Analytica Chimica Acta 501 (2004) 113–117

Comparability of copper complexation capacity determination byabsorption by chelating resin column and cathodic stripping voltammetry

Michael Gardner∗, Eleanor van Veen

WRc, Frankland Road, Blagrove, Swindon, Wilts SN5 8YF, UK

Received 2 April 2003; received in revised form 4 September 2003; accepted 15 September 2003

Abstract

A simple method has been developed for the determination of copper complexation capacity. It is based on absorption of non-complexedspecies on a chelating resin column. The limit of detection is for complexation equivalent to 3�g Cu l−1 and the relative standard deviation ofresults is 10%. The method has been shown to be of comparable performance with voltammetric techniques in respect of the ligands detected.The approach offers the possibility of use in the field and of application to the determination of the speciation of other divalent trace metals.© 2003 Elsevier B.V. All rights reserved.

Keywords: Copper complexation capacity; Chelating resin

1. Introduction

It has been established that complexation by naturally oc-curring organic ligands can reduce the toxicity of importanttrace metals to aquatic life[1]. This creates a need for abetter understanding of metal speciation. The importance ofdissolved phase speciation has been acknowledged in theway water quality standards are defined. For example, higherlevels of copper are permitted where organic complexationis present[2]. The basic approach in studying complexationis so-called ligand titration. This involves taking portions ofthe sample under examination and adding increasing quan-tities of the trace metal of interest. As the metal concentra-tion is increased, any free organic ligand naturally present inthe sample becomes associated with metal until the capac-ity for complexation is exceeded. Application of a measure-ment/separation technique which can distinguish between“free” and complexed metal produces a titration curve fromwhich the complexing capacity (CC)—the effective ligandconcentration can be deduced[3]. Knowledge of the com-plexing capacity allows the estimation of the relative pro-portions of different forms of metal, specifically free metalion, inorganic forms of metal and organic complexes. This,

∗ Corresponding author. Tel.:+44-1793-865204;fax: +44-1793-865001.

E-mail address: gardnermj@wrcplc.co.uk (M. Gardner).

in turn, will give a guide to the behaviour and toxicity ofthe metal[4].

There are two main categories of approach to the instru-mental (rather than biological) evaluation of metal specia-tion. The first involves combined separation/analytical tech-niques. These determine the “free” or uncomplexed metaldirectly in the sample. They include various voltammetricand ion selective electrode techniques. The advantage of thisapproach is that the whole determination is achieved in onestep. The disadvantage is that it may be necessary to com-promise with respect to the conditions that best separatefree and complexed metal and those that are optimal for themeasurement step. Taking cathodic stripping voltammetry(CSV) as an example: the addition of a minimum amountof analytical reagent favours the detection of the full rangeof complexing ligands; however, it is desirable, in order toachieve adequate sensitivity of measurement, to add moreof the active reagent. The chosen experimental conditionsare thus a compromise between achieving a precise mea-surement of the free metal fraction and detecting the wholerange of complexing ligands.

The second approach involves distinct separation and an-alytical techniques. These involve separation of free andcomplexed metal and subsequent separate determination ofeach fraction. Clearly these approaches involve two steps,but they can have the advantage that the analytical techniquecan be chosen on the basis of performance (i.e. limit of de-tection, cheapness etc.) alone. Furthermore, if the separation

0003-2670/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/j.aca.2003.09.011

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stage is sufficiently simple, it may be possible to undertake itin the field. This eliminates the need to consider the issue ofsample stability, and makes the storage and transport of theprocessed samples to the laboratory much more straightfor-ward. These approaches include ion exchange, absorption,extraction and precipitation techniques.

Despite the practical advantages of this second approach,relatively few applications have been reported. Crosser andAllen [5] initially proposed the use of resins for the deter-mination of speciation. They demonstrated the principle insynthetic samples. Van den Berg and Kramer[6] used man-ganese dioxide as a weak ion exchange medium. This wastechnically a success but complicated to use on a routine ba-sis. The application of commercial chelating resins has beeninvestigated in a small number of studies. Chakrabarti et al.[7] investigated the kinetics of uptake of cadmium, copperand lead on such resins using a batch technique. A simi-lar batch based approach has been reported by Pesaventoet al. [8,9]. Sedlak et al.[10] used chelating resin columnpartitioning and graphite furnace atomic absorption spec-troscopy to determine the speciation of copper and nickelin seawaters. Amberlite 718, an iminodiacetate resin simi-lar to Chelex 100, has also been used in the field of metalspeciation in a column application[11] and as an electrodemodifier when using an electrochemical speciation tech-nique involving carbon paste electrodes[12]. The purposeof this paper is to describe a technique for the determinationof copper speciation of the second type and to demonstratethat a simple column method could be developed to pro-duce data that were consistent with other more complicatedand widely used methods for CC determination. Data arepresented on the method performance and its comparabilitywith a well-established electrochemical methodology.

2. Method description and performancecharacterisation

The description of the proposed methodology is as fol-lows. This description of the method takes into account theinvestigations of the parameters of its performance that wereinvestigated as part of this work and which are described inthe next section. In summary, the method involves sorptionof the labile fraction of copper (taken as the “uncomplexed”fraction) by passing a portion of the water sample througha short column packed with a chelating resin.

2.1. Method

Water samples were filtered in the field to a pore sizeof 0.45�m and buffered at a pH value within 0.2 oftheir natural pH value using buffers that were shown byblank titrations to be of negligible complexing capacity.For example, for a sample of pH 7.8, 1 M EPPS buffer(N-2-hydroxyethylpiperazine-N′-3-propane sulphonic acid(pKa 8.00)) was prepared in 0.5 M ammonium hydroxide

such that an addition of 150�l buffered a 15 ml sample ofriver water to a value of 7.8 ± 0.2.

Chelex 100 resin of mesh size 100–200 was packed intoa 5 cm Perspex column by wetting the resin with excessdeionised water and injecting it into the column using a sy-ringe. The column was fitted with PTFE omnifit adaptersattached to PTFE tubing at each end. The filter was removedfrom the adapters in order to ease the flow of the samplethrough the columns. At the outflow end of the column, acircle of nylon gauze of a mesh size capable of retainingthe resin was inserted. The resin column dimensions were40 mm× 2 mm. Complexation titrations were carried outby preparing eight 20 ml portions of sample to which in-creasing aliquots of copper were added in the nominal range0–60�g l−1. Spikes were prepared by the addition of a stockstandard solution (5000�g l−1), buffered to pH 3, preparedfrom a commercial stock standard (Merck Spectrosol, Poole,UK). Each portion was equilibrated for 1 h and the portionswere then passed through the column. Flow through the col-umn was regulated either manually (with a syringe) or byperistaltic pump such that the 5 ml sample contact time was2 min±15 s. The first 5 ml of eluate was discarded in order tocondition the column; the next 5 ml was collected and acid-ified to 0.2% acid using a 10% solution of Primar (Merck,Poole, UK). nitric acid. Subsequent determination of la-bile copper concentration (and total dissolved copper con-centration on an uneluted portion) was by graphite furnaceatomic absorption spectroscopy. A single column was usedfor each titration, the samples being presented in order ofincreasing concentration. The fraction of copper complexedby natural organic ligands was calculated as the differencebetween total dissolved and labile concentrations. Data forthe complexation titration were processed as described byvan den Berg[13] and Ruzic [14] to provide estimatesof CC.

The choice of an equilibration time of 1 h was on thebasis that the greater proportion of complexation of copperhas been shown to take place relatively quickly[15], thata relatively short equilibration time is more practicable forfield uses and that the faster reacting ligands are those ofgreatest interest in the reduction of the toxicity of copperdischarges to aquatic life (on the basis that if copper remainsuncomplexed for an extended period it is more likely toexhibit toxicity than if it is complexed quickly).

3. Choice of resin

The desired characteristic of a suitable resin is that theresin should remove labile copper from a solution, but wherethe solution contained ligands which complexed copper, theresin should allow them to pass through the column witha minimal degree of disruption. The cut-off between com-plexed and “free” copper is somewhat arbitrary for most,if not all, practical speciation techniques so it was seen asdesirable that the column technique could be shown to be

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Table 1Resins tested for suitability

Resin Structure/notes Speciation use reference

Amberlite IRC718 Chelating resin with iminodiacetate functionality used for the recovery oftransition metals. Weak acid cation, macroreticular. Similar to chelex resin

Viana et al.[12] Procopio et al.[11]

Amberlite IRC50 Methacrylic acid divinylbenzene matrix. Will selectively adsorb organicbases such as basic andibodies, alkaloids, peptides, amino acids andmetal present in alkaline solution. Used for antibiotic purification andrecovery and copper and nickel recovery. Weak acid cation,macroreticular, carboxylic functionality

Amberlite XAD 1180 Polystyrene resin which adsorbs non-polar or weakly polar substances.Used in conjunction with a pretreatment with 250 nM solution ofAPDC/cetrimide to provide the adsorption sites for labile copper

Abollino et al. (1998) (saline matrices)

Chelex 100 Styrene divinylbenzene matrix. Weak cation chelating resin withiminodiacetic functionality. Mesh sizes (200–400)

Sedlak et al.[10] (seawater)

comparable with other reported techniques. Initial tests wereundertaken on four types of resin (Table 1) The initial con-centration of copper in the sample and the concentration inthe eluant from the column were determined.

All the resins were tested initially by passing a solution of25�g Cu l−1 in deionised water, buffered with EPPS to pH7.9 over a 40 mm×2 mm column. As there were no ligandsin the sample a suitable resin should absorb all the copperin the sample and the concentration in the eluant from thecolumn should be close to zero. The results are shown inFig. 1.

Only the Chelex resin was considered to have performedwell enough to be examined further. The smallest meshsize available (200–400 mesh) presented a problem as itssize caused the column to block and leak. Chelex of meshsizes 200–400 (small), 100–200 (medium) and 50–100(large) were then tested for its retention performance onstandard solutions and river water. The large particle resinwas found to retain only 85% of uncomplexed metal. Thisfell to nearly 75% after elution of portions of river water.The two small particle resins were found to retain simi-lar proportions of uncomplexed metal (98% for a “fresh”resin and 95% for one that had been subjected to severalportions of river water). Subsequent tests confirmed thatafter a complexation titration (eight natural water sam-ples) the retention of uncomplexed metal was not lessthan 95%.

Fig. 1. Concentration of copper in successive 10 ml aliquots of eluantfrom a 25�g Cu l−1 solution of copper in deionised water (pH 7.9, bufferEPPS).

Fig. 2. Complexation capacity (CC,�g Cu l−1) in Thames water deter-mined by four different speciation techniques. Confidence intervals arecalculated as±2 × the standard error estimated for the determinationof complexation capacity. The standard error is estimated using regres-sion methods and reflects the variabilty of the data points on the titrationcurve. A multipler value of 2 is appropriate for the number of degrees offreedom associated with the standard error estimate.

Fig. 3. Comparison of the determination of complexation capacity by CSVand the column technique with Chelex 100 (mesh 100–200). (�) Cleanriver waters from rural areas, (+) river waters sampled near to sewageeffluent outfalls, (×) lowland rivers/estuarine samples, (�) reservoir/lakesamples.

116 M. Gardner, E. van Veen / Analytica Chimica Acta 501 (2004) 113–117

Fig. 4. Results of participation in an interlaboratory test for the determination of copper CC. Samples A–C were river waters, Samples D–F were estuarinewaters and Samples G and H were synthetic. (�) Denotes the consensus values; (�) denotes the mean of duplicate data reported for this technique. Theerror bars show the range of data reported by all laboratories taking part in the test.

4. Comparison of Chelex 100 mesh sizes 200–400,100–200 and 50–100 with cathodic strippingvoltammetry (CSV)

The three mesh sizes of resin were then tested for theirability to determine CC by comparison with an acceptedelectrochemical technique[16–18]. CSV is an electrochem-ical technique used widely to determine ligand concentra-tions in natural waters. The details of the CSV techniqueused are described in Gardner et al.[19]. The ligand con-centration in a sample of water from the R. Thames wasmeasured using CSV, and in duplicate for the three differentChelex mesh sizes. Confidence intervals were calculated as±2 × the standard error estimated for the determination ofcomplexation capacity.Fig. 2 shows that the medium meshsize resin tends to produce values of CC (under the flowconditions used) nearest to those produced by CSV. Thiswas seen as desirable in that it would produce data that wereas closely comparable as possible with the existing body ofdata. It is accepted that CC determination is somewhat em-pirical (there is a range of ligands of differing affinities forcopper) and that different experimental approaches can tendto produce different CC values. However, given this, consis-tency and comparability of data are regarded as an advan-tage.Fig. 3shows a comparison between CC determinationusing the medium mesh size resin and CSV for 18 naturalwaters of various types. Since the data produced by the col-umn technique and those produced by CSV are both subjectto variation, it is not appropriate to use conventional linearregression to assess the degree of agreement between thetwo sets of data. Instead a non-parametric regression tech-nique known as Theil’s method[20] was used. The result-ing regression line (of column on CSV) was of slope 0.99with confidence limits (P = 0.05) from 0.7 to 1.2. The in-

tercept was 0.6, with confidence limits (P = 0.05) from−6to 11�g l−1. This indicates no significant difference can bedetected between the two techniques.

The precision of the column technique was then assessedby carrying out duplicate determinations of CC on the 18water samples examined earlier. There was good agreementbetween duplicate data with a pooled relative standard de-viation of results for levels of CC greater than 10�g Cu l−1

of 10%. The precision of analysis at low CC concentrationindicates a limit of detection of 3�g Cu l−1.

Finally, the technique was used in an interlaboratory testfor the determination of copper CC in waters. Seven labo-ratories in five different countries took part[21]. Five par-ticipants used voltammetric techniques and two absorptivetechniques.Fig. 4shows the results for this column method-ology. Comparability between the CC data reported for thecolumn is within 60% of the reference level established forthe test—in all cases apart from Sample A—an upland waterof high organic content (>10 mg Cl−1). The column methodproduced a higher value than other techniques, probably asa result of occlusion of absorption sites by the high con-centration of organic matter, leading to an overestimate ofcomplexation.

5. Conclusions

A simple method has been developed for the determina-tion of copper complexation capacity. It is based on absorp-tion of non-complexed species on a chelating resin column.The limit of detection is for complexation equivalent to3�g Cu l−1 and the relative standard deviation of resultsis 10%. The method has been shown, for a wide range ofclean surface waters, to be of comparable performance with

M. Gardner, E. van Veen / Analytica Chimica Acta 501 (2004) 113–117 117

voltammetric techniques in respect of the ligands detected.The approach offers the possibility of use in the field and ofapplication to the determination of the speciation of otherdivalent trace metals.

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