Voltammetry of copper species in estuarine waters

J. Electroanal Chem., 164 (1984) 253-264

253

Elsevier Sequoia S A , Lausanne - Printed in The Netherlands

VOLTAMMETRY OF COPPER SPECIES IN ESTUARINE WATERS PART II. COPPER SPECIES REDUCTION AT THE H M D E IN ESTUARINE WATERS

A. N E L S O N and R.F.C. M A N T O U R A

Institute for Marine Envtronmental Research. Natural Enmronment Research Counctl, Prospect Place, The Hoe, Plymouth (Great Brttam) (Recewed 6th August 1983)

ABSTRACT Estuanne waters have been assayed by stripping polarography (pseudopolarography). It ~s shown that adsorptmn m e c h a m s m s are stgmficant m the reducuon of Cu organic associations m estuarme waters Also, by decreasing p H or adding Cu(II) the importance of the CuCI 2 state m the electroreducuon increases until reversible CuCI~- reduction ~s observed. This confirms previous pre&ctions [J Electroanal Chem., 164 (1984) 237]. Voltammetnc tltratlons of estuanne waters with added Cu have been studied In the course of the titration CuCI 2 becomes stabdlsed d u n n g the electroreduction. At a later stage, all hgand sites are tltrated and the full current sensitivity to CuCI2- ~s observed.

INTRODUCTION

In Part I [1] of this series of papers the electrochemistry of copper in chloride media was discussed together with its application to the DPASV assay of Cu(II) species in estuarine waters. In this paper we describe the chemistry of the electrodeposition of Cu from estuarine waters during DPASV measurement and the relation of this to the speciation of Cu in solution. EXPERIMENTAL

The apparatus and reagents used were as described previously [1]. Stripping polarography (pseudopolarography) was employed to investigate the electrochemistry of Cu deposition and in this instance DPASV scans were initiated from either of two quiescent potentials - 6 0 0 and - 1 0 0 0 mV. The instrumental settings for DPSAV were as follows: pulse height, initially 50 then 25 mV; scan rate, initially 6.7 then 5 mV/s; pulse time, 0.2 s; damping, 2. An H M D E drop surface area of initially 1.82, then 1.39 and eventually 0.88 mm 2 was used. Water samples were collected in a polyethylene bucket, filtered through a 0.45 0022-0728/84/$03.00

© 1984 Elsevier Sequoia S.A.

254

~m membrane filter into a 1 1 pyrex glass bottle on the day of sampling and stored frozen. Estuarine waters were acidified by adding 25 /al HCI to 50 ml of sample in a quartz flask and stored overnight prior to measurement. Waters were UV irradiated by pumping together with oxygen gas at 2 m l / m i n through a 25 m (i.d. 2 mm) length of silica coil around a 1 kW UV lamp. In the voltammetric titrat~ons, 50 ml of the sample were equilibrated with the metal titrant in a quartz flask for a mimmum of 12 h prior to analysis. Acidified samples were titrated with a 5 rain equilibration time after each metal addition. RESULTS AND DISCUSSION

Cu species reduction at the H M D E m estuarme water

Figure 1 shows stripping polarograms for Cu reduction in estuarine waters at different pH values. Although details of the curves vary from sample to sample various features are common. d

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255

The presence of a limitmg current becomes more apparent with decrease in pH. This defines an initial reduction wave with a measurable half-wave potential (El/2). The height of this wave increases with decrease in pH and at low pH it increases markedly. At more negative potentials the current increases in a linear manner. The first wave shows a trend to reversible character w~th decrease in pH and an anodic shift m E]/2. At pH 2.5, reversible reduction is occurring indicated by a log plot slope of 60 mV per decade showing a one-electron transfer. This same stripping polarographic response is seen in a sample at natural pH with sufficient added Cu to remove organic ligand competition to Cu electrodepositlon (see final section). At natural pH and ambient Cu, reduction of Cu(II) takes place m an irreversible manner, Decrease of pH or addition of Cu increases the stability of the CuCI 2 state and its importance m the electrodeposition process [1]. At low pH, liberation of Cu(II) from complexes is indicated by the marked increase in wave height. Referring to the acidified samples, for reverstble diffusion controlled complex reduction, tt may easily be shown [2] from the Lingane equaUon that:

d e]/2/d

log [X] = j X 60/n

(1)

where j is the co-ordination number of the complex, n ts the electron transfer number and [X] is the concentration of ligand. For a series of acidified estuarine waters of varying salinity, E~/2 for the Cu electroreduction shows a linear relation-

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- E ! /mY 2 Fig. 2. Estuarlne waters + added Cu (sufficient to remove organic hgand compet~hon to Cu electrodeposltlon) E]/2 vs. - l o g [ C l - ] for Cu reduction, I m g / m l gelatin, HMDE surface area 0 88 mm 2. (0) Estuarine waters; ( × ) estuarlne water, sahmty 5%o, dosed to 0.19 M [Cl- ] with KCI. Instrument settings. pulse height, 25 mV, scan rate, 5 m V / s

256 ship with log [C1-]. The same is observed for estuarme media at natural pH where the organic ligand competition to Cu electrodeposit~on has been removed by addition of Cu (Fig. 2). From this plot a j value of 2 is obtained indicating the reduction of a Cu(I) complex of co-ordination number 2. The coincidence of El~ 2 values of a chloride dosed sample on this plot confirms the Cu(I) species to be CuCI~-. This is ~dentical to the findings of von Stackelberg and von Freyhold [3] in chloride media although the El~ 2 values we observed were considerably more negative. Thus at salinity 59'~ ( - l o g [CI ] = 1.11), El~ 2 for CuCI~ ~ Cu(Hg) is - 1 4 5 mV whereas von Stackelberg and von Freyhold obtained a value of - 2 1 mV (vs. Ag/AgC1/KC1, 3.5 M). The negative shift in E~/2 is due to the additional dependence of the stripping polarographic wave Ew2 on the electrolysis time. This has been shown previously [4] and is expressed in the following equation:

E~/2 = E ° + 60 log( Yo2r8/yR3D ) -- 60 log t mV

(2)

Thus: if we take the value of von Stackelberg and yon Freyhold [3] of E~/2 as equivalent to the redox potential of the CuCl~/Cu(Hg) couple in a solution of 5%o salimty ( - l o g [C1-] = 1.11) and substitute eqn. (2) with values for the variables as used previously [1], we obtain a value of El~ 2 = - 1 5 1 mV for reduction at the HMDE. This is in good agreement with the El~ 2 = - 145 mV observed in this study (Fig. 2) and reconciles yon Stackelberg and yon Freyhold's [3] data concerning CuCl~ reduction in chloride media with the data in this study regarding voltammetric CuCl~ reduction in estuarine waters. The observation that the CuC12 ion controls the electrodeposition of Cu m estuarine waters at natural pH in the absence of organic hgand competition is significant. Accordingly it confirms the previous postulate [1] that CO 2- does not influence the final voltammetric reduction of Cu in these media. It should also be mentioned that minor shifts in the E~ of reversible CuCl~ reduction ( < 10 mV) were observed with variation in pH or in the presence of a surfactant. These are not considered as representing a significant change in reaction mechanism and have not been investigated further.

Adsorption mechamsms m the reduction of Cu(II) species Experimental evidence indicates that adsorption mechamsms are important in the direct reduction of copper complexes in estuarine waters. Stripping polarograms at potentials more negative than the CuCI~ reduction show no defined diffusion limit but increase linearly with decrease in potential (Fig. 1). In some cases, however, a maximum has been observed at ca. - 8 0 0 mV. This has been removed by the addition of gelatin to the electrolyte (Fig. 3). The gelatin retards but does not prevent the adsorption-reduction of complexes by effectively competing for the mercury surface. The relative magnitude of the Cu complex reduction is correlated with the stripping polarographic current at anodic plating potentials (ca. 0 mV). This current is due to adsorptive accumulation of Cu complexes which are reduced to Cu(Hg) during quiescence. Induced reactant adsorption at the mercury electrode has

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already been observed in the Cd2+-fulvic acid polyelectrolyte system [5,6]. The effects of this in pulse polarography have been rigorously analysed [7,8]. Reductton of Cu spectes in U V zrradtated estuarme waters

Stripping polarographic assays of UV irradiated water (Fig. 4) shows that the form of the copper reduction has changed markedly from the natural sample (e.g. curve a in Fig. 1). Noticeably the height has increased and the wave reaches a constant limiting value although in some samples at natural pH, current maxima are observed. The major point illustrated here is that the ligands affecting the electrodepositlon of copper from estuarine waters are of an organic nature and these are altered or removed on UV-irradlatlon. Nonetheless the UV-lrradiated samples extublted a range of additional phenomena in their electrochemical assay. (1) At pH 7-8 the copper was often reduced in an irreversible manner (Fig. 4). At lower pH, the reduction approached reversibility with an anodic shift in El~ 2 to the value characteristic of the sahnity of the sample. There was also an increase in current with decrease in pH. These effects were markedly less significant in copper spiked samples (Fig. 4b). (ii) In the assay of solutions of pH 7-8 there were often indications of adsorption-reduction of copper complexes. On some samples this occurred to such an

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Fig. 4. Stripping polarographlc assays of UV irradiated estuarme waters (sahmty 5%o). 1 mg/l gelatin, HMDE surface area 1.39 mm 2 (a) Polarograms ambient Cu, pH: (a) 7 65, (b) 6.25, (c) 2.50 (+ 25 #1 HCI) (b) Log analysis plot: (O) pH 7 65, ambient Cu, (e) pH 7.65, + 5 / t g / l Cu, (13) pH 6 25, ambient Cu, (m) pH 6.25 + 5 p,g/l Cu, (A) pH 2.50, ambient Cu. Instrument settings; pulse height, 50 mV; scan rate, 6 7 mV/s; qmescent potential, -1000 mV

extent that it was imposstble to obtain any sensible measurements without addition of gelatin. Previous reports [9] have noted the irreversibility of copper reduction tn UV trradiated samples of seawater and attributed this to the reduction of various

259 complex inorganic species of copper. We attribute these effects to residual organic material and by-products of organic oxidation following UV-irradiation influencing the electrodeposition of copper for the following reasons: (1) The observed shifts in E:/2 and decrease in slope on the log plot were less marked in samples with small additions of copper (5/~g/1). (Fig. 4b). (2) The phenomena were irreproducible from sample to sample of the same salinity and showed no relaUonship with sahnity. (3) The adsorption-reduction of complexes strongly suggests that hydrophoblc organic complexes rather than inorganic complexes are involved. (4) In many cases, passage of the UV-treated sample through a reverse phase octadecane SEP PAK column (Waters Associates) significantly reduced the severity of the interferences. Voltammetrtc tttratlons There has been much argument recently on the validity of ASV titrations in natural waters. Voltammetric titrations have been based on the premise that where metal is present with an excess of ligand, only free metal in equilibrium with the complex reacts at the electrode [10]. In this instance the assumption is that the complex is electroinactwe. Thus on sequential addition of metal to the sample, there comes a point where the ligand complexing sites are filled and with further titrant addition the full voltammetric sensitivity for the metal is realised. This is recorded as the breakpoint in the titration. The arguments against this approach are hinged around the concept of the lability of the complex at the Hg electrode providing an increase in the initial slope of the titration curve giving an underestimation of the stability constant of the complex [11]. However, it has also been argued that provided the extent of the lability of the complex is known, the stability constant can be corrected to its true value [12,13]. As we have found under the condiuons used in this study, Cu complexes with organic material, far from being inert to the Hg electrode show a wide range of reacuon. Nonetheless a voltammetric titration has an important application since it provides some information as to the quantity and form of organic ligands in the waters. Accordingly in carrying out such a titration, we propose, here the idea of a potential window. In order to specify the correct potentials for use in the voltammetric titration, the deposition potentials defining the CuCI~ reduction in the stripping polarogram of an estuarine sample need to be identified. Thus it has been shown in the previous section that the El~ 2 of the CuC12 reduction under given experimental conditions can be predicted from a knowledge of the salinity (see example in Fig. 2). The potentials used in the voltammetric titration of estuarine water are based on the assay of peak height currents of Cu plated at potentials E l l 2 + 110 mV, the lower potential representing reduction during quiescence and being used as baseline blanks. Using this approach the effect of any complex adsorption-reduction or direct complex reduction processes will be minimised.

260 Estuarme waters represent an organic Cu complexmg medium and where flcu L is large and [LT] >> [Cu(II)T] it follows that [Cu: L]---[Cu(II)v ]. Thus during a voltammetric titration, [L] will be successively reduced and eqn. (17) of ref. 1 can be rewritten:

X~ -- Xo/flC u

(3)

L [LT-CU(II)T]

The following conclusions can therefore be deduced from eqn. (3). (1) Throughout the voltammetric titration with Cu, [LT-Cu(II)T ] will be successively reduced until CuCI~- controls the electrodepositmn. This will occur before all ligand sites are filled. (2) Either a decrease in flCu L, an increase in Cu : L ~ Cu(II) mobihty or increase in x 0 will bring forward the point at which CuCI~- controls the reduction. (3) During the titration there will be a progressive stability of CuCI~ with decrease in [LT-Cu(II)T ]. This will accelerate the final electron transfer observed as an increase in titration slope and anodic shift of the CuCI~- reduction wave to the reversible E~/2. (4) The titration will not only reflect the mechanism of reduction but

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Fig 5. Voltammetnc utraaon. Estuarlne water (salinity 11%~, DOC 4.79 mg/l), 1 mg/l gelatin (a) T]traUon, potenual window ( - mV) 40-280; ( × ) pH 2 80 ( + 25/tl HC1), quiescent potential - 600 mV; (©) pH 7.58 direct addLtlon of tltrant, quiescent potential -600 mV, (o) pH 7.58 12 h eqmhbratlon of each tltrant addiuonj quiescent potential -600 mV; (A) pH 7.58 12 h eqmhbrat~on, qmescent potential - 1000 mV. (b) Stripping polarograptuc assays of t]trant addmons, log analysis plots, pH 7.58, qmescent potenual -600 mV (×) +8 #g/l Cu, (©) +40 #g/l Cu, (O) +140/Lg/l Cu. HMDE surface area, 0 88 mm2 Instrument settings; pulse height, 25 mV; scan rate, 5 mV/s.

261

also that of oxidation. As a result, at high [LT-CU(II)T ] when electro-oxidation proceeds via a broad stripping peak associated with adsorption of ligand, there will be a depression in the peak height. This will result in a further decrease in slope of the titration. Titrations were carried out in media with the addition of 1 rag/1 gelatin. Quiescent potentials of both -1000 mV and -600 mV were employed. Thus by scanning from - 1000 mV where stripping is to CuCI~- [14], the adsorption effect on the electro-oxidation will be minimised. Figures 5 and 6 show examples of titrations in estuarine waters of salinity 11%o and 23%o respectively. Previous predictions are confirmed and several points are evident. (1) The titration curve is dependent on the equilibration time, after 12 hours there is decreased electrochemical availability of Cu. Thus the slow complexation of added Cu extends over a wide range of Cu additions. (2) The course of the titration curve is complex. The slope increases with Cu addition showing progressive increase in reduction and oxidation rate. Stripping polarographic criteria of reversibility are used to locate the approximate position of CuCI 2 stabilisation during reduction. Subsequent to this point, continued Cu binding of ligand sites is shown by decreased titration slope. At higher [Cu(II)T], the

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262

voltammetrlc sensitivity increases to a maximum value mdlcatlng the titration of all ligands. This current sensitivity is not always equivalent to that in aci&fied samples and is decreased in higher chlorinity. The participation of CuC12 in the electroreduction of Cu : L is dlustrated by the anodic shift of the polarographic wave with increasing [Cu(II)T] tO the final value for reversible reduction of CuCl~-. Because of this, the mcreased titration gradient with salinity is consistent with the increased stability of CuC1] accelerating the overall rate of reduction. Thus in estuarine waters at higher salinittes there is often no significant change in curvature both when CuCl~ is stabdised and when all ligands are titrated. (3) Alteratxon of the quiescent potentml has some effect on the initial gradient of the titration (Figs. 5 and 6). This indicates that adsorption processes have an influence here on the electro-oxidahon [14]. The conclusions concerning the Cu(II) species electrode reaction m voltammetric

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Cu odd=tlon//~jg [-1 Fig 7. Voltammetric UtraUon. Estuarine water (sahmty 3%~, DOC 2 5 mg/1), 1 mg/l gelatin, 12 h equihbratlon. (O) pH 7.05, potential window (+ 40, -240 mV); ( × ) pH 7.28 [Cl-] boosted to 0.19 M with KCI, potentml window - 4 0 to - 2 8 0 mV HMDE surface area, 0 88 mm2 Instrument settings, pulse height, 25 mV; scan rate, 5 mV/s, qmescent potential - 6 0 0 inV.

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F:g 8. Stripping polarograpbac assays of Utrant additions to estuarlne water (sahmty 3%0, DOC 2.5 # g / l ) (a) pH 7.05, (o) +40/*g/l, (x) +80/xg/l, ( i ) +140 #g/l. (b) pH 7.28 (same sample +0.14 M [Cl ] as KC1, total [Cl-] = 0 19 M). (O) + 8/~g/l. (x) + 16 #g/l, ( i ) + 40/~g/l. HMDE surface area, 0.88 mm 2. Instrument settings, pulse height, 25 mV; scan rate, 5 mV/s, qmescent potential, - 6 0 0 mV.

t]trations of estuarine waters were further substantiated in the following experiments. A low salinity (3.1%o 0.048 M [C1-]), sample was divided into 2 aliquots, one of which was dosed with chloride (as KC1) to 0.19 M [CI-]. Both aliquots were assayed by voltammetric titration and associated stripping polarographic plots (Figs. 7 and 8). It is observed that the increased [C1-] advances the point in the titration by ca. 60 ~tg/1 Cu where the CuCI~- reduction is reversible. In addition, the initial titration slope in the chloride dosed samples is higher in accordance with previous considerations. Later similar experiments performed at constant ionic strength (adjusted with KNO 3) gave identical results. CONCLUSIONS

(1) Reduction of ambient copper in estuanne waters occurs primarily by an adsorption mechanism of the Cu(II) organic complex. By decreasing pH or adding Cu(II) the importance of the CuCI~- state in the electroreduction increases as previously predicted [1].

264 (2) In the absence of o r g a m c ligand c o m p e t i t i o n Cu electrodeposttton in e s t u a r m e waters proceeds via a stable C u C l 2 i n t e r m e d i a t e which is reduced reversibly at the electrode. It has been shown that Cu(II) c a r b o n a t e speciation does not influence the final Cu electroreduction in estuarine waters. U V i r r a d i a t e d estuarine waters c o n t a m a multitude of interferences to v o l t a m m e t r l c assay. It is p o s t u l a t e d that these are due to residual and altered organic material present in the solution. (3) Conceptually, v o l t a m m e t r l c titrations of estuarine waters with Cu, represents the response of the electrode reaction to Cu added. In the course of the titration CuC1 ~ becomes stabilised during the electroreduction. This position is a function of the stability constant of the complex, complex dissociation mobility, ligand conc e n t r a t i o n a n d salinity. A t a later stage all h g a n d sites are titrated a n d the full current sensitivity to CuC12 is observed. ACKNOWLEDGEMENTS This work forms p a r t of the Estuarine Ecology P r o g r a m m e of the Institute for M a n n e E n v i r o n m e n t a l Research, a c o m p o n e n t of the N a t u r a l E n v i r o n m e n t Research Council, a n d was partly s u p p o r t e d b y the D e p a r t m e n t of the E n v i r o n m e n t u n d e r C o n t r a c t No. D G R . / 4 8 0 / 6 0 5 . W e thank Drs. W. D a v i d s o n a n d D.R. Kester for very helpful comments. REFERENCES 1 2 3 4 5

6 7 8 9 10 11 12 13 14

A. Nelson and R.F C. Mantoura, J. Electroanal. Chem., 164 (1984) 237 D R Crow, Polarography of Metal Complexes, Acadetmc Press, London, 1969 M von Stackelberg and H von Freyhold, Z. Electrochem., 46 (1940) 120 A. Zmno and S.P. Kounaves, Anal Chem., 49 (1977) 56. H.P. van Leeuwen m W.F. Smyth (Ed)., Electroanalysis m Hygiene, Environmental, Cllmcal and Pharmaceutical Chemistry, Analytical Chemtstry Symposia Series, Vol. 2 Elsevaer,Amsterdam, 1980, p 383 H P. van Leeuwen, Anal. Chem, 51 (1979) 1322. H.P. van Leeuwen, J Electroanal Chem., 133 (1982) 201. H P. van Leeuwen, J. Electroanal. Chem., 135 (1982) 13 A. Zinno and S.P. Kounaves, Anal CbJm Acta, 113 (1980) 79 M. Plavslc, D. KrTmanc and M Bramca, Mar. Chem., 11 (1982) 17 J R. Tuschall and P.L BrezonLk,Anal. Chem, 53 (1981) 1986 M.S. Shuman, Anal Chem., 54 (1982) 998 G.A. Bhat and J H. Weber, Anal Chem., 54 (1982) 2116 A. Nelson and R.F C. Mantoura, J. Electroanal. Chem, 164 (1984) 265.