ANALYST, JUNE 1986, VOL. 111 619 Anodic-stripping Voltammetry of Metal Complexes in Non-aqueous Media After Extraction: Determination of Copper with Salicylaldoxime Jose Aznarez, Juan Carlos Vidal and Jose Maria Rabadan Department of Analytical Chemistry, University of Zaragoza, 50009 Zaragoza, Spain The determination of copper in biological materials and ores was carried out by anodic-stripping voltammetry in non-aqueous media after its extraction with salicylaldoxime into 1,2-dichloroethane and addition to an aliquot of the extract of 5% VNperchloric acid solution in N,N-dimethylformamide as supporting electrolyte. Different instrumental parameters were studied. The sensitivity and selectivity were considerably improved. The precision was 2.9% ( n = 8) for the determination of 2 pg ml-I of copper and the detection limit was 15 ng ml-1 of Cu(ll). The method can be applied to the determination of other elements (Sb, Bi, Cd, Sn and Pb) atthe microgram level and even the simultaneous determination of several elements after their extraction with organic reagents (8-hydroxyquinoline, benzoylphenylhydroxamic acid, sodium diethyldithiocarbamate) (Pb + Cu, Pb + Bi, Pb + Cd, Pb + Cd + Bi and Pb + Cd + Cu).Keywords: Copper determination; anodic-stripping voltammetry; extraction; polarograph y; non-aqueous media Polarography of organic substances in non-aqueous media has been used extensively and is still widely applied. In contrast, inorganic systems have hardly been studied in organic phases, except for metal complexes extracted with organic reagents.1 A preliminary separation by solvent extraction provides good selectivity for many instrumental methods of analysis that tend to suffer from matrix effects or interferences from different elements.This is particularly true of anodic-stripping voltammetry (ASV), where high concentrations of other ions, the formation of intermetallic compounds or the proximity of the stripping peaks prevent the application of this sensitive technique. Several extraction - voltammetric methods for cation analysis have been reported. Polarographic or ASV determi- nations have been made after the dilution of the extract with a polar solvent such as methanol, ethanol or acetic acid [with LiC1, NH4SCN, NaBr or tributylammonium perchlorate (TBAP) as supporting electrolyte] in order to enhance the conductivitity of the solution.Fano et aZ.2 carried out the polarographic determination of Pb, Cd and Zn, extracted into benzene or chloroform as dithizonates, by releasing the respective cation by addition of AgN03 solution in methanol. Co(II), Ni, Cd, Zn and As(II1) do not interfere in the determination of Pb, Cd and Zn as these cations are not extracted into the organic phase as dithizonates. Antimony can be extracted into toluene with tributyl phosphate (TBP) from 2 M HCl solution.3 The determination of Sb was carried out by ASV in mixtures of extracted phase with methanol that was 0.5 M in LiCl, with a detection limit of 2 X 10-8 M of Sb. Bismuth at concentrations down to 10-8 M can be determined by ASV in an organic phase (chloroform - methanol - 0.25 M NH4SCN) after extraction into chloroform with KI and methylene green.The extraction eliminates interferences from Sb, Sn and Cu in the determination of Bi.4 Pb and Sn give close peaks in ASV. However, Sn(IV) can be extracted into benzene from 0.5 M NaI - 1.5 M HC104 - 3 M NaC104 solution.5 The determination of Sn was carried out by ASV on an extract - methanol (containing 0.33 M NaBr) mixture. Pb extracted into chloroform with dithizone can be released by the addition of mercury(I1) chloride and deter- mined by ASV.6 In the determination of Cd by ASV in aqueous solution, Pb, Ni and Zn interfere. By extraction of Cd into acetonitrile [by the salting out effect with (NH4)*S04] as the iodide association complex with TBA, these interferences are eliminated.The detection limit is about 0.2 pg ml-1 of Cd in the extract.7 Molybdenum has been determined by differential-pulse polarography (DPP) after its separation by 8-hydroxyquino- line extraction into dichloromethane with 0.1 M TBAP as supporting electrolyte.8 In this work, the determination of copper was carried out by polarography and ASV after its extraction into 1,2- dichloroethane with different organic reagents. By the addi- tion of 5% V/V HC104 solution in N,N-dimethylformamide (DMF) to the extract, copper is liberated in the form of Cu2+ solvated with DMF and partially as undissociated copper perchlorate. This is due to the high solvation power and dielectric constant (36.5) of DMF. Perchloric acid in DMF also supplies sufficient electrical conductivity to organic media as a supporting electrolyte.The proposed method can be applied to the determination of other elements such as Sb, Bi, Cd, Sn and Pb, and even the simultaneous determination of several elements (such as Pb + Cu, Pb + Bi, Pb + Cd, Pb + Cd + Bi and Pb + Cd + Cu) by ASV. The method is sensitive and very selective and it has been applied to the determination of copper at the parts per million level in biological samples and ores. Experimental Apparatus Direct current polarographic and ASV measurements were made with an LKB Type 3266 Blomgren Polarolyzer, equipped with a Linear Instruments Model 252-A X - Y recorder. A dropping-mercury electrode (DME), a hanging mercury drop electrode (HMDE) (Kemula electrode), a silver - silver chloride (0.1 M KC1 in ethanol) electrode or mercury pool as reference electrodes and a platinum electrode as counter electrode were used to obtain current - potential curves.Dissolved oxygen was removed by bubbling oxygen-free nitrogen, previously saturated with solvent or solvent mixture for 10 min. The nitrogen stream was then directed over the solution surface. In polarographic determinations, the following parameters were used: DME characteristics, rTz = 2.19 mg s-1; t = 3.79 s with open circuit; scan rate, 0.2 V min-1; and sensitivity, 20 pA (full-scale).620 ANALYST, JUNE 1986, VOL. 111 In ASV determinations the instrumental conditions were as follows: sensitivity, 10 yA (full-scale); electrolysis potential (Eelect), -0.70 V; electrolysis time (telect), 6 min; rest time, 30 s; and scan rate, 0.4 V min-1. An Orion Research microprocessor was used for pH measurements with glass - calomel electrodes in the aqueous phase after extraction.Other apparatus consisted of a Haake thermostatic bath (25 "C), a Commercial Instruments Cedar Grove instrument for electrolytic conductivity measurements and Gallenkamp apparatus for mercury distillation. Reagents Analytical-reagent grade reagents were used unless stated otherwise. Doubly distilled water was used in all measure- ments. Copper stock standard solution, 1000 pg ml-1. Prepared by dissolving electrolytic copper (Merck) in HN03 (1 + l), adding 5 ml of HzS04, warming to evolution of white fumes and diluting to 1000 ml in a calibrated flask. Copper working standard solution, 10 pg ml-1.Prepared at moment of use by diluting 10 ml of the above stock standard solution to 1 1. Tartaric acid solution, 40% mlV. Perchloric acid, 70% mlV. Merck. Caution-Perchloric acid is dangerous and appropriate precautions should be taken. However, the 5% solution in DMF used in this procedure is not hazardous. N,N-Dimethylformamide (DMF). This was distilled with anhydrous sodium hydrogen carbonate, collecting the fraction between 148 and 152 "C. Bufler solution (PH 4.80). Acetic acid - 0.1 M sodium acetate . Ascorbic acid solution, 10% m1V. 1,2-Dichloroethane. Merck. Supporting electrolyte solution. A 5% VIV solution of Extraction solution. A 0.2% mlVsolution of salicylaldoxime Standard metal solutions, 1000 pg ml-1. Solutions of diverse perchloric acid in DMF.(Merck) in 1 ,Zdichloroethane. ions were used for interference studies. Procedure Sample treatment Transfer an ore sample (serpentinite type) weighing 0.2-0.5 g to a 100-ml PTFE beaker. Add 3 ml of concentrated hydrochloric acid and then 3 ml of concentrated nitric acid. Warm carefully by boiling on a hot-plate for 5 min, then add 2 ml of concentrated hydrofluoric acid (48% mlV). Evaporate the solution almost to dryness. Repeat the procedure once more. Dissolve the residue in 10 ml of 4 M nitric acid by warming and dilute to 50 ml with water in a calibrated flask. Take a biological sample weighing up to 2 g in a 250-ml beaker and decompose it with 10 ml of concentrated nitric acid on a hot-plate. Evaporate almost to dryness and repeat the procedure once more.Cool and add 10 ml of perchloric acid and heat gently at 170 "C for 15 min. Dilute to 50 ml with water in a calibrated flask. Determination Take a known volume of the sample solution (about 10 ml) in a 100-ml separating funnel, add 2 ml of ascorbic acid solution and adjust the pH by the addition of 10 ml of buffer solution (pH 4.80). Extract with 10 ml of extraction solution, shaking mechan- ically for 5 min. Allow the phases to separate. Place 9 ml of extracted organic layer in the polarographic cell and add 6 ml of perchloric acid solution in DMF. Remove the dissolved oxygen by bubbling through with oxygen-free nitrogen for 10 min. Record the polarogram or the anodic-stripping voltam- mogram at 25 "C under the specified conditions. Results and Discussion D.c.Polarographic Behaviour of Metal Complexes Preliminary polarographic studies were made for Zn( 11) and Cu(I1) complexes with trioctylamine (TOA) , a-benzoin oxime and salicylaldoxime extracted into toluene by the addition to the organic phase of a 0.15 M solution of tetrabutylammonium bromide (TBAB) in DMF as the supporting electrolyte. Table 1 lists the reagents, pH (or acidity) of extraction of the aqueous phase and half-wave potentials obtained under the experimental conditions. In the range of electroactivity available for the toluene - DMF mixture (up to -2.9 V vs. Ag - AgCl electrode) using TBAB as the supporting electrolyte, the extracted metal complexes examined gave d.c. polarograms but at too negative potentials, probably owing to the high formation constants of the complexes.The solvent used for extraction was toluene and the supporting electrolyte was a 0.15 M solution of TBAB in DMF. Generally the d.c. polarographic waves of the metal complexes appeared at fairly negative potentials, near to the discharge of the solvent or supporting electrolyte. They were often badly defined and presented a polarographic maximum and high residual currents. Additionally, the method lost selectivity because the waves of the different metal complexes accumulated in a narrow range of negative potentials. ASV df- terminations cannot be carried out under these conditions. However, the addition of a 5% VIV solution of perchloric acid in DMF to the metal complex extract releases the solvated cation of the metal complex and increases the electrical conductivity of the solutions used as the supporting electrolyte.Polarographic studies were made on 11 metal complexes with 8-hydroxyquinoline7 dithizone , diethyl dithiocarbamate (DDTC), pyrrolidine dithiocarbamate (PDTC) , benzoyl- phenylhydroxamic acid (BPHA) , tributyl phosphate (TBP) , trioctylphosphine oxide (TOPO) , cupferron, a-benzoin oxime and salicylaldoxime extracted into toluene , chloroform and 172-dichloroethane, with the addition of a solution of perchloric acid in DMF as supporting electrolyte. The pH values for these extractions have been given e1sewhere.g-11 Table 2 lists the reagents, pH (or acidity) of extraction, the solvents used for extraction and the half-wave potentials obtained under the experimental conditions. In all instances , the supporting electrolyte used was perchloric acid at a concentration in the final solution of 2% V/V, prepared by the addition to 9 ml of extract of 6 ml of 5% VIVsolution of HC104 in DMF.In the range of electroactivity available for the solvents used (up to -0.95 V vs. Ag - AgCl electrode for CHC13 and 1,2-dichloroethane - DMF or up to -1.30 V for toluene - DMF), most of the metal complexes examined gave d.c. polarograms after extraction. The half-wave potentials were nearly independent of the reagent used for extraction owing to the rupture of the Table 1. Half-wave potentials for Cu and Zn complexes with TBAB as supporting electrolyte Acidity of Cation Reagent aqueous phase EJV Cu . . . . . .TOA 1 . 0 ~ H C l -1.90 Salicylaldoxime pH 4.80 -1.60 a-Benzoin oxime pH 11.80 -2.70* Zn .. . . . .TOA 0.8 M HC1 - 1.65 * Badly defined.ANALYST, JUNE 1986, VOL. 111 621 Table 2. Polarographic data for various metal complexes Cation Reagent As(II1) . . . DDTC Bi . . . . . . 8-Hydroxyquinoline BPHA DDTC Dithizone DDTC BPHA PDTC DDTC Dithizone Cupferron Cd . . . . . . 8-Hydroxyquinoline Cu(I1) . . . . 8-Hydroxyquinoline Salicylaldoxime a-Benzoin oxime Mo(V1) . . . . 8-Hydroxyquinoline BPHA a-Benzoin oxime Ni . . . . . . BPHA Pb . . . . . . 8-Hydroxyquinoline DDTC Dithizone BPHA Sb(II1) . . . . BPHA DDTC Sn(IV) . . . . BPHA Dithizone U(V1) . . . . 8-Hydroxyquinoline BPHA TBP TOP0 Zn . . . . . . BPHA Dithizone * Polarographic maximum appears. t 1,2-DET = 1,2-dichIoroethane. pH or acidity Solvent 5 3.5 4 10 3 6.2 8.9 8.9 7 7 7 3 2 4.8 11.8 2 1 1 M HCI 9.3 8 3 8 8.9 1 M HCI 8.9 1 M HCl 8 8 3.5 0.1 M HN03 Toluene CHC13 CHC13 Toluene Toluene Toluene Toluene Toluene CHC13 CHC13 CHCI3 CHC13 CHC13 CHCl3, toluene or 1,2-DET*t CHCl3, toluene or 1,2-DET CHC13 CHC13 Toluene Toluene Toluene Toluene CHC13, toluene Toluene Toluene Toluene Toluene Toluene Toluene CHC13 CHC13 0.15 M HN03 Toluene 8.9 Toluene 4.5 Toluene EdV -0.15, -0.85* -0.15 -0.12 -0.20 No wave appears -0.95 -0.85 -0.90 -0.10, -0.32 1 Polarographic } curves badly J defined Reduction of reagent (ca.-0.1 V) -0.10, -0.30 -0.10, -0.30 -0.82, -0.43 -0.53, -0.95 -0.50, -0.95 -0.75 -0.65* No wave appears -0.65 -0.10 -0.30* -0.55 -0.25, -0.70 -0.20, -0.65 E,- -0.38, -0.80 No wave appears No wave appears 1 Badly defined waves No wave appears No wave appears metal complex by perchloric acid.The polarographic waves corresponded to slow or irreversible electrode processes and the limiting current was controlled by diffussion (id hc-& = constant). Two polarographic waves with very different diffusion currents appeared for Cu(II) (E+ -0.10 V and -0.30 V) owing to the presence of solvated Cu2+(DMF) and undissociated copper perchlorate in equilibrium (Fig. 1). In order to check this assertion, the influence of perchlorate concentration on the limiting currents (id1 and id2) was studied by the addition of lithium perchlorate to perchloric acid solution in DMF, as shown in Table 3. For the first wave the limiting current must be id1 = K1 [c~2+]solv. and for the second wave id2 = K2 [cu2+ (C104-)2]solv.but the dissociation constant for copper perchlorate should be [C~*+IsOiv. [C104-12 D2+ P O 4 -)2lsolv. Kd = Therefore, Log (f!d2/idl) = 2 log [CIO4-] -I- K The data in Table 3 yield a linear regression equation { Y = A + SX, where Y = log (id&&) and X = log [C104-1) with B = 2.24, A = -0.6460 and the correlation coefficient R = 0.993. Another confirmation of this situation was obtained by using the solubility in DMF of some copper salts, such as the perchlorate, chloride, sulphate and nitrate. The polarographic waves of copper perchlorate with HC104 in DMF were the Table 3. Influence of perchlorate concentration on the limiting currents of Cu [Clo,-]/M Log [c104-] idl/pA id2/@ Log (&/idl) 0.645 -0.1904 2.085 0.182 0.087 -1.0605 0.647 -0.1891 2.102 0.167 0.079 -1.1024 0.710 -0.1487 2.260 0.235 0.104 -0.9830 0.740 -0.1308 2.135 0.260 0.122 -0.9136 0.810 -0.0915 2.080 0.305 0.147 -0.8327 0.870 -0.0605 2.075 0.325 0.157 -0.8041 0.980 -0.0088 1.975 0.450 0.228 -0.6421 1.090 +0.0374 1.930 0.505 0.262 -0.5817 1 I 1 I 1 0 -0.10 -0.30 -0.90 Potent ia IN Fig.1. Polarographic waves of Cu(I1) after its extraction with salicylaldoxime into 1,2-dichloroethane, with addition to the extract of 5% V/V HCIO, - DMF solution. 1, Blank (without couper); 2,18 pg ml-l of Cu (idl 1.71 pA and id2 0.14 PA); and 3,21 pg ml-1 of Cu (idl 1.96 pA and id2 0.16 pA)622 ANALYST, JUNE 1986, VOL. 111 Table 4. Extraction and ASV data for various metal complexes Cation Extraction reagent Extraction solvent As(II1) . . .. . . Bi(II1) . . . . . . Cd . . , . . . c u . . . . . Mo(V1). . . . . . Sb(II1) . . . . . . Sn(IV) . . . . . . Pb . . . . . . DDTC 8-Hydrox yquinoline DDTC BPHA DDTC BPHA 8-Hydrox yquinoline Salicylaldoxime a-Benzoin oxime DDTC 8-Hydrox yquinoline PDTC BPHA Dithizone wBenzoin oxime BPHA BPHA 8-Hydroxyquinoline DDTC BPHA Toluene Toluene Toluene Toluene Toluene Toluene Toluene CHC13 or 1,2-DET CHC13 or 1,2-DET Toluene Toluene Toluene Toluene Toluene Toluene Toluene Toluene Toluene Toluene Toluene Eelect./V -0.75 -0.75 -0.75 -0.75 -1.20 -1.20 - 1.20 -0.70 -0.70 -0.70 -0.70 -0.70 -0.70 -0.70 -1.15 -0.70 -1.00 -0.90 -0.90 -0.90 E,IV No peak -0.26 -0.24 -0.30 -0.75 -0.80 -0.80 -0.08 -0.08 -0.08 -0.12 Badly defined -0.11 Badly defined No peak -0.28 -0.54 -0.55 -0.46 -0.58 Table 5.Extraction and ASV data for simultaneous determination of cations Cations Reagent pH or acidity Solvent Pb, Cu(I1) . . . . . . 8-Hydroxyquinoline Sn(IV),Sb(III) . . . . BPHA Pb, Sb(II1) . . . . . . BPHA Sn(IV),Cu(II) . . . . 8-Hydroxyquinoline Pb, Bi . . . . . . 8-Hydroxyquinoline Pb,Cd . . . . . . BPHA DDTC Pb,Sn(IV),Cu . . . . 8-Hydroxyquinoline Pb,Cd,Bi(III) . . . . DDTC Pb,Cd,Cu(II) . . . . DDTC * Badly defined peaks. 8 1 MHCI 1 5 6.5 8.9 8.9 6.15 8.9 9.3 Toluene Toluene Toluene Toluene Toluene Toluene Toluene Toluene Toluene Toluene E,, V -0.60, -0.16 -0.55, -0.25* -0.61,* -0.11* -0.60, -0.22 -0.66, -0.84 -0.64, -0.90 -0.66, -0.84, -0.30 -0.60, -0.77, -0.10 * - * - same as those obtained by extraction. The formation of complexes or undissociated salts with chloride, sulphate or nitrate was also evident from the shift in the half-wave potential at more negative values.The effect of the water content of DMF on the polaro- graphic curves was studied. Water at levels up to 10% V/Vin DMF did not influence appreciably either the polarographic wave or the limiting currents. Solvents for Extraction Toluene (E = 2.4; b.p. 111 "C) has been used as an extraction solvent because a toluene - DMF - HC104 mixture gives a wider range of electroactivity (up to - 1.3 V) than chloroform (E = 4.8; b.p. 61.3 "C) or 1,2-dichloroethane (E = 10.2; b.p. 83.6 "C) (about -0.95 V in the presence of HC104 solution in DMF). However, copper complexes were better extracted into chloroform owing to their greater solubility in this solvent. Finally, 1,2-dichloroethane was preferred because the volatility of chloroform led to a loss of solvent during the elimination of oxygen by bubbling nitrogen, in spite of its previous saturation.The solubility of metal complexes in 1,2-dichloroethane was similar to that in chloroform and there were no difficulties in the extraction process. Anodic-stripping Voltammetry in Non-aqueous Solution From the polarographic waves obtained in the above proce- dure, it was possible to design an ASV method for the determination of copper and other ions by using a hanging mercury drop electrode (HMDE). The electrolytic potential applied in the first stage was 0.3 V more negative than the corresponding half-wave potential. However, this electrolytic Table 6. Effect of Cu concentration and teleCt.on the AS\' peak Cu(I1) concentration/ pg ml-1 telect.Jmin ASV peak i,lpA 0.405 10 1.17 1.220 6 2.31 4.560 4 5.94 6.250 2 4.04 Table 7. Tolerance limits in the determination of 40 pg of Cu(I1) Tolerance Element limithg Ion : Cu ratio (mlm) Mn(II), Al, Mg, Pb, Ca . . . . 40 1000 Zn,Sn(IV). . . . . . . . . . 30 750 Fe(III), F- . . . . . . . . . . 20 500 Ni,Cd,Co . . . . . . . . . . 10 250 V ( V ) . . . . . . , . . . . . 6 150 Bi(III), Mo(VI), Ti(IV), Sr, Ba . . 4 100 potential was verified as giving the maximum ASV peak height. The electrolysis time used was from 1 to 15 min, depending on the concentration of the extracted solution. The scan rate was 6.3 mV s-1. The results are given in Table 4. Simultaneous Determination of Different Elements by ASV The determination of different elements simultaneously extracted by the same reagent was studied.A suitable pH value of the aqueous solution must be used for quantitative extraction with the respective reagent. Table 5 lists the mixtures studied, extraction reagents, pH (or acidity) of the aqueous phase, solvents used and peak potentials ( E p ) for each element.ANALYST, JUNE 1986, VOL. 111 623 ~ ~~~~~ Table 8. Results for the determination of copper in biological samples and ore Relative standard Sample Cu present, Yo Cu found, YO * deviation, Yo * Vitis vinifera . . . . . . . . 0.0887 0.071 2.2 Pyrus malus . . . . . . . . O.OlS? 0.019 2.5 Olea europea . . . . . . . . 0.0047 0.004 2.6 Gossypium herbaceum . , . . 0.0027 0.002 3.0 Calf liver .. . . . . . . Mussels . . . . . . . . - 0.001 - 0.001 3.5 3.5 Serpentinite (IGS-22) . . . . 0.106$ 0.102 2.9 * Average of five determinations. t Approximate values according to CII (Comitk Inter-Instituts pour 1’Analyse Foliare, France). $ Certified value by Institute of Geological Sciences (London). I I I 1 -0.10 -0.60 -0.77 - 1 PotentialiV Fig. 2. ASV peaks obtained for the simultaneous determination of Cd, Pb and Cu(I1) (Ep = -0.77, -0.60 and -0.10 V, respectively) after their extraction with DDTC into toluene, by addition to the extract of 5% V/V HC104 ; DMF solution. Approximate concentra- tions in the measured solution: Cd 2.70 pg ml-1, Pb 2.40 yg ml-1 and Cu 1.60 yg ml- I PotentialiV Fig. 3. ASV peak for Cu(I1) after its extraction with salicylaldoxime into 1,Z-dichloroethane, with addition to the extract of 5% V/V HC104 - DMF solution.Concentration of Cu(I1) in the measured solution, 0.50 yg ml-1 (peak intensity, i, = 3.90 pA) The instrumental parameters for ASV determinations were as follows: telect, = 6 min; Eelect, = -1.1 V (vs. Ag - AgCl electrode); rest time, 30 s; scan rate, 6.3 mV s-1; and sensitivity, 10 pA (full-scale). Fig. 2 shows the ASV voltam- metric curves for Pb + Cd + Cu(I1). Determination of Copper by ASV Copper(I1) ions were extracted with salicylaldoxime into 1,2-dichloroethane from aqueous solution buffered at pH 4.80. After separation of the phases, 9 ml of organic phase were mixed with 6 ml of 5% V/V solution of perchloric acid in DMF. The ASV peak for copper(I1) in 172-dichloroethane - DMF - HC104 is shown in Fig.3. The instrumental parameters were as follows: Eelect, = -0.70 V; telect, = 6 min; rest time, 30 s; and sensitivity, 5 pA (full-scale). Results for different concentrations of copper(I1) and electrolysis times are shown in Table 6. The other parameters were the same as mentioned above. A caIibration graph was constructed for an electrolysis time of 6 min and the same instrumental parameters. The graph of peak height versus concentration of copper was linear from 0.1 to 4.5 pg ml-1 of Cu(I1) (peak intensity from 0.18 to 8.52 PA). The peak intensity for blank solutions due to copper contami- nation of the chemicals was less than 0.05 pA. The detection limit, as three times the standard deviation of the blanks, was 15 ng ml-1 of copper. The precision of the determination of 40 pg of copper(I1) (2.67 pg ml-1 in the final measured solution) was 2.9% (eight replicate determinations). Interference Study The effects of different ions on the determination of 40 pg of copper (2.67 pg ml-1 in the measured solution) were studied. Tolerance limits, defined as the amount of interferent that did not give an error larger than 5%, are given in Table 7. Applications The method was applied to the determination of copper in biological samples and a nickel ore (serpentinite, IGS-22) certified to contain 0.106% of copper. The results are given in Table 8. References 1. 2. 3. 4. 5. 6. Budnikov, G. K . , and Makhovich, N. A., Russ. Chem. Rev., 1980, 49, 74. Fano, V., Licci, F., and Zanotti, L., Microchem. J . , 1974, 19, 163. Nghi, T. V., and Vydra, F., Anal. Chim. Acta, 1975, 80, 267. Nghi, T. V., and Vydra, F., J . Electroanal. Chem., 1976, 71, 325. Nghi, T. V., and Vydra, F., J . Electroanal. Chem., 1976, 71, 333. Nghi, T. V., and Vydra, F., J . Electroanal. Chem., 1977, 78, 167.624 ANALYST, JUNE 1986, VOL. 111 7. Nagaosa, Y., and Yamada, T., Talanta, 1984,31, 371. 8 . Nagaosa, Y., and Kobayashi, K . , Talanta, 1984,31, 593. 9. Minczewski, J., Chwastowska, J. , and Dybczynski, R., “Sepa- ration and Preconcentration Methods in Inorganic Trace Analysis,’’ Ellis Horwood, Chichester, 1982. Elements,” Ellis Horwood, Chichester, 1976. 11. Zolotov, Yu. A . , Bodnya, V. A., and Zagrunina, A. N., CRC Crit. Rev. Anal. Chern., 1983, 14, 92. Paper A51223 10. Marczenko, Z., “Spectrophotometric Determination of Received June 21st, 1985 Accepted January 15th, 1986