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Copper potentiometric sensors based on copper complexes containing thiohydrazone and thiosemicarbazone ligands
Analytica Chimica Acta 385 (1999) 143±149
Copper potentiometric sensors based on copper complexes containing thiohydrazone and thiosemicarbazone ligands M. JesuÂs Gismerab, M. Antonia Mendiolaa, JesuÂs Rodriguez Procopiob,*, M. Teresa Sevillab a
b
Departamento de QuõÂmica InorgaÂnica, Facultad de Ciencias, Universidad AutoÂnoma de Madrid, Madrid 28049, Spain Departamento de QuõÂmica AnalõÂtica y AnaÂlisis Instrumental, Facultad de Ciencias, Universidad AutoÂnoma de Madrid, Madrid 28049, Spain Received 24 June 1998; received in revised form 26 November 1998; accepted 30 November 1998
Abstract Carbon paste ion-selective electrodes based on complexes of copper with the macrocyclic 3,4,10,11-tetraphenyl1,2,5,8,9,12,13-octaaza-cyclotetra-deca-7,14-dithizone-2,4,9,11-tetraene, L1, and benzilbisthiosemicarbazone, L2, were constructed for copper determination. The calibration graphs were linear for a wide concentration range (110ÿ5± 110ÿ2 mol lÿ1). The limits of detection for Cu±L1 (1:1) and (1:2) complexes were pCu5.2 and 4.8, whereas for Cu±L2 (1:1) and (1:2) complexes were pCu5.6 and 4.8, respectively. Selectivity coef®cients were tabulated. The response times for these electrodes were fast and stable potentials were obtained within 2±18 s for concentrations higher than 10ÿ4 mol lÿ1. The electrodes were applied to potentiometric titrations of humic acids with copper(II) and show promise for the determination of complexation characteristic of these compounds. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Copper potentiometric sensors; Thiohydrazone; Ion-selective electrode; Thiosemicarbazone
1. Introduction Copper is an essential element that is also toxic at elevated concentration. Its reactivity and biological uptake are strongly in¯uenced by the free ion concentration that is controlled by the extent of Cu complexation with ligands. Potentiometric measurements with a copper ion-selective electrode (ISE) allow directly to determine free ion concentration in water samples. In view of this fact, researchers in the speciation ®eld have attempted to develop sensors for its determination with high selectivity. *Corresponding author. Tel.: +34-1-3974932; fax: +34-13974931; e-mail: [email protected]
For copper determination, solid membrane electrodes based on copper sulphide [1±4], tungsten oxide [5], ion exchangers [6,7] and copper(II) complexes [8± 14] as electroactive material have been tried as copper potentiometric sensors. In order to obtain a better selectivity, membrane electrodes based on macrocyclic polyethers [15,16] and polymethyldiene [17] have been developed. Sulphur ligands co-ordinate well with the transition metal cations, such as copper or cadmium. Thiohydrazone and thiosemicarbazone ligands form stable complexes with the transition metals, which makes these ligands suitable candidates for ion-selective electrodes. In view of this, we have attempted to characterize and apply this type of complexes in copper sensing electrodes.
0003-2670/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0003-2670(98)00840-X
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M. JesuÂs Gismera et al. / Analytica Chimica Acta 385 (1999) 143±149
The aim of this work is to report the behaviour of potentiometric carbon paste modi®ed electrodes based on several copper complexes with two derivatives of thiocarbazide and thiosemicarbazide: 3,4,10,11-tetraphenyl-1,2,5,8,9,12,13-octaaza-cyclotetra-deca-7,14dithizone-2,4,9,11-tetraene (L1H4) and benzilbisthiosemicarbazone (L2H6) (Fig. 1). These ligands, as a result of their structure, bind metal cations with varying strength [18] and they are suitable as electroactive materials for use in ion-selective electrodes. The advantages to use a carbon paste electrode were ease of construction, increased portability, robustness and economy. 2. Experimental 2.1. Chemicals In Fig. 1 are shown the structures of ligands and complexes used in this work. Ligands were prepared by the reported method [18]. The complexes were obtained by mixing the ligands with methanolic solutions of copper(II) nitrate (electrode A) and chloride (electrodes B, C and D) in stoichiometric molar ratios. The complexes were characterized on basis of nuclear magnetic resonance, IR, electronic and mass spectral studies, conductance and elemental analysis [18]. Copper nitrate (Fluka, Switzerland) and other metal nitrates and reagents (Carlo Erba, Italy) were AR grade. Aqueous solutions were prepared using ultrapure Milli-RO-MilliQ water. Two humic acids (HA) were investigated, one purchased from Fluka (Switzerland, 600-1000 g/mol) as acid form and other from Aldrich (USA) purchased as sodium salt. Fluka humic acid was dissolved in a minimum amount of 10ÿ3 M NaOH until no pH variation was observed and then ®ltered over a 0.45 mm membrane ®lter (Durapore, Waters, USA). 2.2. EMF measurements All electrode potentials and pH measurements were made with a Metrohm pH/ion meter (model 692) using an Orion double junction electrode (model 90±02) as the reference electrode. The cell was maintained at 2518C by means of a Gricel thermostat (model 28). The performance of the electrodes was investigated by
Fig. 1. Structures of ligands and complexes.
measuring the potential of copper nitrate solutions prepared in the concentration range 10ÿ2±10ÿ7 M. If necessary, pH adjustments were made using a diluted solution of nitric acid or sodium hydroxide. A Metrohm combined pH electrode (model 6.0233.100) was used for pH measurements. The activities of metal ions were based on activity coef®cients ( ) data calculated from the extended Debye± HuÈckel [19]. Selectivity coef®cients (KCu,M) for ions were determined by the mixed solution method [15]. 3. Electrode preparation The modi®ed carbon paste electrodes were prepared by mixing spectrographic graphite (Aldrich, USA, powder 1±2 mm) and powdered complex, in a 80:20 (w/w) ratio, in an agate mortar. Then paraf®n oil (Fluka, Switzerland) was added and the mixture
M. JesuÂs Gismera et al. / Analytica Chimica Acta 385 (1999) 143±149
was mixed until a uniform paste was obtained. The paste was packed in the end of a polypropylene syringe (ca. 1 mm deep and 2 mm in diameter) and provided with an unmodi®ed carbon paste±copper wire contact. Appropriate packing of the carbon paste was achieved by pressing the surface electrode against a ®lter paper. The same electrode surface could be used repeatedly for at least four weeks if stored in the air and keeping it free of contamination. For activation, the electrode was immersed in ultrapure water, 0.1 M NaNO3 solution and water (15 min each) prior to immersion in the sample solution. When contamination of the surface occurs, the response of the electrode can be restored by removing a thin layer of the electrode surface and pressing the new surface against a ®lter paper. 4. Humic acid titration procedure The electrode was calibrated before each titration by adding known volumes of 0.1 M Cu(NO3)2 solution, with ionic strength adjusted to 0.1 M with NaNO3, to 50.0 ml of 0.1 M NaNO3 solution and measuring the potentials. Titration of 50.0 ml of HA solutions (concentration range 150±400 mg lÿ1, prepared daily by weight of the sodium salt and dissolution in 0.1 M NaNO3) were carried out at 2518C. The total Cu(II) ion concentration ranged from 510ÿ5 to 510ÿ3 M. The pH value of solution was maintained constant at pH 6.0 by addition of very small volumes of 0.1 M NaOH. 5. Results and discussion 5.1. Effect of complex structure on electrochemical properties Electrodes based on the four suggested complexes were prepared to evaluate their feasibility in the development of ISE applications. The complexes were selected taking into consideration their stability and their three-dimensional structure for the cation complex formation. The copper complexes have been characterized spectrophotometry in the visible region in solution and solid state, and by cyclovoltammetry
145
on a carbon electrode [21]. The visible spectra of complexes in the solid state show for complexes A and D pseudotetrahedral geometry, while the complexes B and C show an important distortion to a square-planar structure. The electrochemical studies show that in complexes D and C (derived from the open L2 ligand) the copper is bonded to the ligand through the imine nitrogen and sulphur atoms. This can be deduced from the very close shape of their cyclic voltammograms and from the potential for the reversible Cu(II)/Cu(I) process, ÿ0.575 V and ÿ0.600 V vs. Ag/AgCl, respectively. The complexes derived from macrocyclic L1, complexes B and A, are more easily reduced to the corresponding Cu(I) complexes, as suggested by the more positive potential for the Cu(II)/Cu(I) redox process observed for these complexes (0.050 V and ÿ0.050 V vs. Ag/AgCl, respectively) than in complexes C and D. Also, A and B complexes decompose by reduction to Cu0 at not very negative potentials. These facts suggest that in A and B complexes the metal±ligand bond is not very strong. Potential response graphs for the Cu2-selective electrodes, for variable ionic strength, are shown in Fig. 2. Electrodes based on complex A and C exhibit a linearity range of 10ÿ2±10ÿ5 M for Cu2 activity, whereas electrodes based on complex B and D produced a straight line in the range 10ÿ2±10ÿ4 M. Electrodes A and B, based on the cyclic ligand, had sub-Nernstian slopes of 15.1 mV per decade (r20.994) and 22.4 mV per decade (r20.997), respectively. On the other hand, electrodes C and D, based on the open ligand, had slopes of 26.2 mV per decade (r20.997) and 27.2 mV per decade (r20.996), respectively. The detection limit obtained for electrode C, pCu5.6, was lower than limits of detection obtained for all others electrodes: pCu5.2, 4.8 and 4.8 for electrodes A, B and D, respectively. The E0 potentials of electrodes are in the order B>A>C>D (Fig. 2). This order agrees with the relative solubility of the complexes in those electrodes. The response times for the electrodes were measured by a 10-fold increase in copper concentration from 10ÿ4 to 10ÿ3 M in presence of a 0.1 M NaNO3 as electrolyte. The electrodes based on complexes A, B and D reached a steady potential within 17, 6 and 3 s, respectively. The response time of electrode C, 2 s, is the shortest among the electrodes, probably owing to
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Fig. 3. Potential of copper ISEs in 0.1 mM Cu2 solution as a function of pH. Fig. 2. Potential response of copper ISEs as a function of copper ion concentration.
the rapid rate of complex formation. The potential variation of the electrodes was within 0.5 mV in the same day, and within 1 mV in day-to-day results. For reliable measurements, restandardization for each test was accomplished. All electrodes, in absence of Cu(II), present high sensitivity to pH changes because the ligands are weak acids. However, in the presence of copper(II) only the electrode based on complex B shows similar beha-
viour (Fig. 3). The electrodes A, C and D have a low dependence on pH, between pH 3 and 7. This behaviour can be associated to the higher deprotonation of the ligand L1 in complex B opposite to other compounds. The selectivity coef®cients of the four electrodes for various cations were evaluated for the mixed solution method and compared in Table 1. As can be shown, the electrode based on the complex C has the best selectivity. This electrode is superior to the others because of its higher stability. Although complex D has a lower solubility than the other complexes,
Table 1 Selectivity coefficients, KCu,i, for various interfering cations for copper-selective electrodes based on the four complexes at a copper concentration of 110ÿ3 M Interferent
KCu,i Electrode A
ÿ3
Electrode B
Electrode C ÿ4
Na K NH4
2.810 0.089 0.045
0.028 0.020 0.045
8.810 5.610ÿ4 1.110ÿ3
Ca2 Mg2 Ba2 Pb2 Cd2 Zn2 Hg2
0.25 0.16 8.2 1.6 0.15 0.085 1.6104
0.17 0.32 8.1 0.18 0.14 0.33 2104
0.060 0.020 0.011 0.12 0.028 0.040 1.9
Al3 Fe3
0.05 0.12
0.046 0.075
0.012 0.015
Electrode D 2.810ÿ3 8.910ÿ3 0.020 0.080 0.032 0.70 0.17 0.16 0.25 160 0.0029 0.023
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the presence of only one copper in the molecule permits to bond the ligand with others cations and consequently a change in potential can be observed. The presence of two coppers in complex C prevents this interaction, leading to higher selectivity. The lower selectivity observed for the electrode A can be due to the ionic behaviour of the complex [18] that permit a higher interaction of ligand in the complex with other ions in solution. The interference of chloride and sulphate was investigated for all electrodes under study. The values for log KCu, Na for the electrode C were ÿ1.9 (chloride) and ÿ1.4 (sulphate) when employing 0.1 M sodium halide solutions. On the other hand, it was found that chloride causes a strong interference for electrodes A, B and D, with log KCu, Na higher than 0.8. Similar selectivity coef®cients were observed for sulphate for these three electrodes than electrode C. Electrode C is less sensitive by a factor of 103 to alkali, 102 to alkaline earth and 101±102 to heavy metal cations. This electrode shows a large interference by Hg2 ions, but lower than the other assayed electrodes. Based on the above results, it can be deduced that the selectivity for a particular ion depends on the relative stability of the bond with the ligand and the protective effect of the presence of two copper ions in the molecule. The presence of two copper ions in the molecule completes the coordination sites of the ligand and prevents the bond of the ligand with interfering ions. The electrode C shows a similar selectivity for alkali and alkaline earth ions as commercial electrodes, but commercial electrodes shows better selectivities by a factor of 100 for transition metal cations than the electrode C [9]. 5.2. Potentiometric titration of humic acids Electrode C was evaluated as an indicator electrode in potentiometric titration of humic acids. Plots of E vs. [Cu]t for the titration of an Aldrich humic acid solution with Cu2, at pH 6.0, are shown in Fig. 4. It is shown in Fig. 4 that, in complexing media such as humic acid solutions, we can get linearity in the calibration graph at lower concentration levels than in non-complexing media. The lowest linear concentration observed by using nitriletriacetic acid and EDTA buffer solutions [23] was in the order of 510ÿ9 mol lÿ1 (Fig. 5).
147
Fig. 4. Variation of the potential of ion-selective electrode C with change in the total concentration of copper in presence 0.4 g lÿ1 Aldrich HA (pH 6.0).
To show the suitability of electrode for speciation studies, the data obtained for several titrations of humic acid were modelled as follows. For calculation of mean equivalent weight (MEW) and average stability constants from experimental data, in this ®rst work the method proposed by Buf¯e et al. [20] was used. At the lowest concentration of added Cu(II), the 1:1 complex is predominantly formed: Cu2 Hx L $ CuL xH
(1)
for which the average stability constant, 1 , can be de®ned as 1
x
CuLH Cu2 Hx L
(2)
Where [x] denotes concentration of species x in mol lÿ1. Combining this equation with mass balances of metal and ligands, the following expression can be obtained: fLgt H MEW MEW 1 Cu2 t Cu2 t ÿ 1
(3)
Where {L}t is expressed in g lÿ1, is the degree of complexation of Cu2 ([Cu2]t/[Cu2]f), and the subscripts t and f refer to the total and free concentrations. Then, from the plot of the left-hand side of the Eq. (3) against /[Cu2]t, MEW can be obtained from the intercept and the conditional stability constant from the slope, using the value of MEW calculated before.
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148
Fig. 5. Calibration graphs for copper(II)-selective electrode C (&) in metal buffer solutions and (*) in unbuffered solutions.
At higher concentrations of added Cu(II), the 1:2 complex can be formed: Cu2 2Hx L $ CuL2 2xH with a stability constant 2
2
(4)
de®ned as
CuL2 H 2x Cu2 Hx L
(5)
It is possible to show that MEW
ÿ1 fLgt 1 2 2x fLgt H H MEW
(6)
Plotting MEW ( ÿ1)/{L}t vs. {L}t/MEW, for various x concentrations of L, the values for 1 =H and 2x 2 =H can be obtained. Fig. 6 shows the variation of Y fLgt =Cut = ÿ 1 vs. /[Cu2]t. It can be seen that the plot is not linear to low abscissa values, suggesting the formation of 1:1 and 1:2 complexes. Using the MEW values obtained by extrapolation of the linear portion at low Cu2 concentrations obtained in Fig. 6, values x 2x for log 1 =H and log 2 =H can be obtained for the humic acid under study and for
Fig. 6. Variation of Y{L}t/[Cu]t /( ÿ1) with /[Cu2]t.
solutions containing between 150 and 400 mg lÿ1 of organic matter. In Table 2 the complexation data for the two humic acids are summarized. The obtained results are in good agreement with data obtained by other authors in studies on interaction of copper with humic substances [22]. For Fluka humic acid, the calculated molecular weight are in close agreement with the range of molecular weight reported by the supplier.
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149
Table 2 Experimental values for complexes of HAs with Cu2 (temperature: 258C; ionic strength: NaNO3 0.1 M pH 6.0) HA
Cc (mol gÿ1)
MEW (g molÿ1)
log 1/[H]x
log 2/[H]2x
Aldrich Fluka
3.010ÿ3 1.510ÿ3
590 863
4.70.2 5.70.2
6.60.3 7.40.2
Cc: complexing capacity. MEW: calculated molecular weight. i: equilibrium constant for complexing site i.
6. Conclusion Ion-selective electrodes based on thiohydrazone and thiosemicarbazone copper complexes exhibited rapid response, adequate sensitivity and selectivity for copper ions. From results, it can be seen that the electrode based on the complex C (Fig. 1) is superior to the others in relation with sensitivity, selectivity and limit of detection. These results may be due to higher complex stability. These electrodes allow to investigate the complexation characteristic of natural ligands with copper. Acknowledgements The authors are grateful to the Government of the Comunidad AutoÂnoma de Madrid for ®nancial support of this work (Project no. 06M/041/96) and for the grant to technician formation (FINNOVA'97 Plan) to Eva Gomez -EspanÄa. References [1] J. Kouljenovic, V. Martinac, N. Radic, Anal. Chim. Acta 231 (1990) 137. [2] S.M. Stankovic, V.M. Javanovic, M.S. Javanovic, J. Serb. Chem. Soc. 55 (1990) 125. [3] M. Neshkova, Anal. Chim. Acta 273 (1993) 255. [4] R. De Marco, D.J. Mackey, A. Zirino, Electroanalysis 9 (1997) 330.
[5] Z. Chen, P.W. Alexander, Electroanalysis 9 (1997) 141. [6] S.P. Awasthi, T.C. Thambi, M. Sunderesan, J. Electrochem. Soc. 39 (1990) 34. [7] A.K. Jain, P. Singh, L.P. Singh, Ind. J. Chem. 33A (1994) 272. [8] K. Ren, Talanta 36 (1989) 767. [9] S. Kamata, H. Murata, Y. Kubo, A. Bhale, Analyst 114 (1989) 1029. [10] G.A. Askerov, F.M. Chyragov, D.G. Gambarov, A.M. Ayubova, Zh. Anal. Khim. 47 (1992) 337. [11] K. Nagashima, J. Guo, T. Hobo, Bunseki Kagaku 41 (1992) 153. [12] C.Q. Sun, Y.P. Sun, Y. Zhang, H.D. Xu, J.C. Shen, Anal. Chim. Acta 312 (1995) 207. [13] M. Pleniceanu, M. Preda, N. Huresam, L. Simoiu, Anal. Lett. 29 (1996) 1485. [14] S.X. Xia, D. Xiao and R.Q. Yu, Fensi Huaxue, Anal. Abstr. 1995 3D41. [15] S. Kamata, K. Yamasaki, M. Higo, A. Bhale, Y. Fukunaga, Analyst 113 (1988) 45. [16] Z. Brzozka, Chem. Anal. (Warsaw) 35 (1990) 415. [17] K.A. Jain, V.K. Gupta, B.B. Sahoo, L.P. Singh, Anal. Proc. 32 (1995) 99. [18] P. Souza, M.A. Mendiola, A. Arquero, V. Fernandez, E. Gutierrez-Puebla, C. Ruiz-Valero, Z. Naturforsch 49B (1994) 263. [19] J. Buffle, Complexation reactions in aquatic systems, Ellis Horwood, Chichester, 1988. [20] J. Buffle, F.L. Greter, W. Heardi, Anal. Chem. 49 (1977) 216. [21] Z. Durackova, M.A. Mendiola, M.T. Sevilla, A. Valent, Bioelectrochem. Bioener. (1999) in press. [22] F.J. Stevenson, Humus Chemistry, 2nd ed., Wiley, Chichester, 1994. [23] E.H. Hansen, C.G. Lamn, J. Ruzicka, Anal. Chim. Acta 59 (1972) 403.
Copper potentiometric sensors based on copper complexes containing thiohydrazone and thiosemicarbazone ligands M. JesuÂs Gismerab, M. Antonia Mendiolaa, JesuÂs Rodriguez Procopiob,*, M. Teresa Sevillab a
b
Departamento de QuõÂmica InorgaÂnica, Facultad de Ciencias, Universidad AutoÂnoma de Madrid, Madrid 28049, Spain Departamento de QuõÂmica AnalõÂtica y AnaÂlisis Instrumental, Facultad de Ciencias, Universidad AutoÂnoma de Madrid, Madrid 28049, Spain Received 24 June 1998; received in revised form 26 November 1998; accepted 30 November 1998
Abstract Carbon paste ion-selective electrodes based on complexes of copper with the macrocyclic 3,4,10,11-tetraphenyl1,2,5,8,9,12,13-octaaza-cyclotetra-deca-7,14-dithizone-2,4,9,11-tetraene, L1, and benzilbisthiosemicarbazone, L2, were constructed for copper determination. The calibration graphs were linear for a wide concentration range (110ÿ5± 110ÿ2 mol lÿ1). The limits of detection for Cu±L1 (1:1) and (1:2) complexes were pCu5.2 and 4.8, whereas for Cu±L2 (1:1) and (1:2) complexes were pCu5.6 and 4.8, respectively. Selectivity coef®cients were tabulated. The response times for these electrodes were fast and stable potentials were obtained within 2±18 s for concentrations higher than 10ÿ4 mol lÿ1. The electrodes were applied to potentiometric titrations of humic acids with copper(II) and show promise for the determination of complexation characteristic of these compounds. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Copper potentiometric sensors; Thiohydrazone; Ion-selective electrode; Thiosemicarbazone
1. Introduction Copper is an essential element that is also toxic at elevated concentration. Its reactivity and biological uptake are strongly in¯uenced by the free ion concentration that is controlled by the extent of Cu complexation with ligands. Potentiometric measurements with a copper ion-selective electrode (ISE) allow directly to determine free ion concentration in water samples. In view of this fact, researchers in the speciation ®eld have attempted to develop sensors for its determination with high selectivity. *Corresponding author. Tel.: +34-1-3974932; fax: +34-13974931; e-mail: [email protected]
For copper determination, solid membrane electrodes based on copper sulphide [1±4], tungsten oxide [5], ion exchangers [6,7] and copper(II) complexes [8± 14] as electroactive material have been tried as copper potentiometric sensors. In order to obtain a better selectivity, membrane electrodes based on macrocyclic polyethers [15,16] and polymethyldiene [17] have been developed. Sulphur ligands co-ordinate well with the transition metal cations, such as copper or cadmium. Thiohydrazone and thiosemicarbazone ligands form stable complexes with the transition metals, which makes these ligands suitable candidates for ion-selective electrodes. In view of this, we have attempted to characterize and apply this type of complexes in copper sensing electrodes.
0003-2670/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0003-2670(98)00840-X
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M. JesuÂs Gismera et al. / Analytica Chimica Acta 385 (1999) 143±149
The aim of this work is to report the behaviour of potentiometric carbon paste modi®ed electrodes based on several copper complexes with two derivatives of thiocarbazide and thiosemicarbazide: 3,4,10,11-tetraphenyl-1,2,5,8,9,12,13-octaaza-cyclotetra-deca-7,14dithizone-2,4,9,11-tetraene (L1H4) and benzilbisthiosemicarbazone (L2H6) (Fig. 1). These ligands, as a result of their structure, bind metal cations with varying strength [18] and they are suitable as electroactive materials for use in ion-selective electrodes. The advantages to use a carbon paste electrode were ease of construction, increased portability, robustness and economy. 2. Experimental 2.1. Chemicals In Fig. 1 are shown the structures of ligands and complexes used in this work. Ligands were prepared by the reported method [18]. The complexes were obtained by mixing the ligands with methanolic solutions of copper(II) nitrate (electrode A) and chloride (electrodes B, C and D) in stoichiometric molar ratios. The complexes were characterized on basis of nuclear magnetic resonance, IR, electronic and mass spectral studies, conductance and elemental analysis [18]. Copper nitrate (Fluka, Switzerland) and other metal nitrates and reagents (Carlo Erba, Italy) were AR grade. Aqueous solutions were prepared using ultrapure Milli-RO-MilliQ water. Two humic acids (HA) were investigated, one purchased from Fluka (Switzerland, 600-1000 g/mol) as acid form and other from Aldrich (USA) purchased as sodium salt. Fluka humic acid was dissolved in a minimum amount of 10ÿ3 M NaOH until no pH variation was observed and then ®ltered over a 0.45 mm membrane ®lter (Durapore, Waters, USA). 2.2. EMF measurements All electrode potentials and pH measurements were made with a Metrohm pH/ion meter (model 692) using an Orion double junction electrode (model 90±02) as the reference electrode. The cell was maintained at 2518C by means of a Gricel thermostat (model 28). The performance of the electrodes was investigated by
Fig. 1. Structures of ligands and complexes.
measuring the potential of copper nitrate solutions prepared in the concentration range 10ÿ2±10ÿ7 M. If necessary, pH adjustments were made using a diluted solution of nitric acid or sodium hydroxide. A Metrohm combined pH electrode (model 6.0233.100) was used for pH measurements. The activities of metal ions were based on activity coef®cients ( ) data calculated from the extended Debye± HuÈckel [19]. Selectivity coef®cients (KCu,M) for ions were determined by the mixed solution method [15]. 3. Electrode preparation The modi®ed carbon paste electrodes were prepared by mixing spectrographic graphite (Aldrich, USA, powder 1±2 mm) and powdered complex, in a 80:20 (w/w) ratio, in an agate mortar. Then paraf®n oil (Fluka, Switzerland) was added and the mixture
M. JesuÂs Gismera et al. / Analytica Chimica Acta 385 (1999) 143±149
was mixed until a uniform paste was obtained. The paste was packed in the end of a polypropylene syringe (ca. 1 mm deep and 2 mm in diameter) and provided with an unmodi®ed carbon paste±copper wire contact. Appropriate packing of the carbon paste was achieved by pressing the surface electrode against a ®lter paper. The same electrode surface could be used repeatedly for at least four weeks if stored in the air and keeping it free of contamination. For activation, the electrode was immersed in ultrapure water, 0.1 M NaNO3 solution and water (15 min each) prior to immersion in the sample solution. When contamination of the surface occurs, the response of the electrode can be restored by removing a thin layer of the electrode surface and pressing the new surface against a ®lter paper. 4. Humic acid titration procedure The electrode was calibrated before each titration by adding known volumes of 0.1 M Cu(NO3)2 solution, with ionic strength adjusted to 0.1 M with NaNO3, to 50.0 ml of 0.1 M NaNO3 solution and measuring the potentials. Titration of 50.0 ml of HA solutions (concentration range 150±400 mg lÿ1, prepared daily by weight of the sodium salt and dissolution in 0.1 M NaNO3) were carried out at 2518C. The total Cu(II) ion concentration ranged from 510ÿ5 to 510ÿ3 M. The pH value of solution was maintained constant at pH 6.0 by addition of very small volumes of 0.1 M NaOH. 5. Results and discussion 5.1. Effect of complex structure on electrochemical properties Electrodes based on the four suggested complexes were prepared to evaluate their feasibility in the development of ISE applications. The complexes were selected taking into consideration their stability and their three-dimensional structure for the cation complex formation. The copper complexes have been characterized spectrophotometry in the visible region in solution and solid state, and by cyclovoltammetry
145
on a carbon electrode [21]. The visible spectra of complexes in the solid state show for complexes A and D pseudotetrahedral geometry, while the complexes B and C show an important distortion to a square-planar structure. The electrochemical studies show that in complexes D and C (derived from the open L2 ligand) the copper is bonded to the ligand through the imine nitrogen and sulphur atoms. This can be deduced from the very close shape of their cyclic voltammograms and from the potential for the reversible Cu(II)/Cu(I) process, ÿ0.575 V and ÿ0.600 V vs. Ag/AgCl, respectively. The complexes derived from macrocyclic L1, complexes B and A, are more easily reduced to the corresponding Cu(I) complexes, as suggested by the more positive potential for the Cu(II)/Cu(I) redox process observed for these complexes (0.050 V and ÿ0.050 V vs. Ag/AgCl, respectively) than in complexes C and D. Also, A and B complexes decompose by reduction to Cu0 at not very negative potentials. These facts suggest that in A and B complexes the metal±ligand bond is not very strong. Potential response graphs for the Cu2-selective electrodes, for variable ionic strength, are shown in Fig. 2. Electrodes based on complex A and C exhibit a linearity range of 10ÿ2±10ÿ5 M for Cu2 activity, whereas electrodes based on complex B and D produced a straight line in the range 10ÿ2±10ÿ4 M. Electrodes A and B, based on the cyclic ligand, had sub-Nernstian slopes of 15.1 mV per decade (r20.994) and 22.4 mV per decade (r20.997), respectively. On the other hand, electrodes C and D, based on the open ligand, had slopes of 26.2 mV per decade (r20.997) and 27.2 mV per decade (r20.996), respectively. The detection limit obtained for electrode C, pCu5.6, was lower than limits of detection obtained for all others electrodes: pCu5.2, 4.8 and 4.8 for electrodes A, B and D, respectively. The E0 potentials of electrodes are in the order B>A>C>D (Fig. 2). This order agrees with the relative solubility of the complexes in those electrodes. The response times for the electrodes were measured by a 10-fold increase in copper concentration from 10ÿ4 to 10ÿ3 M in presence of a 0.1 M NaNO3 as electrolyte. The electrodes based on complexes A, B and D reached a steady potential within 17, 6 and 3 s, respectively. The response time of electrode C, 2 s, is the shortest among the electrodes, probably owing to
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Fig. 3. Potential of copper ISEs in 0.1 mM Cu2 solution as a function of pH. Fig. 2. Potential response of copper ISEs as a function of copper ion concentration.
the rapid rate of complex formation. The potential variation of the electrodes was within 0.5 mV in the same day, and within 1 mV in day-to-day results. For reliable measurements, restandardization for each test was accomplished. All electrodes, in absence of Cu(II), present high sensitivity to pH changes because the ligands are weak acids. However, in the presence of copper(II) only the electrode based on complex B shows similar beha-
viour (Fig. 3). The electrodes A, C and D have a low dependence on pH, between pH 3 and 7. This behaviour can be associated to the higher deprotonation of the ligand L1 in complex B opposite to other compounds. The selectivity coef®cients of the four electrodes for various cations were evaluated for the mixed solution method and compared in Table 1. As can be shown, the electrode based on the complex C has the best selectivity. This electrode is superior to the others because of its higher stability. Although complex D has a lower solubility than the other complexes,
Table 1 Selectivity coefficients, KCu,i, for various interfering cations for copper-selective electrodes based on the four complexes at a copper concentration of 110ÿ3 M Interferent
KCu,i Electrode A
ÿ3
Electrode B
Electrode C ÿ4
Na K NH4
2.810 0.089 0.045
0.028 0.020 0.045
8.810 5.610ÿ4 1.110ÿ3
Ca2 Mg2 Ba2 Pb2 Cd2 Zn2 Hg2
0.25 0.16 8.2 1.6 0.15 0.085 1.6104
0.17 0.32 8.1 0.18 0.14 0.33 2104
0.060 0.020 0.011 0.12 0.028 0.040 1.9
Al3 Fe3
0.05 0.12
0.046 0.075
0.012 0.015
Electrode D 2.810ÿ3 8.910ÿ3 0.020 0.080 0.032 0.70 0.17 0.16 0.25 160 0.0029 0.023
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the presence of only one copper in the molecule permits to bond the ligand with others cations and consequently a change in potential can be observed. The presence of two coppers in complex C prevents this interaction, leading to higher selectivity. The lower selectivity observed for the electrode A can be due to the ionic behaviour of the complex [18] that permit a higher interaction of ligand in the complex with other ions in solution. The interference of chloride and sulphate was investigated for all electrodes under study. The values for log KCu, Na for the electrode C were ÿ1.9 (chloride) and ÿ1.4 (sulphate) when employing 0.1 M sodium halide solutions. On the other hand, it was found that chloride causes a strong interference for electrodes A, B and D, with log KCu, Na higher than 0.8. Similar selectivity coef®cients were observed for sulphate for these three electrodes than electrode C. Electrode C is less sensitive by a factor of 103 to alkali, 102 to alkaline earth and 101±102 to heavy metal cations. This electrode shows a large interference by Hg2 ions, but lower than the other assayed electrodes. Based on the above results, it can be deduced that the selectivity for a particular ion depends on the relative stability of the bond with the ligand and the protective effect of the presence of two copper ions in the molecule. The presence of two copper ions in the molecule completes the coordination sites of the ligand and prevents the bond of the ligand with interfering ions. The electrode C shows a similar selectivity for alkali and alkaline earth ions as commercial electrodes, but commercial electrodes shows better selectivities by a factor of 100 for transition metal cations than the electrode C [9]. 5.2. Potentiometric titration of humic acids Electrode C was evaluated as an indicator electrode in potentiometric titration of humic acids. Plots of E vs. [Cu]t for the titration of an Aldrich humic acid solution with Cu2, at pH 6.0, are shown in Fig. 4. It is shown in Fig. 4 that, in complexing media such as humic acid solutions, we can get linearity in the calibration graph at lower concentration levels than in non-complexing media. The lowest linear concentration observed by using nitriletriacetic acid and EDTA buffer solutions [23] was in the order of 510ÿ9 mol lÿ1 (Fig. 5).
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Fig. 4. Variation of the potential of ion-selective electrode C with change in the total concentration of copper in presence 0.4 g lÿ1 Aldrich HA (pH 6.0).
To show the suitability of electrode for speciation studies, the data obtained for several titrations of humic acid were modelled as follows. For calculation of mean equivalent weight (MEW) and average stability constants from experimental data, in this ®rst work the method proposed by Buf¯e et al. [20] was used. At the lowest concentration of added Cu(II), the 1:1 complex is predominantly formed: Cu2 Hx L $ CuL xH
(1)
for which the average stability constant, 1 , can be de®ned as 1
x
CuLH Cu2 Hx L
(2)
Where [x] denotes concentration of species x in mol lÿ1. Combining this equation with mass balances of metal and ligands, the following expression can be obtained: fLgt H MEW MEW 1 Cu2 t Cu2 t ÿ 1
(3)
Where {L}t is expressed in g lÿ1, is the degree of complexation of Cu2 ([Cu2]t/[Cu2]f), and the subscripts t and f refer to the total and free concentrations. Then, from the plot of the left-hand side of the Eq. (3) against /[Cu2]t, MEW can be obtained from the intercept and the conditional stability constant from the slope, using the value of MEW calculated before.
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148
Fig. 5. Calibration graphs for copper(II)-selective electrode C (&) in metal buffer solutions and (*) in unbuffered solutions.
At higher concentrations of added Cu(II), the 1:2 complex can be formed: Cu2 2Hx L $ CuL2 2xH with a stability constant 2
2
(4)
de®ned as
CuL2 H 2x Cu2 Hx L
(5)
It is possible to show that MEW
ÿ1 fLgt 1 2 2x fLgt H H MEW
(6)
Plotting MEW ( ÿ1)/{L}t vs. {L}t/MEW, for various x concentrations of L, the values for 1 =H and 2x 2 =H can be obtained. Fig. 6 shows the variation of Y fLgt =Cut = ÿ 1 vs. /[Cu2]t. It can be seen that the plot is not linear to low abscissa values, suggesting the formation of 1:1 and 1:2 complexes. Using the MEW values obtained by extrapolation of the linear portion at low Cu2 concentrations obtained in Fig. 6, values x 2x for log 1 =H and log 2 =H can be obtained for the humic acid under study and for
Fig. 6. Variation of Y{L}t/[Cu]t /( ÿ1) with /[Cu2]t.
solutions containing between 150 and 400 mg lÿ1 of organic matter. In Table 2 the complexation data for the two humic acids are summarized. The obtained results are in good agreement with data obtained by other authors in studies on interaction of copper with humic substances [22]. For Fluka humic acid, the calculated molecular weight are in close agreement with the range of molecular weight reported by the supplier.
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149
Table 2 Experimental values for complexes of HAs with Cu2 (temperature: 258C; ionic strength: NaNO3 0.1 M pH 6.0) HA
Cc (mol gÿ1)
MEW (g molÿ1)
log 1/[H]x
log 2/[H]2x
Aldrich Fluka
3.010ÿ3 1.510ÿ3
590 863
4.70.2 5.70.2
6.60.3 7.40.2
Cc: complexing capacity. MEW: calculated molecular weight. i: equilibrium constant for complexing site i.
6. Conclusion Ion-selective electrodes based on thiohydrazone and thiosemicarbazone copper complexes exhibited rapid response, adequate sensitivity and selectivity for copper ions. From results, it can be seen that the electrode based on the complex C (Fig. 1) is superior to the others in relation with sensitivity, selectivity and limit of detection. These results may be due to higher complex stability. These electrodes allow to investigate the complexation characteristic of natural ligands with copper. Acknowledgements The authors are grateful to the Government of the Comunidad AutoÂnoma de Madrid for ®nancial support of this work (Project no. 06M/041/96) and for the grant to technician formation (FINNOVA'97 Plan) to Eva Gomez -EspanÄa. References [1] J. Kouljenovic, V. Martinac, N. Radic, Anal. Chim. Acta 231 (1990) 137. [2] S.M. Stankovic, V.M. Javanovic, M.S. Javanovic, J. Serb. Chem. Soc. 55 (1990) 125. [3] M. Neshkova, Anal. Chim. Acta 273 (1993) 255. [4] R. De Marco, D.J. Mackey, A. Zirino, Electroanalysis 9 (1997) 330.
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