Study of cadmium complexation at high ligand concentrations

Marine Chemistry, 28 (1989) 227-239 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

227

Study of Cadmium Complexation at High Ligand Concentrations* M. B R A N I C A I,I.P I Z E T A I,G. B R A N I C A - J U R K O V I C

2 and M. ZELI(~ ~

~Center for Marine Research Zagreb, "Rudjer Bo~kovi~" Institute, POB 1016, 41001 Zagreb, Croatia (Yugoslavia) 2Institute for Medical Research and Occupational Health, POB 291, 41001 Zagreb, Croatia (Yugoslavia) (Received February 13, 1989; revision accepted July 6, 1989)

ABSTRACT Branica, M., Pi~eta,I.,Branica-Jurkovid,G. and Zelid,M., 1989. Study of cadmium complexation at high ligandconcentrations.Mar. Chem., 28: 227-239. Using square-wave voltammetry, cadmium complexation with chloride ions and small organic molecules has been studied at high ligandconcentrationsand high ionicstrengths,i.e.conditions that can be found or expected in nature (seawater,body fluids,etc.) Over a relativelywide range of ionic strengths (I= 0.5-6 tooll-i) the number of chloridecomplexes found and theirstabilityconstants were in a very good agreement with the literaturedata. With allorganic ligands (glycine,diglycineand triglycine),three consecutive complex species have been found at I--0.1 tooll-i instead of two as published previously.The firststabilityconstants were in satisfactoryagreement with literaturevalues. All measurements have been performed over ligand concentration ranges as wide as possible. Only in such a case can allthe complexes be detected and theirstabilityconstants calculated.

INTRODUCTION

Nowadays, marine chemists are not satisfied with determination of total metal concentrations only. Although such data can be important, they do not give any information on speciation, i.e. the forms in which a dissolved metal really exists in a natural water. However, each form, with its own chemical and toxicological properties, can be important for the understanding of biogeochemical cycles and the prediction of pollution effects (Bernhard et al., 1986). At the present 'state of the art' direct measurement of all of the species is not possible. However, using available information on total metal and ligand con*Presented at the 10th International Symposium "Chemistry of the Mediterranean", May 1988, Primo~ten, Yugoslavia.

0304-4203/89/$03.50

© 1989 Elsevier Science Publishers B.V.

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M. BRANICA Eft' AI,

centrations, together with the corresponding stability constants, one can calculate the distribution of any metal in a complex system, supposing that all possible interactions are included. Of course, all the values taken into account should be correct and this is difficult to achieve. Working on the same simple systems different investigators have managed to find different numbers of complexes and corresponding stability constants depending upon the experimental conditions. According to Goldberg (1986), as much as 80% of the published stability constants are wrong. This is one of the reasons why we have tried to reinvestigate cadmium complexation with chloride ions and small organic molecules (glycine, diglycine and triglycine ), and searched for the origin of the discrepancy mentioned above. Cadmium, as one of the most toxic metals, has been studied intensively, especially its concentration, distribution and speciation in natural waters (Raspor, 1980 ). The chloride ion, which is the most abundant inorganic ligand in seawater, is a very important complex-forming agent; amino acids and small peptide molecules can be treated as very simple models in the study of metalprotein interactions. Voltammetric methods are widely used for the study of metal ions in aqueous systems because they can give information on total metal concentration (Niirnberg, 1984) at trace levels, and simultaneously differentiate between labile and inert complexes (Florence, 1986). They are also very convenient for studies in model systems and for determination of stability constants (DeFord and Hume, 1951 ). In comparison with other voltammetric methods, squarewave voltammetry, used in our experiments, has the additional advantage of high speed (Osteryoung and Osteryoung, 1985)~ EXPERIMENTAL All chemicals used were of analytical grade, produced by Merck (Darmstadt), Ventron (Karlsruhe) and Fluka (Buchs). Deionized water obtained in a Millipore Milli-Q column deionizer was used for solution preparation. The measuring system consists of a polarographic analyser (PAR 384B ), the corresponding static mercury drop electrode assembly (PAR 303A) and a technical computer (HP 9816S) together with a double disc drive (HP 9121D ), a graphics printer (HP 82986A) and a six-pen plotter (HP 7475A), as described earlier {Branica et al., 1986 ). A software package '384-ALLMET' was developed for communication between the computer system and PAR 384B (with the corresponding cell) for graphical presentation and evaluation of electrochemical data (Pi~eta and Branica, 1988). Square-wave voltammetric measurements were performed in a three-electrode system consisting of hanging mercury drop, platinum wire and saturated Ag-AgC1 electrodes in a solution which had been deaerated by nitrogen bubbling for 15 min.

CADMIUM COMPLEXATION AT HIGH LIGAND CONCENTRATIONS

229

For the study of cadmium complexation with chloride ions the experiments were designed as follows. Starting with metal ions in pure perchlorate solution, the peak potential was measured after successive additions of the chloride solution until the concentrations of the two anions became equal. A new solution of cadmium ions in pure chloride medium was then placed in the polarographic cell and measurements after additions of pure perchlorate solution were repeated until the concentrations of the two anions became equal. Ionic strength, pH value and metal ion concentration were kept constant throughout the measurements. Experimental details of cadmium complexation studies, including organic ligands, have been published elsewhere (Branica-Jurkovid and Simeon, 1989). TREATMENT OF DATA When a metal ion forms more than one complex with some ligand M+L=ML

(1)

M+2L=ML2

(2)

or in general M+nL=ML,

(3)

each of the complexes can be characterized by its overall stability constant, which is defined as ft.

[ML,] [M] [L]"

-

(4)

If all the complexes are labile and if the metal ion can be reduced or oxidized reversibly it is possible to calculate stability constants by means of the wellknown equation (DeFord and Hume, 1951 ) Fo=exp ~--~AEp+ln =

-

= 1 + ~ B.[LI"

(5)

(6)

after measuring the half-wave or peak potential shift as a function of the ligand concentration. In the expressions above, Epf stands for the peak potential in non-complexing medium and Epc corresponds to the peak potential in complex-forming solution. F, n, R and T have their usual meaning: Faraday, number of electrons, gas constant and thermodynamic temperature, respectively. The last term in parenthesis corresponds to the ratio of the peak heights in a solution without

230

M. BRANICA ET AL

ligand and one in which complexes are present. It was omitted in our calculations. Equation (5) can be solved either graphically, as was usually practised in the past, or by curve-fitting using a computer program (Pi~eta et al., 1987). In the latter case the polynomial that best fits the experimental points should be found. The degree of such a polynomial then gives the number of complexes, and its coefficients correspond to the stability constants. The criteria for judgement of 'the best-fitting function' among all examined possibilities are as follows: (1) for obvious reasons it is necessary that all coefficients ,8, should be positive; (2) if two functions with positive coefficients are possible, that with a better variance of fit is chosen; (3) a coefficient with a confidence interval greater than the coefficient itself is taken as not being significantly different from zero, supposing that the accuracy of measurement is satisfied. RESULTS AND DISCUSSION

Complexation with chloride Results of cadmium ion complexation in the chloride solutions ( I = 4 mol l- 1) are presented in Fig. 1. The points were measured experimentally and the curve was constructed by the a b o v e - d e s c r i ~ procedure, using the computer

-3

-2

-I

0

.

.

.

.

Log IcL~ Fig. 1. C a d m i u m peak potential shifts in chloride medium. Total metal concentration: 5 X I0 -s tool I- ~,ionic strength: 4 tool I- i,pH-- 2.

CADMIUMCOMPLEXATIONAT HIGH LIGAND CONCENTRATIONS

231

TABLE 1 Stability constants of cadmium chloro complexes as a function of the ligand concentration range (I=4 tool 1-1, p H = 2 )

[Cl ] * 0.004-4.0 0.004-1.0

log fll

logf12

logf13

logf14

Reference

1.66+0.1

2.4 +0.1

2.8 +0.3

2.2+0.3

SmithandMartell(1976)

1.57+_0.07 1.57 +_0.03

2.42+_0.07 2.39 +- 0.04

2.66+_0.06 2.73 +- 0.03

2.0+_0.1 -

T h i s work T h i s work

*Concentration range not defined.

program developed in our laboratory (Pi~eta et al.,1987). Four stabilityconstants calculated in this way (Table 1 ) are in very good agreement with values published in "Critical Stability Constants" by Smith and Martell (1976). However, such resultscan be obtained only ifthe chloride concentration range is as wide as possible. In general, the lowest ligand concentration should be one that stillgives peak potential shift whereas the highest one is limited by the value of the ionic strength or ligand solubility.However, if only experimental points measured at lower chloride concentrations are taken into account, i.e.if [CI] ~<1 tool 1-1, three stabilityconstants instead of four are obtained. This simple example (Table 1) indicates how the complex with the highest ligancy, ifnot present at a high percentage, can easilybe overlooked. Measurements at high ligand concentrations are especially important iffin is lower than fiN- i because in such a case the highest complex is never present at the m a x i m u m concentration (100%). In the previously described example of cadmium complexation with chloride (f14< P3 ),CdCl24- cannot be identified although at a ligand concentration of I tool l- i itmakes up ~ 10% of the total metal. This indicates that each complex should be present at some minimal percentage for detection. That is why high ligand concentrations, which increase the percentage of the ultimate complex species, can help us to 'see'it. The problem is more serious if one of the lower complexes appears in small quantities. In such a case the program cannot distinguish it as a separate species, and gives for the stabilityconstants a set of values that is unacceptable because one of them is statisticallyinsignificantor even negative. The lowest percentage that stillpermits calculation of acceptable stabilityconstants depends upon several factors,one of them being accuracy of experimental results. In working with modern polarographic instruments, the accuracy is governed by the lowest possible scan increment, which is usually 1 m V (BAS 100A, Bioanalytical Systems, West Lafayette) or 2 m V ( P A R 384B). For accurate determination of stabilityconstants the lowest increment should be an order of magnitude lower because in the present conditions a small contribution from a lower complex species to the total peak potential shift can be masked by experimental error.

232

M. BRAN|CA ET A t

Preliminary calculations indicate that, at present, a weak complex such as metal halogenide cannot be distinguished as a separate species if its maximum percentage does not exceed 20% of the total metal. At different ionic strengths the number of chloro complexes whose stability constants can be measured is not the same. We have tried to investigate several systems in the range of high electrolyte concentrations (I=0.5-6 mol l 1). From Fig. 2 it is obvious that the first and second complex formation can be followed in all solutions. However, for the third stability constant to be calculated, ionic strength should be at least 1 mol 1-', and for the fourth one 4 mol l- 1 Obviously, these higher complexes are not very important in ordinary seawater conditions but should be expected in brine from solar saltworks. In all the measurements, chloride concentration should be much higher than that of the metal ion because in such a case we can take free and total ligand as equal. This assumption is only partially correct, depending upon the quantity and nature of the basic electrolyte, i.e. the ability of its cation to form ion pairs and reduce the free ligand concentration. In Table 2 we can see the results for cadmium complexation with chloride ions in different M(C104),,-MCI,, systems, M being Li +, Na +, Mg 2+ or A13+, together with some previously published values (Mironov et al., 1963). Going from lithium to aluminium, formation constants becomes lower, in agreement with the fact that in lithium chloride solutions ion pairing is not pronounced whereas in sodium and magnesium media its importance increases (Johnson and Pytkowicz, 1978). For aluminium chloride solutions we could not find any literature data on ion pairing or even complex formation. The finding that formation constants are strongly influenced by the character of 'inert' cations is not new. It has been described for the case of lead complexation with chloride ions (Byrne and Miller, 1984; Millero and Byrne, 1984). A

S

P i

} i i

!

i

3

i

J o

~

Fig. 2. Dependence of the stability constants ((A) first and second, (B) third and fourth) upon the square root of the ionic strength (mol l - 1) in Cd2+-NaCtO4-NaC1 solutions.

CADMIUMCOMPLEXATIONAT HIGHLIGANDCONCENTRATIONS

233

TABLE 2 Influence of the basic electrolyte composition on the values of stability constants in M (Cl04)nMC1. systems; ionic strength 4 tool 1-1, p H = 2 M

log fll

log f12

log fl3

log f14

Reference

Li Li Na Mg Al

1.73_+0.04 1.77+_0.02 1.57+_0.07 1.51_+0.03 1.11 _+0.03

2.3 _0.1 2.56_+0.05 2.42_+0.07 1.91+_0.09 1.65 _+0.03

3.16_+0.04 3.19_+0.07 2.66+_0.06 2.1 _ + 0 . 1 1.81 _+0.01

2.5 _ + 0 . 1 2.5 _0.1 2.0 _+0.1 1.80_+0.08 -

Thiswork Mironovetal. (1963) This work This work This work

Complexation with organic ligands According to the literature data (Table 3) at low ionic strengths the cadmium ion can make two different complexes with glycine, diglycine and triglycine. However, when we tried to fit experimental points, measured over the widest possible ligand concentration range, with the second-degree polynomial, the resulting curve was always of the type shown in Fig. 3 by a dotted line. Obviously, one cannot be satisfied with such a fitting; however, with the thirdorder polynomial the results seem much better (Fig. 3, full line) and the variance is lower, 0.0070 instead of 0.0683. This result indicates that one more complex should be present in the solution. Fitting with a polynomial of higher degree would not improve the situation because stability constants not significantly different from zero, or even negative values would appear. Of course, such results are mathematically correct, but physically they are meaningless. In all three cases (Cd-Gly, Cd-GlyGly and Cd-GlyGlyGly), the situation was nearly the same,i.e, three instead of two complexes were found, introducing serious changes in the distribution diagrams. In Figs. 4 and 5 these curves are given for Cd-Gly and Cd-GlyGlyGly systems as degree of complex formation versus pH value because in such a form they reflect the experimental procedure, in which total ligand concentration was kept constant. Similar diagrams for cadmium complexation with diglycine can be found elsewhere (Branica-Jurkovid and Simeon, 1989). For calculation of the free ligand concentrations, the protonation constants (Martell and Smith, 1974) were used. Taking into account stability constants published in the literature (Table 3) at pH values higher than 7, the complex of ML2 type should be predominant in all three systems. However, according to our results, ifpH is higher than 8.5 for glycine or 7.8 for diglycine and triglycine, the third complex is most abundant. It is important to stress that introduction of the third species causes some

2:~4

M. BRANICA ET AI,

TABLE 3 Stability constants of cadmium complexes with glycine, diglycine and triglycine Ionic log fll strength

log f12

log fl~

Method"

Reference

10.68

e.m.f./pH (selec~d value) POT

Evans and Monk ( 1955 ) Martell and Smith ( 1974 ) Perkins (1954) Anderegg (1961) Branica-Jurkovid and Simeon ( 1989 )

Gly I} 0 0.01 0.1 0.1

4.80 4.69 4.4 4.239 _+0.008 4.05 +_0.18

8.83 8.40

0.15 0.670 0.720 1.0 2.0

3.96 + 0.03 4.19 ± 0.04 4.14 5.68±0.1

7.25 ± 0.03 7.27 _+0.02 7.60 8.45_+0.1

GlyGly 0 0.01 0.1

3.33 _+0.01 3.2 2.72 ± 0.08

5.87 ± 0.01

0.15 0.8

2.95 2.76 ± 0.17

5.82

POT NMR

5.85 ± 0.05

e.m.f./pH POT SWV

9.94

GlyGlyGly 0 3.30 ± 0.01 0.01 2.0 0.1 2.39 ± 0.17 0.15 0.8

7.759 ± 0.011 7.37 ± 0.25 9.66 ± 0.31

2.70 2.69+_0.12

4.83 _+0.13

4.90 ± 0.12 5.3

9.74

6.54 ± 0.03

6.42 _+0.08

pH SWV

DC DPP DPP (selectedvalue) DC

Li et al. (1956) Simoes Goncalves et al. (1983) Simoes Goncalves et al. (1983) Martell and Smith (1974) Smith et al. (1962)

e.m.f./pH POT SWV

Evans and Monk (1955) Perkins {1954) Branica-Jurkovid and Simeon ( 1989 ) Li and Chen (1958) Rabenstein and Libich {1972 )

pH/DC NMR

Evans and Monk (1955) Perkins {1954) Branica-Jurkovid and Simeon ( 1989 ) Li and Chen (1958) Rabenstein and Libich {1972)

"e.m.f. = electromotive force; POT = potentiometry; SWV = square wave voltammetry; DC = direct current polarography; DPP = differential pulse polarography; NMR = nuclear magnetic resonance.

change even in the pH range where the fLrst complex and free metal are the only ions present at significant concentrations. Although distribution diagrams give a wide pH range, the experiments were performed from pH 4 to pH 8.6. At lower values the peak potential shift was neither expected nor recorded and at higher ones hydrolysis of cadmium ions should be taken into account as well. In the existing literature (Rabenstein and Libich, 1972) complex species of the CdHL 2+ type are mentioned and their stability constants are presented, with values of ~ 10 for the reaction Cd2+ + HL = CdHL 2+ at I - 0.8 tool l - 1. If so, formation of corresponding ions should cause a significant peak potential shift (ziE> 10 mV) in the pH range where the ligand exists in the HL form only. However, this is not the case. By comparison of the peak potential shift

CADMIUM COMPLEXATION AT HIGH LIGAND CONCENTRATIONS

235

100

/7 / '

., ,,J/'

>'

z" // •

'

4

"/

'

J

'

-~

tog[GtyGLy]

Fig. 3. C a d m i u m peak potential shiftas a function of diglycine concentration. Experimental points fitted with a polynomial of the second (dotted line) and third order (fullline). Total metal concentration: 6 X 10-5 tool I- i,total ligand concentration: 0.1 tool l- i,p H = 4.0-8.6, basic electrolyte: 0.1 tool I-I KN03.

with a ligand distribution among three different species (H2L +, H L and L- ), in dependence on the p H value, one can see that the measured effect follows the rising part of the curve that corresponds to the percentage of L-. This means that C d H L 2+ complexes most probably do not exist under the experimental conditions described. W e feel that the relatively high standard error obtained in calculation of some constants is a problem. T w o purely experimental reasons can be found for this.First,each solution was prepared and measured separately. However, as pointed out by Crow (1984), in such a case errors caused by the difficultyof exactly restoring electrode parameters after replacement of the working solution can appear. That is why a titrationprocedure, i.e.stepwise addition of the ligand, should be preferred and used when possible. Second, because only the p H value was different in the series of solutions, whereas concentrations of total ligand and basic electrolyte were kept constant, ionic strength was not strictlythe same in all samples, owing to the differentpercentage of the H2L +, H L and L- species at different acidities. In Table 3, literaturedata on stabilityconstants are presented, together with results by Branica-Jurkovid and Simeon (1989) published elsewhere in more detail. It seems that some discrepancy originates from the experimental con-

2;:;6

M. BRANI< A ET A |

~. . . . . . . . . . . . . . .

1.0~-

0 . . . . . . . . .~. . . . . . . . .

2

Q6~ i

,,,D n

A

o.2~

2

"°I

10

'12

pH

,,4,n L

B

LI [

pH Fig. 4. Distribution of cadmium species in the glycine solution (0.1 mol l-i). (A) Calculations based on the literature data (log K H =9.57, log K P =2.36, log fil=4.24, log p2=7.71 ) and (B) experimentally determined stabilityconstants (logPl = 4.05,log ~2 = 7.37, log ~s = 9.66).

ditions used in earlier studies, i.e.ligand concentrations may not have been high enough or perhaps the ligand to metal ratio was too low. From the environmental point of view, CdL~- ions found in our experiments are not important when considering cadmium speciation in natural waters because the high ligand concentrations necessary for their formation are not to be expected. However, in body fluids,amino acids, peptides and proteins are present in much higher quantities, i.e.tris-ligandcomplexes might have some

biological significance, especially after administration of peptide-based drugs (Dutta, 1989).

CADMIUM COMPLEXATION AT HIGH LIGAND CONCENTRATIONS

237

2

Q6

A

Q2

'

~

~

6

pH

8

1'o

'

12

B

i

'

~

'

~.

6

8

"~

~:

pH Fig. 5. Distribution of c a d m i u m complexes in triglycine solution (0.05 tool 1-1). Calculations based on (A) literature data (log K H = 7.90, log K H = 3.27, log fll = 2.70, log fi2 = 5.30 ), and (B) experimentally determined stability constants (log fll = 2.39, log f12= 4.90, log f13= 6.42 ).

CONCLUSION

Determination of stability constants of metal complexes should be performed over as wide a ligand concentration range as possible. Only in such a way can the maximum number of complexes be detected. This is important because omission of the species of the highest ligand number has a strong effect on the distribution diagram, even in the ligand concentration range where such a complex species is not present at a significant percentage. Only a few determinations of stability constants have been performed in this way. At least, in

238

M. Bf~ANICA ET A~,

u s i n g v o l t a m m e t r i c m e t h o d s , m a x i m u m ligand c o n c e n t r a t i o n s s h o u l d be used, b e c a u s e a v e r y high l i g a n d / m e t a l r a t i o does n o t p r o d u c e a n y of t h e difficulties w h i c h m a y occur w i t h o t h e r e x p e r i m e n t a l m e t h o d s . T h e role of so-called i n e r t c a t i o n s s h o u l d also be t a k e n into a c c o u n t , a n d e x p e r i m e n t a l design s h o u l d include t i t r a t i o n p r o c e d u r e s w h e n e v e r possible. ACKNOWLEDGEMENTS T h i s s t u d y is a c o n t r i b u t i o n to t h e j o i n t p r o j e c t ' E n v i r o n m e n t a l R e s e a r c h in A q u a t i c S y s t e m s ' o f t h e I n s t i t u t e o f A p p l i e d P h y s i c a l C h e m i s t r y , N u c l e a r Res e a r c h C e n t e r ( K F A ) , Jiilich, a n d t h e C e n t e r for M a r i n e R e s e a r c h Zagreb, " R u d j e r Bo~kovid" I n s t i t u t e , Zagreb. F i n a n c i a l s u p p o r t f r o m t h e I n t e r n a t i o n a l B u r e a u of K F A , Jiilich, w i t h i n t h e f r a m e w o r k o f t h e b i l a t e r a l a g r e e m e n t bet w e e n t h e F.R.G. a n d Yugoslavia, is g r a t e f u l l y a c k n o w l e d g e d . T h i s s t u d y was p a r t i a l l y s u p p o r t e d b y t h e U.S. N a t i o n a l Science F o u n d a t i o n a n d S e l f - M a n a g e m e n t C o m m u n i t y for Scientific R e s e a r c h o f S.R. C r o a t i a , t h r o u g h f u n d s m a d e a v a i l a b l e to t h e U . S . - Y u g o s l a v J o i n t B o a r d on Scientific a n d T e c h n i c a l C o o p e r a t i o n u n d e r P r o j e c t J F P 679.

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Li, N.C., White, J.M. and Yoest, R.L., 1956. Some metal complexes of glycine and valine. J. Am. Chem. Soc., 78: 5218-5222. Martell, E.A. and Smith, R.M., 1974. Critical Stability Constants, Vol. 1: Amino Acids. Plenum, New York. Millero, F.J. and Byrne, R.H., 1984. Use of Pitzer equations to determine the media effect on the formation of lead chloro-complexes. Geochim. Cosmochim. Acta, 48:1145-1149. Mironov, V.E., Kulba, F.Ya. and Nazorov, V.A., 1963. Effect of extrasphere cations on complex formation between cadmium and chloride ions. Zh. Neorgan. Khim., 8:916-922 (in Russian). Niirnberg, H.W., 1984. Trace analytical procedures with modern voltammetric determination methods for the investigation and monitoring of ecotoxic heavy metals in natural waters and atmospheric precipitates. Sci. Total Environ., 37: 9-34. Osteryoung, J.G. and Osteryoung, R.A., 1985. Square wave voltammetry. Anal. Chem., 57: 101All0A. Perkins, J.D., 1954. A study of some simple peptide complexes with zinc and cadmium ions in aqueous solution. Biochem. J., 57: 702-704. Pi~eta, I. and Branica, M., 1988. Computer automation of polarographic analyzer PAR 384B and development of specific implementation software. J. Electroanal. Chem., 250: 293-299. Pi~.eta, D., Pi~eta, I. and Branica, M., 1987. Electrochemical speciation of trace metals by computer data fitting. Proc. IXth Int. Symp. Computer Aided Design and Computer Aided Manufacturing, Zagreb, 291-296. Rabenstein, D.L. and Libich, S., 1972. Nuclear magnetic resonance studies of the solution chemistry of metal complexes. V. Cadmium, zinc and lead complexes of polyglycine peptides. Inorg. Chem., 11: 2960-2967. Raspor, B., 1980. Distribution and speciation of cadmium in natural waters. In: J.O. Nriagu (Editor), Cadmium in the Environment. Part I. Wiley, New York, pp. 147-236. Simoes Goncalves, M.L.S., Valenta, P. and Niirnberg, H.W., 1983. Voltammetric and potentiometric investigations on the complexation of Cd{II) by glycine in seawater. J. Electroanal. Chem., 149: 249-269. Smith, J.H., Cruikshank, A.M., Donoghue, J.T. and Pysz, J.F., 1962. Polarographic study of cadmium (II)-amino acid complexes. J. Inorg. Chem., 1: 148-150. Smith, R.M. and Martell, A.E., 1976. Critical Stability Constants. Vol. 4: Inorganic Complexes, Plenum, New York.