Anal. Chem. 1992, 64, 1769-1776
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Stripping Voltammetry of Metal Complexes: Interferences from Adsorption onto Cell Components Jose M. Diaz-Cruz and Miquel Esteban' Department of Analytical Chemistry, University of Barcelona, Av. Diagonal 647, 08028 Barcelona, Spain
Marc A. G. T. van den Hoop and Herman P. van Leeuwen Laboratory for Physical and Colloid Chemistry, Wageningen Agricultural University, Dreijenplein 6, 6703 HB Wageningen, The Netherlands
The Influence of the adwrptlon of metals by cell components on the voltammetrlc study of metal complexes was studled for the systems Zn/PMA and Cd/PMA (PMA, poly(methacry1k acid)) at dMerent metal concentratlon levels. Glass, poly(fluoroethykne), Teflon, polymethacrylate, nylon, and polystyrene celk were tested In a wlde range of pH values. The rorults of thk work allowed the establlshment of general guldellnes and speclflc procedures to mlnlmlze the effect of adsorption In the study of such systems.
INTRODUCTION Adsorption of metals onto sampling, storing, and measuring containers has been recognized as a big problem for determining total concentrations in environmental and analytical investigations, and it has been extensively studied, mainly by radiochemical methods.lV2 To avoid this problem, samples are, for example, often stored in Teflon or polyethylene containers at a very low pH and low temperature (4 "C). In metal speciation studies, however, the distribution of heavy metals over the different species in their natural environment is of interest. Changing the conditions of the sample solution changes the original speciation and is therefore not allowed. Voltammetry, especially anodic stripping voltammetry (ASV), which has been shown to be an important tool in environmental metal speciation studies,3-5 allows direct speciation measurements in samples with heavy metal ion concentrations down to the sub-ppb levels without altering the ionic solution chemistry. Unfortunately, adsorption of heavy metals can seriously affect voltammetric responses, as indicated by Davison et al.6 They have investigated the ASV response of Pb2+, Zn2+, and Cd2+ at a concentration level of 5 X 10-8 mol L-1 over a certain range of pH and ionic strength values. It was found that the current decreaseswith increasing pH in a pH range where hydrolysis is not significant. This finding is not strange, considering that the surface of the commonly used glass cells is negatively charged due to the dissociation of the surfacial silica groups at pH higher than 3. The negative charge substantially increases with pH
* To whom correspondence should be addressed.
(1) Zief, M.; Mitchell, J. W. Contamination Control in Trace Element Analysis; Wiley-Interscience: New York, 1976. (2) Benes, P.; Majer, V. Trace Chemistry of Aqueous Solutions: General Chemistry and Radiochemistry; Czechoslovak Academy of Sciences: Prague, 1980. (3) Florence, T. M.; Batley, G. E. CRC Crit. Rev. Anal. Chem. 1980, 9,219-296. (4) Florence, T. M. Talanta 1982,29, 345-363. (5) Florence, T. M. Analyst 1986, 111,489-505. ( 6 ) Davison, W.; de Mora, S. J.; Harrison, R. M.; Wilson, S. Sci. Tot. Enuiron. 1987, 60, 35-44. 0003-2700/92/0364-1769$03.00/0
and so does the attractivity for counterions. Because of their charge, divalent heavy metal ions are bound preferentially as compared to the often used monovalent cations from the supporting electrolyte. For the analysis of natural samples, which normally contain very low concentrations of heavy metals and have a slightly acidic or sometimes even basic pH, the glass cells seem to be a priori not very suitable for voltammetric experiments. Under slightly acidic conditions, the surface of cells made of hydrophobic organic polymers is also negatively charged due to the adsorption of OH- ions or by dissociation of functional groups in the surface. As a consequence, such polymeric materials are also able to adsorb heavy metals under natural water conditions and are also a priori unsuitable for voltammetric speciation studies. Irrespective the nature of the container material, metal adsorption shall be affected by the presence of (supporting) electrolytes and/or complexing agents. Cations from the supporting electrolyte compete with the divalent heavy metal ions for adsorption, resulting in a decrease of the heavy metal adsorption with increasing electrolyte concentration. Adsorption will generally decrease with increasing concentration of complexing agents because of the competition between the adsorption and the complexation processes. The time dependence of the adsorption is of practical importance because, for the low concentrations considered, typical equilibration times are not small as compared to the usual duration of complete voltammetric experiments (on the order of 10 min). Adsorption kinetics are determined by the rate of the two steps involved: (i) the transport of the ion to the adsorbent surface, and (ii) the interfacial adsorption process itself. In fact, for divalent heavy metal ions, the very adsorption step ii seems to be fast, although it may be followed by a certain diffusion toward deeper layers of the adsorbent. For most practical systems, however, the kinetic features are complicated,' and most studies are referred to the equilibrium. The aim of the present paper is to study the adsorption behavior of heavy metals onto various cell surfaces in the presence of macromolecular homofunctionalligands as models for natural complexing agents. This necessarily includes a study in the absence of ligand in order to find conditions to minimize adsorption and in order to simplify the treatment of the problem by comparison between data obtained in the presence and in the absence of ligand. The voltammetric response of solutions containing either Zn(I1) or Cd(I1) ions has been studied by differential pulse anodic stripping vol(7) van Leeuwen, H. P. Dynamic Aspects of Metal Speciation in Aquatic Colloidal Systems. In Environmental Particles;Buffle, J., van Leeuwen, H. P., Eds.; IUPAC Environmental Analytical Chemistry Series, Vol. 1; Lewis Publishers: Chelsea, MI, 1992.
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tammetry (DPASV)for metal ion concentration levels varying from 10-5 to 10-7 mol L-1 over a range of pH values for different types of cells. For the Zn, Cd/poly(methacrylic acid) (PMA) systems, DPASV-monitored complexation experiments have been carried out under a variety of experimental conditions. Adsorption effects will be quantitatively accounted for, and some general guidelines will be extracted from the data.
THEORETICAL SECTION
4 vs CL*data. In order to determine K values, the first part of the obtained complexation curve (relatively low CL*values) appears to be the most adequate, whereas for determining t (= D M ~ D M the ) region at high CL*values is the most informative.lo By using the equations above, and if p and c are known, it is possible to get expressions for the fractions of free and complexed metal with respect to the total concentration in solution (cT*)
The basic scheme of a metal ion, M, which (i) can be adsorbed onto the cell and electrode material (Mad),(ii) can be reversibly reduced to the metal atom MO on the mercury electrode, and (iii) can associate with a ligand, L, to give the electroinactive complex, ML, can be summarized in the form Still, for a labile system without adsorption phenomena and with a large excess of L, K can be related to the potential shift of the peak (AE,) through the equationegg
Maa
11
M+L=ML
(nF/RT)AE,= -In (DIDM)’- In (1 + KcL*)
which, written in terms of the Leden function of order zero,
M0
If adsorption is not present or negligible, the total bulk concentration of metal in solution (cT*)is essentially constant and equal to the s u m of the M and ML bulk concentrations (CM* and CML*, respectively) = cM* + cML*
(1) If adsorption cannot be neglected, a part of the metal initially present in solution, (cT*)o, is lost onto the cell surface. In this case, and with Ac,d denoting the decrease in CT* cT*
(CT*)O
+
+ AC,d
= CM* CML*
= CT*
+ ACad
(2)
Dividing CM*, CML*, and h a d by (cT*)o, one obtains the corresponding fractions of free M, ML complex, and adsorbed M, denoted as f ~~ M, L and , fad, respectively. In the stripping voltammetric study of these systems it is useful to define the normalized peak current 4 as the ratio between the peak currents obtained for a M solution in the presence and in the absence of L
4 = Z,(with L)/I,(without L) = I/Io
(3)
In the absence of adsorption, for the case of a labile complex and a large excess of L, 4 can be related to the complexation constant K through the equation819 4 = (D/DM)’ = (1 + &cL*)’/(l
+ KcL*)’
(4)
where
Fo,leads to a linear relationship between FOand CL* with a slope equal to K
Fo = exp[-(nF/RT)AE, - In 41 = 1 + KC,*
= DML/DM
K = cML*/cM*cL*
(5) (6)
(7) with Di and D denoting the diffusion coefficient of species i and the mean diffusion coefficient of the complex system, respectively. The parameter p depends on the nature of the mass transport during the preelectrolysis step; it is l/2 for semiinfinite linear diffusion and 2/3 for laminar convective diffusion. Experimentally, the moat convenient procedure to determine c,p, and K (by using eq 4) is through successiveadditions of the ligand L to a metal ion solution, which yield a set of (8)de Jong, H.G.; van Leeuwen, H. P.; Holub, K. J . Electroanal. Chem. Interfacial Electrochem. 1987,234,l-16,17-29; 1987,235,l-10. Caeassas,E.; de Jong, H.G.; van Leeuwen, H. P. Anal. (9)Esteban, M.; Chrm. Acta 1990,229,93-100.
(11)
When stripping voltammetric measurements are carried out in a M/L system in the presence of adsorption, the application of eqs 4-11 becomes more involved. This is not the case for peak potentials. Equation 10 shows that the potential shift (AE,)is determined by the ratio CM*/CT* in solution (through D and KcL*),and this ratio is maintained essentially constant if an excess of L is present. Measuring the peak current in the presence of L (Z) is not a problem, either. It must be done a t a pH value which is fixed by the system itself. However, it is not clear at which pH the current in absence of L (Io) should be measured, and this causes a big uncertainty in the calculation of 4 because of the adsorption-controlled pH dependence of Io. From a practical point of view, the test of IOshould be done in order to maintain the applicability of eqs 4 and 11. These equations were obtained by assuming that the intensity measured in a M/L system is proportional to the product of the total concentration of M in solution and the mean diffusion coefficient of the system to the power p899 (12)
Io: P c T * 6
(10)
By assuming that the proportionality factor only depends on voltammetric conditions, eq 12 shows that eqs 4 and 11 are valid only if I and Io refer to the same CT* value. This can be realized by using in these equations a corrected 4 function calculated in the form
where I1 is the peak current measured in absence of L at a pH where adsorption is absent and is the peak current measured after changing the MIL solution for a noncomplexing medium (the supporting electrolyte without M, and at a pH value where adsorption is not noticed). This approach allows the calculation of the metal fractions from current measurements: ~
(10)van den Hoop, M. A. G . T.;Leus, F. M. R.; van Leeuwen, H.P. Collect. Czech. Chem. Commun. 1991,51,96-103.
ANALYTICAL CHEMISTRY, VOL. 64, NO. 17, SEPTEMBER 1, 1992
In the absence of ligand, f~ is just (1 - fad). The f~ vs pH plots are then useful to determine the pH ranges in which adsorption takes place and also to compare the behavior of different ion solutions in contact with different materials. Plots of f~ vs t (although also useful for comparisons) are especially adequate to get information about adsorption kinetics and to estimate the time necessary to reach the equilibrium. In the presence of ligand, f~ vs CL*,~ M vs L CL*,and fad vs CL*are the most interesting dependences, in order to know how L affects the adsorption of M. For convenience we finally defined 41
= III,
42 = I/&
(18)
with I and I1 as before, and I2 denoting the peak current measured in the absence of L a t a pH value equal to that attained in the presence of L. The normalized peak current 41 verifies eqs 4 and 11 when the addition of L supposes a total desorption of any adsorbed M. The same is true for +z when L is not able to desorb the adsorbed M at all (compared with a solution of M). As a consequence, 41 I 4,,,, I 4z, provided the ligand itself is not adsorbed so that it is unable to induce metal adsorption.
EXPERIMENTAL SECTION Chemicals and Instrumentation. Adsorption experiments were carried out by using cells made of different materials. Glass cells (provided by Metrohm),poly(fluoroethy1ene)cells (provided by EG&G PAR) and homemade cells of Teflon, polymethacrylate, nylon, and commercial ordinary polystyrene were tested. The poly(methacry1ic acid) solutions (PMA) were obtained from BDH. The average molecular mass, according to BDH, was 26 OOO g mol-'. Stock solutions of approximately0.1 mol L-l (in monomeric units) were prepared by dilution with water, and the total amount of carboxylic groups was determined by conductometric acid-base titration. These solutions were stored in the dark at 4 "C. Buffer stock solutions of different pH values were prepared by addition of the required volume of nitric acid (Merck a.r.) to solutions containing tris(hydroxymethy1)aminomethane(Tris) (Merck a.r.) and diluting with water until a total concentration 0.05 mol L-l in Tris. Titrisol potassium hydroxide solutions (Merck a.r.) were used in the conductometric titrations and for the partial neutralization of PMA. All other reagents employed were Merck a-r.: nitric acid, zinc(I1)nitrate, cadmium(I1)nitrate, and potassium nitrate (used as a supporting electrolyte). Water was obtained from a Millipore Super-Q system or a Culliganwater purification system. Differential pulse anodic stripping voltammograms were obtained with (i) a Metrohm 663 VA stand controlled by a homemade "Quick Step" polarograph (Wageningen)attached to a Hewlett-Packard 3497A data acquisition unit and a HewlettPackard 85B personal computer or (ii) a Metrohm 646 VA Processor attached to a Metrohm 647 VA stand. System i was also connected to a Metrohm 665 Dosimat for the automatic addition of PMA solutions and to a Knick Multi-Calimatic pHmeter for the pH measurement after each addition. System ii was attached to a Metrohm 665 Dosimat and to an Orion SA 720 pH-meter. In all cases,working, reference,and counter electrodes were HMDE, Ag/AgCl,KCl..,, and glassy carbon, respectively. Glassy carbon was preferred because of the notorious adsorption of metal ions onto the Pt counter electrode in ASV measure-
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ments." Pulse durations of 25 ms (system i) and 40 ms (system ii), pulse heights of 50 mV, and deposition potentials of -1150 mV for Zn and -850 mV for Cd were employed. The preelectrolysis time and the rest period observed were 1 or 2 and 0.5 min, respectively, and the scan rates in the stripping step were 4 mV s-l (system i) and 10 mV s-l (system ii). Measurements were done at 25 "C. Purified nitrogen was used for deaeration of the sample solutions. Conductometrictitrations of the PMA solutions were done at 25 "C with a Wayne Kerr B905 automatic precision bridge attached to a HP85 personal computer and a 665 Dosimat, or with an Orion 120 conductometer coupled to a 665 Dosimat. Procedures. (Beforeeach experiment,cellswere treated with HN03(1:l) during 24 hand, after that, with a solution containing KN03 at the same concentration to be used in the experiment and at a pH value close to 4.) ( a ) Measurement of the Dependence of the Voltammetric Signal on pH (ficr us pH Plots). Solutions containing zinc(I1)or cadmium(I1) nitrate, KN03 (0.01 mol L-l if not indicated otherwise),and the required amount of Tris and HN03to fix the initial pH value of the solution are titrated with KOH solutions. In somecases,Tris is not used, and pH is fixed only by the addition of diluted HN03. Simultaneous measurement of voltammograms and pH is done in the beginning of the titration and after each addition. ( b ) Measurement of the Dependence of the Voltammetric Signal on Time ( f ~us t Plots). First some blank voltammograms are recorded for a pH value fixed such that noticeable adsorption is absent (pH < 5). Then a small volume of concentrate Tris buffer solution is added to reach a given pH, and some voltammograms are recorded at intervals of 6 min. ( c ) Voltammetric Titrations. Solutions of PMA are partially neutralized with KOH to a certain degree of neutralization, an. Solutions containing zinc(I1)or cadmium(I1) nitrate and KN03 (0.01 mol L-l if not indicated otherwise) are prepared at an acidic pH (lower than 5) by addition of HN03, and they are placed in the voltammetric cell. A t this point, some voltammetric measurements are done (in order to obtain Il). By means of KOH addition, the pH is adjusted as closely as possible to the pH of the titrant, and voltammetric measurements are done until the signal becomes stable and reproducible (in order to obtain I*). Then, aliquots of the titrant PMA solution (containingthe same KN03 concentration) are added to the metal ion solution, and the DPASV voltammograms are recorded after each addition (in order to obtain I). Peak currents are corrected for dilution due to the addition of polymethacrylate solution. The complete procedure is automatized. ( d ) Exchange Experiments. The exchange experiments are conducted in three steps: Step a. Blank solutions of zinc(I1)or cadmium(I1) nitrate, at 0.01 mol L-l KN03 concentration and at pH < 5, are prepared as described before, and several voltammetric measurements are made. In this way, the current value 11is obtained. Then, the pH is fixed at a value close to that of the titrant PMA solution by using KOH or Tris buffer solution. New voltammetric measurements yield 1 2 values. Step b. The necessary amount of PMA solution (alsoprepared as described before) is added, and new measurements are done, which yield I values. Step c. The solution is removed, and the cell and the electrodes are rinsed with pure water. Immediately after rinsing, a solution containing only KN03(at the same concentration and acidic pH
