentration of C U ( N O ~the ) ~ electrode had previously been in contact with. Figure 2 shows a series of response curves given by the coarsely polished electrode in 10-'M copper nitrate. Before taking each curve, the electrode was pretreated and cleaned, first by immersing in 10-5Mcopper nitrate for 1 minute (to calibrate the meter, and also to subject the electrode to a fairly high level of contamination), then rinsed for 10 seconds with distilled water, and finally cleaned either by immersion in distilled water for a specific time,, or by cleaning to a particular potential in 0.025M HzS04. The slopes of the curves in Figure 2 are explained as follows. It is postulated that some contaminating copper ion remains on the electrode after exDosure t o the 10-'M solution. The response curves with early potentials more positive than the steady-state potentials are indicative that the corresponding cleaning procedures did not re-
Or1
0 I
0
z
I
4 TIME, MINUTES
2
6 TIME, MINUTES
8 59
60 240 241
I
6
Figure 2. Dependence of electrode response to C u ( N 0 3 ) 2 upon extent of cleaning
Figure 3. Potential-time response curves for various copper ion
concentrations lO-’M
In each case, the electrode was exposed to 10-5M Cu(N03)2 for 1 minute before cleaning. Methods of cleaning are as follows: Curve A , 10second rinse with triply distilled water; Curve B, 5-minute immersion in triply distilled water; Curve C, cleaned to +25 mV in 0.025M H2SO4; Curve D. cleaned to 0.0 mV in 0.025M H2S04; Curve E, cleaned to -25.0 mV in 0.025M H2S04
move all of the copper ion contamination. The gradual decrease in early potential response with increased cleaning intensity is taken to mean that cleaning removes contaminant cupric ion from the electrode surface. In addition, however, intensive cleaning can actually reduce the surface concentration of copper ion below the equilbrium concentration, since the early potential in such cases falls below the equilibrium potential. On immersion of the cleaned electrode into water, the gradual rise of potential to the equilibrium value is interpreted as a buildup of the surface copper ion concentration to the equilibrum value. For the coarsely polished electrode, the response curves are not critically dependent on cleaning potentials below -25 mV. Thus, the standard cleaning potential of -25.0 mV was selected for the coarsely polished electrode rather than a more negative one in order to reduce the time required for cleaning. Similarly, a cleaning potential of -50.0 mV was selected for the finely polished electrode. The data of Figure 2 emphasize the need for adequate cleaning of the electrode in order to obtain electrode behavior which is not critically dependent on cleaning procedure. Effect of Polishing. The copper ion-selective electrode as received had undergone undefined previous use. Its surface was mottled with some shallow pits, and with a slight surface porosity that results from extended and heavy use. Measurements of early potentials and rates of approach to steady state in pure water solutions were repeated with the coarsely polished electrode. Compared to the unpolished electrode, magnitudes of the early potentials were not much affected, but rates of approach to steady state were noticeably increased. The rate of cleaning in 0.025M HzS04 was greatly increased. It was postulated that the polishing had removed a thin, porous surface layer of AgzS, left after leaching out of CuS through heavy use of the electrode. A study of the effect of polishing the Orion copper ionselective electrode has been reported by Johansson and Edstrom ( 5 ) ,with their effort directed toward response to supermicromolar concentrations of copper ions. In this work, polishing of porous electrodes was also observed to markedly improve the reproducibility and stability of the response. Dependence of Potential-Time Response Curves upon Copper Ion Concentration. It was observed early in this (5) G . Johansson and K. Edstrom. Talanta, 19, 1623 (1972)
work that both the early potential and the rate of. approach to the steady-state potential were dependent upon the copper ion concentration. With a well-cleaned electrode, considerable differences were noted between 10. 3 is a reproduction of reand 10-8M C u ( N 0 3 ) ~ Figure corded potential-time curves for the electrodes immersed to 10-9M Cu(N03)z. in solutions ranging from From Figure 3, it may be concluded that: (1) The steady-state potentials are not sensitively dependent upon copper ion concentrations below 1 0 ~ ~ M (2). The early potential and rate of approach to steady-state are sensitively dependent upon copper ion concentration down to nominal concentrations of 10-9M or lower. It is important to reemphasize that the early potential and rate of approach to steady-state are also critically dependent upon the extent to which the electrode has been cleaned in dilute sulfuric acid. Adsorption of Copper Ion by the Electrode. Qualitative tests were first performed to determine if the electrode adsorbed and carried copper ions. The electroactive surface was masked with Parafilm (American Can Company, Neenah, Wis.), and then immersed in a 10-3M Cu(NO3)z solution for 15 minutes. After removal from the solution, the electrode was rinsed with triply distilled water from a wash bottle, the parafilm removed, the electrode rinsed again from the wash bottle, and then placed in another batch of triply distilled water. The early potential, rate of approach to steady-state, and the steady-state potential were taken as indicators of carryover of cupric ions. The experiment was repeated with the non-electroactive surface masked. These experiments showed a definite tendency of the electroactive surface of the electrode, but not the inactive surface, to carry copper ions through a moderate washing procedure. The work also supports the need for proper cleaning with sulfuric acid to ensure thorough removal of copper ions from the electrode surface. Figure 4 is a series of potential-time curves, obtained by repeated immersion of the electrode in the same 100-ml batch of 10T7MCu(N03)2, with sulfuric acid cleaning between runs. The decrease in potential from curve to curve corresponds to a diminution of copper ion concentration in the solution as the number of immersions increases. These experiments are consistent with the hypothesis that copper ion is removed from solution by adsorption upon the electrode. Equilibrium Nature of the Steady-State Response. Figure 5 shows potential-time curves for electrodes that were cleaned to different extents before immersion in Cu(NO3)p solutions ranging from to 10-7M. For curves approaching steady-state from more negative potentials, standard cleaning procedure for the coarsely polished electrode was used. For curves approaching steadystate from more positive potentials, the electrode was ANALYTICAL CHEMISTRY, VOL. 46, NO. 7, JUNE 1974
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0-
-1
> I
J-
9
z -50w
2 3 4 TIME, MINUTES
I
0
1
560
61
Figure 5. Approach to steady-state potential in dilute copper solutions I
0
1
I
I
I
2 3 4 TIME, MINUTES
I 5
Figure 4. Removal of copper ion from solution by adsorption on the electrode Lettered curves represent successive immersions of the electrode in the same 100-ml batch of lO-'M Cu(N03)2, with sulfuric acid cleaning between immersions. Curve labeled H20 represents final immersion in triply-distilled water
simply rinsed with triple-distilled water from a wash bottle between immersions. The data show that the steadystate potential can be approached from potentials corresponding to either higher or lower surface copper concentrations than the steady-state concentration, and that the steady-state therefore fepresents a state of true chemical equilibrium. In several runs, the stirring rate at steady-state was stepped between 90 and 300 rpm, in an attempt to find if the potential was dependent on stirring rate. No significant effect was observed, indicating further that the steady-state potential is truly an equilibrium potential. This independence of the steady-state potential is at variance with the behavior of the Orion fluoride electrode, whose response depends on stirring a t low ionic strengths, but not a t high ionic strengths (6). Nature of the Rate-Controlling Processes T h a t Affect the Potential Response of the Electrode. An experiment was performed to show that the long time required to reach steady-state in dilute copper ion solutions was not simply due to slow saturation of the bulk of solution by dissolution of the electrode material. Potential-time response curves were recorded for the electrode immersed in 10-7M C u ( N 0 3 ) ~solutions with volumes of 50, 100, and 200 ml. The response curves all showed the same rate of approach to steady-state potential and were all within a half millivolt of the same potential a t the end of 10 minutes. If bulk saturation of the solution had been the controlling factor in potential response, the rate of approach to steady state should have been significantly different for the three different volumes. A 10- ?M Cu(N03)Z solution was chosen for these measurements as an intermediate concentration whose rate of approach to steady-state was of a magnitude convenient to measure. In another experiment, shown in Figure 6, the electrode potential was permitted to progress toward steady state in 10-?M Cu(NO3)z. After 5 minutes, the electrode was removed and transferred to a fresh 100-ml portion of 10-'M Cu(N03)2, about 2 seconds being required for the transfer. Only a slight dislocation in the response curve was noted when the electrode was transferred, indicating that saturation of the bulk solution by the dissolution of electrode material is not a controlling factor in the slow rate of approach to steady-state. The rate-controlling process must ( 6 ) K . Srinivasan and G.A. Rechnitz, Anal. Chem., 40, 509 (1968)
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A N A L Y T I C A L C H E M I S T R Y , VOL. 46, N O . 7 , J U N E 1974
0
1
2
3
4 5 6 7 TIME, MINUTES
8
9
1
0
Figure 6. Approach to steady-state potential in 1 0 - 7 M C u ( N 0 3 ) ~
be associated with the electrode surface itself, rather than with the bulk of the solution. At potentials removed from the steady-state, the stirring rate definitely affected the rate of approach to the steady-state potential, indicating that a liquid film mass transfer process definitely influences the overall rate process. Effect of Air. In additional experiments, it was shown that the small hump in Figure 6 becomes larger when the electrode is allowed to be in contact with air for longer periods of time during the transfer from one solution to the other. For a 15-minute air exposure, a large hump is obtained (as much as 30 mV above steady-state potential), indicating that free Cu(I1) is generated a t the electrode surface in air. This is in line with general knowledge that moist insoluble metal sulfides like CuS are slowly oxidizable to soluble sulfates by air. In addition, Smith and Manahan (7) have noted severe instability of the copper ion-selective electrode in the presence of oxidizing agents, and for the attainment of stable potentials, they make their measurements in a medium containing a mild reducing agent (formaldehyde). All of the data in this paper were obtained without control of air oxidation. Any method for the quantitation of cupric ion a t submicromolar concentration levels would probably require a t least some control of air oxidation during the measurements. CONCLUSIONS Hypothesis to Explain the Response of Copper Ion-Selective Electrodes at Submicromolar Concentration Levels. On the basis of the preceding exploratory work, it is postulated that a t very low concentrations, there is a surface concentration of copper ions a t the electrode that is in adsorption or ion-exchange equilibrium with the bulk concentration of copper ions in solution. It is the surface concentration that determines the electrode potential. This concept is similar to the hypothesis of Hansen, (7) M . J. Smith and S. E. Manahan, Anal. Chem., 45, 836 (1973)
Table I. Effect of' Interferences on the Rate of Change of Early Electrode Potential Cation (1O-SM) added to 10-:M Cu(N0a)r
Rate of change of early potential, mV/min
None
38 25 23 18 77 62
K+
Ca2+ Zn2+ ~ 1+ 3
Fe3 +
Indicated copper ion molarity
1.0 x 6.0 X 5.4 x 3.9 x 2.1 x 1.7 X
10-7 10-8 10-8 10-8 10-7
lo-;
10-9
Lamm, and Ruzicka (8) that the ion-sensitive surface alone determines the response of the electrode. Pungor and Toth (9) have also demonstrated the importance of surface ion-exchange reactions in the response mechanism of ion-selective electrodes. When the bulk concentration of copper ions is changed, the rate a t which adsorption equilibrium is achieved is slow-minutes to hours-and the rate controlling process, therefore, probably involves slow reactions or rearrangements in the electrode surface itself. At very low concentrations of copper ion, the adsorption equilibrium is predominant, and the electrode potential is related to the bulk concentration through the adsorption equilibrium, which determines the surface concentration and the electrode potential. At high concentrations (supermicromolar), the adsorption process reaches saturation and is no longer predominant in determining the surface concentration of copper ion, which approaches the bulk concentration. In this case, the electrode potential depends upon the bulk concentration of copper ion according to the Nernst equation, with an increase in potential of 30 mV per decade increase in Concentration. This hypothesis is consistent with the previously described exploratory experiments. However, it is only tentative, and further work is needed to test it and to elucidate the nature of the ionic adsorption process. Analytical Promise. The mechanism of the response of the copper ion-selective electrode to submicromolar concentrations of copper ion is not clear. However, the dependence of both potential and rate of approach to steadystate upon copper ion concentration, time, and preparatory cleaning of the electrode is clear and reproducible, and indicates analytical promise for the determination of submicromolar concentrations of copper ion.
(8) E. H. Hansen, C. G. Lamm, and J. Ruzicka. Ana/. Chim. Acfa, 59, 403 (1972) (9) E. Pungor and K. Toth, PureAppl. Chem., 34,105 (1973).
10-8 10-7 NOMINAL COPPER MOLARITY
10-6
Figure 7. Dependence of the rate of change of early electrode potential on copper ion concentration
Figure 7 is a working curve taken with the finely polished electrode which shows the dependence of the rate of change of early potential on copper concentration. The solid line is the average of four runs, with the individual data points shown. Correction for the rate of change of early potential with the electrode immersed in triply distilled water (the blank) straightens the curve considerably in the lower concentration ranges. Table I shows the effect of various cations on the rate of change of response potential in a 10-7M copper solution. Even with the 100-fold excesses of the other cations, the error in copper concentration is only about 2-fold in the worst case. Apparently the rate process that affects the rate of change of early potentials in the submicromolar concentration region is similar in selectivity to the equilibrium process that affects the equilibrium potential in the supermicromolar region. The concentration-dependent phenomena observed in the submicromolar concentration region are rate phenomena, and their behavior indicates that measurements in a deaerated flowing system might provide the easiest and most reproducible utilization for analytical purposes. Such a system is presently being constructed. ACKNOWLEDGMENT We wish to acknowledge the cooperation of D. R. Keeney of the U. W. Soils Department for the extended use of his electrodes. Received for review October 9, 1973. Accepted January 24, 1974. This research was supported in part by an Office of Water Resources Reseach Grant, No. A-053-WIS. DED is a member of the Air Force Institute of Technology Education Program.
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