Complexation of Nickel with 2-(Aminomethyl) pyridine at High Zinc

Article pubs.acs.org/jced

Complexation of Nickel with 2‑(Aminomethyl)pyridine at High Zinc Concentrations or in a Nonaqueous Solvent Mixture Markku Laatikainen,†,* Katri Laatikainen,† Satu-Pia Reinikainen,‡ Helena Hyvönen,§ Catherine Branger,∥ Heli Siren,‡ and Tuomo Sainio† †

Laboratory Separation Technology, Lappeenranta University of Technology, P.O. Box 20, FI-53851 Lappeenranta, Finland Laboratory of Chemistry, Lappeenranta University of Technology, P.O. Box 20, FI-53851 Lappeenranta, Finland § Laboratory of Inorganic Chemistry, University of Helsinki, P.O. Box 55, FI-00014, Helsinki, Finland ∥ MAPIEM, University of Toulon, EA 4323, 83957 La Garde, France ‡

ABSTRACT: Complexation of Ni2+ with 2-(aminomethyl)pyridine (amp) has been studied at 22 °C using absorption spectroscopy. Effect of high Zn2+ concentration on complex formation in aqueous sulfate solutions and complexation of Ni(ClO4)2 with amp in a nonaqueous methanol/2-methoxyethanol mixture were investigated. The spectra were analyzed using least-squares minimization technique (LS) and partial least-squares method (PLS) to obtain the stability constants and distribution of the complex species. Moreover, complex formation was studied qualitatively by means of continuous variation analysis. The data measured at zinc concentrations of 0.100−1.84 mol·kg−1 indicate only minor deviations from the equilibria calculated using literature stability constants. At the highest zinc concentration, the equilibrium is slightly shifted to higher complexes but no evidence was found of changes in the octahedral complex geometry. Octahedral 1:1, 1:2, and 1:3 complexes were observed also in the nonaqueous solvent mixture but the stability constants were smaller than in aqueous solutions. Moreover, the equilibrium was shifted toward higher complexes, and a higher concentration of the 1:3 complex was obtained in nonaqueous than in aqueous solution. The results are discussed in view of the separation of nickel from concentrated ZnSO4 solutions and of the design of ion-imprinted materials.

1. INTRODUCTION Complexation of transition metals with 2-(aminomethyl)pyridine (amp) in aqueous solutions has been widely studied and the stability constants can be found in standard compilations.1 In particular, amp and Ni2+ form 1:1, 1:2, and 1:3 complexes with octahedral geometry and the stability constants have been reported, e.g., by Garcia-Espana et al.2 and by Goeminne and Eeckhaut.3 When combined with the protonation constants of amp, the behavior at different pH values is obtained. In a similar way, complex formation in multimetal solutions can be predicted using the stability constants of individual metals. However, this approach is less accurate in highly asymmetric mixtures containing a large excess of other complexing metals than nickel. On the other hand, stability constants in nonaqueous or aqueous−organic solvent systems are rarely known and methods are needed to find their values. Complex formation is traditionally analyzed by means of potentiometric titration, conductivity, calorimetric, and spectroscopic data. The procedures are well-established and a large number of methods have been developed to treat the experimental data. Spectroscopic methods allow highly sensitive measurements at various solution conditions, and electronic absorption spectra are commonly used in complexation studies. Decomposition of the spectra can be done with least-squares minimization methods (LS).4,5 In strongly overlapping systems, © XXXX American Chemical Society

chemometric methods including partial least-squares (PLS), principal component analysis (PCA), or various factor analysis methods are also used. For example, Zarei et al.6 have used PLS to analyze UV−vis spectra of Fe, Ni, and Co complexes with 1(2-pyridylazo)-2-naphtol. Rank annihilation factor analysis has been applied in decomposition of spectra containing strongly overlapping absorptions of metal and ligand.7,8 The same methods can be applied to solutions containing also other solvents than water.9,10 In this study, complex formation of nickel with amp is investigated using absorption spectroscopy in the visible range, and the experimental spectra are analyzed using the LS and PLS methods. The measurements were made either in the presence of a large excess of zinc or in a nonaqueous mixture of methanol and 2-methoxyethanol. Complex formation is measured by keeping the overall concentration of nickel and amp constant and by varying the mole ratio. This procedure was selected, because it also allows continuous variation analysis of complexes formed at different conditions. The results are compared with calculated data obtained using literature values of stability constants. The novelty of this study Received: February 18, 2014 Accepted: May 28, 2014

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dx.doi.org/10.1021/je500164h | J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Typically 14 spectra at different mole ratios were recorded. All experiments were made at room temperature (22 ± 1 °C). All solutions were prepared on molar basis because molar concentrations are conventionally used in complexation studies. The molar concentrations c were converted to molalities m using experimentally determined and literature values for solution densities. Effect of Zn2+ was studied at pH 5.0 ± 0.1 and using sulfate matrix. NiSO4·6H2O was used as the nickel source. Four sample series were made with constant zinc concentration of (0, 0.100, 0.805, and 1.84) mol·kg−1 [(0, 0.100, 0.800, and 1.800) mol·L−1]. When zinc was not present, the supporting ionic strength Is was adjusted with Na2SO4. In other zinc concentrations supporting electrolyte was not used because of precipitation problem in concentrated zinc solutions. Influence of organic solvent was measured using methanol/2methoxyethanol mixture as solvent and Ni(ClO4)2·6H2O as the nickel source. Methanol mole fraction in the solvent mixture is xMeOH = 0.66. Perchlorate salt was used instead of sulfate because of better solubility in the organic solvent. The absorption spectra of the Ni-amp complexes were measured at room temperature with UV−vis spectrophotometers (Agilent 8453, Jasco V670) using a quartz cuvette with a light path of 1 cm. The concentrations of the sample solutions are estimated to be correct within 2 %. The accuracy of the spectra was tested by repeating a measurement several times over a period of hours. The average absolute difference in the wavelength range (400− 1000) nm was 0.001 absorbance units. 2.2.2. Continuous Variation Method. The continuous variation method originally introduced by Job18 is most suitable for cases, where only one complex is formed. When plotting the absorbance, A, as a function of the ligand mole fraction, x, the coordination number, NL, is simply obtained from the position of the maximum (eq 1). Following the normal procedure, total molar concentration was kept constant. On molal basis, this is not true but because of the low nickel and ligand concentrations used here, the difference is relatively small. In the presence of highest zinc concentration (1.84 mol·kg−1), for example, the molal concentration of nickel is 0.102 mol·kg−1 when the molar concentration is 0.100 mol·L−1.

lies in treatment of strongly overlapping spectral data measured at nonstandard conditions. The first case considered in this study is motivated by our earlier findings in hydrometallurgical separation with a chelating adsorbent containing anchored amp as functional group.11,12 In hydrometallurgical applications of chelating ion exchange and adsorption, typically very small amounts of impurity metals are removed from the concentrated target metal solution. In particular, solution purification in zinc production involves removal of nickel at concentrations, which are 104 times lower than the zinc concentration. At such extreme conditions, unexpectedly low nickel uptake has been noticed and reevaluation of the complex stability in homogeneous solution was considered essential.11 Although behavior in the free solution is not directly comparable with the solid adsorbent case, it gives insight in the processes taking place in the solid phase. Nevertheless, very little attention has been paid to the behavior of the ligand groups at conditions comparable with the actual separation process. The second case is related to synthesis of ion-imprinted polymers, where the template (in this case Ni2+) is incorporated in the polymerization mixture.13,14 Nonaqueous solvents are typically used and knowledge on complex formation in such environments is needed for optimal design of the reaction mixture. The ligand amp is not as such polymerizable but it is used here as a readily available analogue for, e.g., its vinylbenzyl derivative. Continuous variation method has been used to analyze complex formation between the target metal and the selected ligand in the polymerization solvent.15,16 However, only qualitative information can be obtained. Shamsipur et al.,17 on the other hand, have applied quantitative spectral analysis to verify formation of the desired stable Cu 2+ complex incorporated then in the ion-imprinted resin. However, actual distribution of the copper species in the polymerization mixture was not discussed and the ligand/Cu2+ ratio was simply taken from the maximum in the absorbance vs composition plot. In this sense the treatment is equivalent with the continuous variation method. The idea of this study is to introduce a method to analyze the actual complex distribution before polymerization with ion imprinting technique. The method reported here will be used in our forthcoming studies dealing with development of highly selective separation materials for hydrometallurgical applications.

NL = xmax /(1 − xmax )

(1)

However, as shown already by Vosburgh and Cooper,19 the method can be applied to more complicated systems provided that the wavelengths are selected appropriately.20,21 In the Ni2+amp system studied here in the visible wavelength range, the ligand is nonabsorbing and there are four absorbing species: [Ni(H2O)6]2+, [Ni(amp)(H2O)4]2+, [Ni(amp)2(H2O)2]2+, and [Ni(amp)3]2+. Below, the water (or solvent) ligands are omitted in the formulas for simplicity. The reaction sequence and the definition of the stepwise stability constant K are shown in eq 2, where cθ = 1 mol·L−1 is the unit molar concentration.

2. EXPERIMENTAL AND COMPUTATIONAL METHODS 2.1. Materials. The ligand 2-(aminomethyl)pyridine shown in Figure 1 (purity 99 %) was obtained from Sigma-Aldrich.

Figure 1. Chemical structure of 2-(aminomethyl)pyridine (amp).

Reagent-grade solvents methanol and 2-methoxyethanol as well as the metal salts ZnSO4·6H2O, NiSO4·6H2O, Ni(ClO4)2· 6H2O, and Na2SO4·10H2O were supplied by Sigma-Aldrich. All chemicals were used without further purification. All solutions were prepared using deionized water (conductivity less than 0.1 μS/cm). 2.2. Methods. 2.2.1. Complexation Measurements. Complexation between nickel and soluble amp was studied in separate batches by keeping the combined concentration of the nickel and amp constant at 0.100 mol·kg−1 (0.100 mol·L−1).

K1

K2

K3

Ni 2 + ⇄ [Ni(amp)]2 + ⇄ [Ni(amp)2 ]2 + ⇄ [Ni(amp)3 ]2 + c[Ni(amp)n ] θ c Kn = c[Ni(amp)n‐ 1]camp (2)

Only deprotonated amp is assumed to exist in the nonaqueous solution but protonation in aqueous solution is competing with complex formation. The protonation constants given in Table 2 B



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indicate that only the primary amine of amp can protonate at conditions used in this study but it is assumed for simplicity that even this partially protonated form amp-H is inactive in eq 2. Furthermore, the anions SO42− and ClO4− are assumed to be excluded from the structure of the inner-sphere complexes [NiLn]2+, where L is amp or solvent. Variation of the absorbance A with x can be then described by eq 3,19 where ε and c are the extinction coefficient and molar concentration. The constant total concentration of Ni2+ and amp is represented by ctot, and x = camp0/ctot. The summation includes the complexes [Ni(amp)]2+, [Ni(amp)2]2+ and [Ni(amp)3]2+. dA′ = dx

∑ (εj(λ) − εNi(λ)) j

dcj dx

A′ = A − εNi(λ)(1 − x)ctot

Figure 2. Absorption spectra of Ni2+-amp system at different ligand mole fractions x. T = 22 °C, pH 5.0, Is = 0.100 mol·kg−1, and ctot = 0.100 mol·L−1.

(3)

With suitable choice of wavelengths, maxima of all complexes can be found. In aqueous solutions, 650 nm was selected for the 1:1 complex, because the extinction coefficients of the 1:2 and 1:3 complexes are nearly equal to that of Ni2+ and eq 3 reduces to eq 4. The maximum in A′ thus coincides with the maximum in [Ni(amp)]2+concentration. dc dA′ ≈ (ε1:1(650 nm) − εNi(650 nm)) 1:1 dx dx

Table 1. Step-Wise Stability Constants log K Measured in This Study for the Ni2+-amp System in Aqueous Solution and in the Absence of Zinca log K

(4)

The value for the 1:3 complex was 515 nm in order to minimize interference by [Ni(amp)2]2+. Analysis of the 1:2 complex is most difficult because of strong overlapping of all three absorptions. The wavelength of 590 nm was selected on the basis of trial-and-error. The situation in the nonaqueous solvent mixture will be discussed in Section 3.2. 2.2.3. Estimation of Stability Constants. Stability constants were estimated using the HypSpec program based on the leastsquares (LS) minimization scheme.4 In principle, extinction coefficients and concentrations of all absorbing components are estimated. In this study, however, extinction coefficients of uncomplexed nickel were measured separately and kept constant, while the values for the complexes [Ni(amp)]2+, [Ni(amp)2]2+, and [Ni(amp)3]2+ were unknown. Typically 200 wavelengths between (450 and 1000) nm were used and all 14 spectra were calculated simultaneously. Typical set of experimental spectra is shown in Figure 2. The spectrum at x = 1 clearly shows that absorption by the ligand can be neglected. The LS method was first tested with Ni2+-amp system in aqueous solution and in the absence of zinc (mZn0 = 0). The results are given in Table 1. The 95 % confidence intervals given in Table 1 were estimated as ±2σ, where σ is the standard deviation. The literature values listed in Table 2 were used as initial guesses for estimation of the stability constants. The values obtained in this study for Is = 0.100 mol·kg−1 (0.100 mol·L−1) compare well with the literature values, although log K2 and log K3 tend to be slightly smaller than reported earlier.2 It is possible that the difference is due to the counterion, because the data of Garcia-Espana et al.2 was measured in nitrate solutions. Sulfate anion forms outer-sphere complexes with transition metals, while nitrate has much weaker complexing activity. When considering the experimental accuracy, however, the difference is too small to make any detailed conclusions. When the measurements were made at

complex

Is = 0.100 mol·kg−1

Is = 2.03 mol·kg−1

[Ni(amp)]2+ [Ni(amp)2]2+ [Ni(amp)3]2+

7.18 ± 0.05b 6.12 ± 0.09 4.92 ± 0.16

7.14 ± 0.06 6.37 ± 0.08 5.00 ± 0.09

T = 22 °C, pH 5.0. bThe uncertainty was estimated as twice the standard deviation.

a

Table 2. Literature Values of Step-Wise Stability Constants log K for H−Ni−Zn-amp Systema log K complex +

[H(amp)] [H2(amp)2]+ [Ni(amp)]2+ [Ni(amp)2]2+ [Ni(amp)3]2+ [Zn(amp)]2+ a

−1

Is = 0.15 mol·L , ref 2

Is = 0.5 mol·L−1, ref 3

8.649 2.084 7.128 6.231 4.985 5.30

8.79 2.31 7.11 6.41 5.14 5.37

T = 25 °C.

high ionic strength (Is = 2.03 mol·kg−1 (2.00 mol·L−1)), the equilibrium is slightly shifted toward [Ni(amp)3]2+ and this tendency is also observed in values of Goeminne and Eeckhaut3 measured at Is = 0.5 mol·L−1. The cumulative stability constant log β3 = Σ log Kj (j = 1−3) increase slightly from 18.2 at Is = 0.100 mol·kg−1 to 18.5 at Is = 2.03 mol·kg−1. Same trend is observed also in the literature values; log β3 = 18.3 at Is = 0.15 mol·L−1 and log β3 = 18.6 at Is = 0.5 mol·L−1.2,3 It seems, therefore, that the ionic strength is the main factor in increased stability. The extinction coefficients estimated for [Ni(amp)]2+, [Ni(amp)2]2+, and [Ni(amp)3]2+ at different ionic strengths are shown in Figure 3. The data obtained at an initial zinc concentration of 0.100 mol·kg−1 (dash-dotted lines) and in methanol/2-methoxyethanol solution (dotted lines) are included for comparison. In 1:1, 1:2, and 1:3 complexes, two intense absorptions due to spin-allowed transitions as well as the weak absorption of the spin-forbidden transition suggest an C



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In the present study, the PLS method is used to estimate the species distribution by using the absorption spectra. Same experimental data were used as in the LS method. No background correction was used, and the spectrum of noncomplexed nickel was filtered from the data. Extinction coefficients estimated for one case by PLS and LS are compared in Figure 4.

Figure 4. Extinction coefficients for [Ni(amp)]2+ obtained using LS (solid line) and PLS (dashed line). Is = 2.03 mol·kg−1, T = 22 °C, pH 5.0. Figure 3. Extinction coefficients of [Ni(amp)]2+ (A), [Ni(amp)2]2+ (B), and [Ni(amp)3]2+ (C). T = 22 °C, pH 5.0. Solid lines: mZn0 = 0, Is = 0.100 mol·kg−1; dashed lines: mZn0 = 0, Is = 2.03 mol·kg−1; dashdotted line: mZn0 = 0.100 mol·kg−1, Is = 0; dotted line: mZn0 = 0, Is = 0, 50 vol % methanol/2-methoxyethanol solution.

Good agreement is found in the position of the peaks but the absolute values estimated by PLS are about 15 % higher.

3. RESULTS AND DISCUSSION 3.1. Influence of Zinc Concentration on Formation of Ni-amp Complexes. In this section, formation of Ni-amp complexes at different zinc background concentrations is investigated. First, the system was qualitatively studied using the method of continuous variation. Because several complexes coexist in the solution, the analysis was made at specific wavelengths as discussed in section 2.2.3. The results are depicted in Figure 5 for each initial zinc concentration. It is evident from Figure 5 that all three Ni-amp complexes can be detected at zinc concentrations up to 0.805 mol·kg−1. The results shown for [Ni(amp)3]2+ at the highest concentration of 1.84 mol·kg−1 are uncertain and the data points may also represent absorption by the [Ni(amp)2]2+ complex. Next the experimental spectra were analyzed using the LS method described in section 2.2.3. Complexation of zinc is not accessible by absorption spectroscopy in the visible wavelength range and therefore no attempts were made to evaluate the stability constants for Zn2+. In order to simplify calculations, only [Zn(amp)]2+ is assumed to be present in the solution and the stability constants shown in Table 2 were used. Test calculations, where also [Zn(amp)2]2+ was included, showed that the influence on Ni-amp stability constants was very small. In a similar way, protonation constants were not re-evaluated but the values reported earlier and listed in Table 2 were used in calculations. Therefore, the values shown in Table 3 for the Ni-amp complexes are conditional values. Simultaneous evaluation of extinction coefficients and stability constant for all three complexes was possible only at the lowest zinc concentration, mZn0 = 0.100 mol·kg−1. At higher zinc concentrations, amount of [Ni(amp)3]2+ and at mZn0 = 1.84 mol·kg−1 even of [Ni(amp)2]2+ becomes too small for reliable estimation of both ε and K values. For these cases, the

octahedral geometry.22 Third spin-allowed transition overlaps with the charge-transfer absorption and is not shown here. The spectra of the individual species at the lowest ionic strength and in the absence of zinc are very similar to those reported by Garcia-Espana et al.2 They found the middle maximum at (635, 562, and 533) nm for complexes [Ni(amp)]2+, [Ni(amp)2]2+, and [Ni(amp)3]2+, while the corresponding values in Figure 3 are 618, 562, and 530 nm. Moreover, the increase in peak intensity from [Ni(amp)]2+ to [Ni(amp)2]2+ and [Ni(amp)3]2+ in Figure 3 agrees well with the earlier data.2 In contrast to the main maxima due to spinallowed transitions, absorption due to the spin-forbidden transition at 730 nm in Figure 3A shifts to lower energies as the number of amp ligands around the nickel ion increases. Increasing ionic strength shifts the peaks of [Ni(amp)2]2+ and [Ni(amp)3]2+ (Figure 3B,C) downfield and at the same time the intensity decreases. Small changes in opposite direction are observed for the 1:1 complex. Down-field shift is also found when the complexes are formed in organic solvent rather than in water and the positions of the maxima are (620, 564, and 541) nm. In summary, the LS method can be successfully used in analysis of the strongly overlapping spectral system of Ni2+ and amp. Both the extinction coefficients and stability constants obtained in this study compare favorably with results reported earlier. 2.2.4. Partial Least Squares Modeling. Partial least squares (PLS) regression is one of the most well-known multivariate methods applied in analyzing various spectroscopic data. The PLS method has been extensively presented in the literature,23 and no details are given here. D



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Figure 6. Influence of zinc concentration on distribution of Ni species. Ni2+ (circles), Ni(amp)]2+ (triangles), [Ni(amp)2]2+ (squares), and [Ni(amp)3]2+ (diamonds). mZn0 = 0.100 mol·kg−1 (A), 0.805 mol·kg−1 (B), and 1.84 mol·kg−1 (C). T = 22 °C, pH 5.0. Open and filled symbols represent the PLS and LS models, respectively. Continuous lines were calculated with the stability constants of Goeminne and Eeckhaut.3

Figure 5. Continuous variation plots for complex formation of Ni2+ with amp in the presence of Zn2+. [Ni(amp)]2+ at 650 nm (circles), [Ni(amp)2]2+ at 590 nm (squares), [Ni(amp)3]2+ at 515 nm (triangles). cNi0 + camp0 = 0.1 mol·L−1, T = 22 °C, pH 5.0. The vertical dashed lines represent maxima of 1:1(x = 0.5), 1:2 (x = 0.67), and 1:3 (x = 0.75) complexes. Solid lines are drawn to connect the experimental points. A: mZn0 = 0 mol·L−1, Is = 2.03 mol·kg−1; B: cZn0 = 0.100 mol·kg−1, Is = 0; C: cZn0 = 0.805 mol·kg−1, Is = 0; D: cZn0 = 1.84 mol·kg−1, Is = 0.

For comparison, the compositions calculated using the stability constants of Goemine and Eeckhaut3 (Table 2) are given as continuous lines. Because of this comparison, the data are plotted against molar rather than molal concentrations. As discussed earlier, however, error made in replacement of cNi0 by mNi0 is less than 2 %. The general trend in Figure 6 is that the amount of higher nickel complexes decreases markedly in the presence of a competing metal. In particular, the concentration of [Ni(amp)3]2+ becomes negligible as the zinc concentration is around 1 mol·kg−1 or higher. As a result, the average amp/Ni2+ mole ratio is strongly decreased. In general, the results obtained with the LS and PLS methods agree well but at low values of cNi0 (or mNi0), the PLS model tends to result in inconsistent

extinction coefficients of 1:2 and 1:3 complexes were taken from data measured without zinc but at high ionic strength. When comparing the Ni-amp stability constants obtained at mZn0 = 0.100 mol·kg−1. with the values of Garcia-Espana et al.2 (Table 2) and those measured in this study for mZn0 = 0 and Is = 0.100 mol·kg−1 (Table 1), the values are similar. As mZn0 increases, log K1 of nickel decreases while log K2 and log K3 increase. This effect is at least partly due to increasing ionic strength, because similar trend was observed also in the presence of noncomplexing electrolyte Na2SO4 (see Table 1). Distribution of nickel species obtained with LS and PLS methods from the experimental spectra are shown in Figure 6.

Table 3. Step-Wise Stability Constants log K Obtained for Ni-amp in the Presence of Zinc and in Nonaqueous Solutiona log K aqueous solution (pH 5.0) complex 2+

[Ni(amp)] [Ni(amp)2]2+ [Ni(amp)3]2+ a

0

mZn = 0.100 mol·kg 7.14 ± 0.02 6.33 ± 0.05 5.02 ± 0.04

b

−1

mZn0

= 0.805 mol·L

methanol/2-methoxy-ethanol (xMeOH = 0.66)

−1

mZn0

7.06 ± 0.03 6.36 ± 0.10 5.48 ± 0.08

= 1.84 mol·kg 7.01 ± 0.03 6.50 ± 0.02 6.07 ± 0.02

−1

mZn0 = 0 6.43 ± 0.04 5.48 ± 0.08 5.51 ± 0.03

T = 22 °C, Is = 0. bThe uncertainty was estimated as twice the standard deviation. E



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values as shown for [Ni(amp)]2+ in Figure 6C and for [Ni(amp)2]2+ in Figure 6A. Moreover, inspection of the data in Figure 6 shows that experimental concentrations of noncomplexed Ni2+ and the three Ni-amp complexes differ only slightly from the calculated data. At the highest zinc concentration of 1.84 mol·kg−1, however, the calculated values somewhat overestimate the stability of [Ni(amp)]2+ and underestimate the stability of [Ni(amp)3]2+. The difference stems from the increase in logK3 value observed in Table 3. In summary, the stability constants determined for the Niamp system at moderate ionic strengths describe surprisingly well the behavior of Ni2+ and amp in the presence of high concentration of Zn2+. A wide Zn/Ni mole ratio of 1−150 was covered and only at highest zinc concentrations, slight deviations from calculated solution compositions were observed. No indications were found that the complex structure and coordination number changes. As discussed in the Introduction, the starting point for studying the Ni−Zn-amp system was the behavior found in amp-functionalized solid materials. Assuming that amp anchored on solid surface behaves similarly as the free ligand, the results imply that the complexation mechanism remains practically unchanged in the presence of high concentration of Zn2+. However, the average amp/Ni2+ mole ratio is expected to decrease in accordance with the data shown in Figure 6. At the same time, the average value for nickel binding constant decreases markedly, because the cumulative stability constant is much lower for the 1:1 constant than for the higher complexes. The decrease is clearly seen in Figure 5 by the gradual increase in the concentration of the noncomplexes nickel as the zinc concentration increases. Due to heterogeneous distribution of the ligand groups in solid matrix, actual ligand/metal ratio is not known. Measurements made earlier using reflectance absorption spectroscopy12 suggest a value between 1 and 2, when no zinc is present in the system. Assuming a value of 1.7, 70 % of nickel is bound as [Ni(amp)2]2+ and 30 % as [Ni(amp)]2+. Because of limited mobility of the ligands in solid matrix, formation of the 1:3 complex is highly improbable. Using the values of Table 2, the average stability constant for this composition is log K(aver) = 11.6. If 1.84 mol·kg−1 of zinc is present, an average ligand/ metal ratio around 1.2 can be estimated from Figure 6. In this case, log K(aver) is only 8.3 suggesting a decrease of more than three decades in the nickel binding constant. The values of Tables 1 and 2 are not directly applicable to complexation on solid surfaces but decrease in average ligand/metal ratio and in average binding constant gives a qualitative explanation for the observed trends. 3.2. Formation of Ni-amp Complexes in Methanol/2Methoxyethanol Mixture. As shown in Figure 3, the spectra of the Ni-amp complexes in the nonaqueous solvent mixture are similar to those obtained in aqueous solution indicating that octahedral geometry is formed in both cases.24 The maxima are shifted downfield by 10−20 nm and the effect is comparable with that induced by high ionic strength in aqueous solutions. The continuous variation plot depicted in Figure 7 clearly shows formation of the same complexes as found in aqueous solutions. Formation of the 1:1, 1:2, and 1:3 complexes was analyzed using λ = (620, 590, and 540) nm, respectively. Stability constants were estimated using the LS method. Protonation of amp was omitted and the ligand was assumed to exist as free base. The values given in the last column of Table 3 clearly show that K1 and K2 are distinctly lower and K3 higher

Figure 7. Continuous variation analysis of the Ni(ClO4)2-amp system in methanol/2-methoxyethanol mixture. [Ni(amp)]2+ at 620 nm (circles), [Ni(amp)2]2+ at 590 nm (squares), [Ni(amp)3]2+ at 540 nm (triangles). cNi0 + camp0 = 0.1 mol·L−1, T = 22 °C. Continuous lines are drawn to connect the experimental points.

than the values measured in aqueous solutions. The differences are more easily seen in Figure 8, where distributions of the nickel species in 50 vol % methanol/2-methoxyethanol mixture and in aqueous solution (Is = 0.100 mol·kg−1) are compared. For consistency, the distributions are again given against cNi0 but practically same results are obtained using mNi0. As shown in Figure 8, formation of [Ni(amp)]2+ is quite similar in both cases but marked differences are found for the

Figure 8. Distribution of Ni species in methanol/2-methoxyethanol mixture (xMeOH = 0.66) (open symbols) and in aqueous solution (Is = 0.100 mol·kg−1, pH 5) (filled symbols). The data were obtained by the LS method. T = 22 °C. Ni2+ (circles), [Ni(amp)]2+ (triangles down), [Ni(amp)2]2+ (squares), and [Ni(amp)3]2+. F



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higher complexes. Much less of the 1:2 complex is found in the nonaqueous solvent and the equilibrium is shifted to [Ni(amp)3]2+. Consider, for example, the solution compositions at amp/Ni2+ mole ratio of 3 (cNi0 = 0.025 mol·L−1 (mNi0 = 0.025 mol·kg−1)). In the aqueous solution, about 65 mol % of nickel exists as [Ni(amp)]2+ and much smaller amounts (10 and 25 mol %, respectively) of the 1:1 and 1:3 are found. No evidence of other complexes was detected. The situation is quite different in the methanol/2-methoxyethanol mixture; more than 90 mol % of nickel is in the form of [Ni(amp)3]2+. The information contained in Figure 8 is the key in design of ion-imprinted separation materials. In contrast to cases, where only one complex is formed, the conditions must be carefully selected to obtain a desired complex stoichiometry and geometry in more complicated systems, like the Ni2+-amp case considered in this study.

This study was financially supported by the Academy of Finland, Research Foundation of Lappeenranta University of Technology, Finnish Cultural Foundation, South Karelia Regional Fund, and Foundation of Magnus Ehrnrooth. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful to B.Sc. Timka Silvonen and B.Sc. Juuso Huittinen for assistance in the experimental and analytical work.

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4. CONCLUSIONS In this study, complex formation between Ni2+ and 2(aminomethyl)pyridine (amp) was investigated at nonstandard conditions using electronic absorption spectroscopy. The spectra were measured in aqueous solutions containing high concentrations of zinc sulfate and in a nonaqueous solvent mixture. The experimental spectra were analyzed qualitatively with the continuous variation method and quantitatively by means of least-squares minimization (LS) and partial leastsquares regression (PLS). The LS method was found suitable for treatment of strongly overlapping spectra of the Ni-amp system. Stability constants consistent with literature data were obtained for aqueous solutions of moderate and high ionic strengths. Applicability in more complicated systems was demonstrated with Zn2+ as nonabsorbing competitive central atom and methanol/2methoxyethanol as nonabsorbing and nonaqueous solvent. It was shown that the stability constants of the NiSO4-amp system change only slightly when a very high concentration of competing metal ion is present. Behavior of highly asymmetric mixed-metal system can be thus described surprisingly well using the single-metal parameters. The results obtained in this study can be used to qualitatively explain complexation also on solid surfaces. For example, the unexpectedly low nickel uptake from concentrated ZnSO4 solutions on amp-functionalized silica stems from changes in ligand/metal ratio and average stability constant rather than from changes in individual stability constants. This result has also a more general meaning for chelating ion exchange and adsorption in hydrometallurgical applications, because typically small concentrations of impurities are to be separated from concentrated metal salt solutions. In ion-imprinting, complex formation between metal template and ligand before attachment to a solid matrix is one of the key issues. In this study we have demonstrated that the complex distribution in a nonaqueous solvent can be evaluated by the LS method even for a strongly overlapping spectral system. Optimal conditions for formation of the desired complex can be thus easily found.

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Complexation of Nickel with 2-(Aminomethyl) pyridine at High Zinc

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