Critical Evaluation of the Copper(I1) Solubilization Method for the Determination of the Complexation Capacity of Natural Waters P. G. C. Campbell,” Marc Bisson, Robert GagnB,’ and Andr6 Tessier Universit6 du Qubbec, INRS-Eau, C.P. 7500, Ste-Foy, Qubbec, Canada G7V 4C7
Thermodynamic calculations indicate that for realistic llgand M), the copper( 11) solubilization method concentrations ( 5 is insensitive to most monomeric blogenlc ligands and underestimates the true complexatlon capacity of natural waters. Experimental results show that the conditions required by the copper( 11) solubillzatlon method favor the removal of dlssolved organic carbon by coprecipitatlon. I n the absence of a final ultrafiltration step, the method is subJect to interference by colloidal Cu( 11)-humlc specles; a peptlration mechanism, involvlng a stablllzlng adsorptive lnteractlon between colloidal Cu(I1) hydroxide and the humic materials, is proposed to explain this Interference.
T h e chemical speciation of trace metals in the aquatic environment has a determining influence on their transport, reactions, and biological availability (1-5). Of primary importance in determining the speciation of a given trace metal in solution are t h e nature and concentration of the various ligands present. Provided t h a t the “available” ligand concentration, [L]A, is greater than the total trace metal concentration, [M]T, the degree of complexation will be independent of the metal concentration and will depend only on the concentration of the individual ligands and the magnitude of the appropriate stability constants (6). Inorganic ligands of importance in determining trace metal speciation are relatively few in number (1, 7) and their concentrations are readily determined. In contrast, many organic compounds of potential complexing ability are known t o occur in natural waters, for example: amino acids, proteins, monosaccharides, polysaccharides, porphyrins, hydroxamic acids, fulvic acids, humic acids (8-10). In recognition of the difficulties inherent in identifying and quantifying all the organic ligands present in a given water sample, several workers have developed the “complexation capacity” concept (11-15). As the term implies, complexation capacity is a measure of the ability of a water sample to complex or mask the trace metals present. Acceptance of the complexation capacity concept implies that for a given metal it is possible t o group together the various ligands present and represent their behavior by means of certain “average” properties. Several experimental procedures, varying greatly in manipulative complexity, have been proposed for measuring the complexation capacity of natural waters. These can be grouped into chemical methods (11-14), based on such physicochemical techniques as solubility determinations and complexometric titrations, and biological methods (4, 15). In this paper we present a critical evaluation of the copper(I1) solubilization method proposed by Manahan and co-workers (11, 16, 171, and compare the results obtained with this technique t o those obtained with a reverse colorimetric method. Present address, Ministere des Richesses naturelles, Service de la Qualite des eaux, Complexe Scientifique,Ste-Foy,Quebec, Canada G1P 3W8. 2358
ANALYTICAL CHEMISTRY, VOL. 49, NO. 14, DECEMBER 1977
EXPERIMENTAL General Analytical Procedures. Stock solutions were prepared using reagent-grade chemicals and deionized water (Millipore Milli-$3 RO/Milli-Q2 system). Dissolved organic carbon was determined with a Beckman Total Organic Carbon Analyzer (Model 915A). Ammonia and total reduced nitrogen determinations were performed colorimetrically (18) on a Technicon AutoAnalyzer (Model AA-111, and dissolved organic nitrogen was obtained by difference. Iron analyses were also performed on a Technicon AutoAnalyzer: after digestion of the sample with thioglycolic acid and reduction, iron(I1) reacts with 2,4,6-tri(Z’-pyridy1)-5-triazine to form a colored complex (19). Copper was determined by atomic absorption spectrophotometry (Varian Techtron Model AA-5). Conventional techniques, involving direct aspiration of the aqueous sample into an airacetylene flame, were used for complexation capacity measurements. Determination of copper concentrations in filtered river water required preconcentration; a chelation-extraction step involving ammonium pyrrolodine dithiocarbamate and methyl isobutyl ketone was employed (20). Complexation Capacity, Principles. The experimental procedure introduced by Kunkel and Manahan (11) for the determination of complexation capacity is based on the solubilization of copper(I1)by complexing agenb at alkaline pH values. Under the experimental conditions defined by Kunkel and Manahan, the solubility of inorganic Cu(I1) reaches a minimum of approximately 2.4 X lo-’ M (15 kg Cu L-I) in the pH range 10-11. In a natural water sample, any increase in copper concentration above this value is attributed to solubilization or complexation involving species other than hydroxo or carbonato ligands. For comparison purposes, complexation capacity values were also obtained with a copper(I1) titration procedure and a reverse colorimetric method. The copper titration procedure involves the measurement of electrochemically labile copper by differential pulse anodic stripping voltammetry after a number of ionic copper spikes have been allowed to equilibrate with the available ligands in the water sample (13). The colorimetric method, essentially identical to that used to determine nitrilotriacetic acid (NTA) in aqueous solution @ I ) , is based on the complexation of zinc(I1) with zincon (2-carboxy-2’-hydroxy-j/-sulfoformazylbenzene) The procedure involves treatment of the water sample with a cation-exchange resin followed by addition of the zinc-zincon reagent (molar ratio 1:13,buffered to pH 9.2). The extent of zinc binding to natural ligands present in the sample is calculated from the reduction a t equilibrium in zinc-zincon complexation (21, 22). Complexation Capacity, Copper Solubilization Method. The experimental procedure described by Kunkel and Manahan (11) was carried out in triplicate. To each of three 50-mL portions of the filtered water sample was added 5 mL of 5 X lo-’ M CuS04, followed by sufficient 5 X M Na2C03to raise the pH to a stable value in the range 9.9-10.1. The samples were heated in a hot water bath (ca. 100 “C) for 1 h. To avoid a significant decrease in pH during this treatment, with a concomitant increase in the solubility of inorganic Cu(II), a carbon dioxide-free atmosphere was maintained in the sample flasks by circulating prehumidified nitrogen over the samples during both the heating and cooling steps. Once cooled to room temperature, the contents of each reaction flask were quantitatively transferred t o a volumetric flask and diluted to 100 mL with deionized water (pH 10, Na2C0,). After thorough mixing, this suspension was filtered through a 0.45-km membrane filter. To avoid copper contamination of the filtrate, the membrane employed for this filtration I
Table I. Complexation Capacity of River Water as Measured b y t h e Copper(I1) Solubilization Method Complexation capacity (kmol Cu L-’) Mean R.ange
No. of
River Yamaska
Saint-Francois
Station Y-1 Y-2 Y-3 Y-4 Y-5 Y-6
ssmples 6 5 4 5
SF-4
6 6 6 6 4 3
SF-5
4
SF-6
6
SF-1
SF-2 SF-3
1.3 2.7 1.0
0.4-3.2
0.7 4.1
0.4-1.0
0.4-4.8 0.6-1.7 1.9-9.6
