Liquid membrane ion-selective electrodes for potentiometric dosage of coper and nickel

J. Serb. Chem. Soc. 70 (2) 269 276 (2005) UDC 541.135.5:543.554:546.562+546.742 JSCS 3271 Original scientific paper Liquid membrane ion-selective electrodes for potentiometric dosage of coper and nickel MARIA PLENICEANY *, MARIAN ISVORANU and CEZAR SPINU University of Craiova, Faculty of Chemistry, A. I. Cuza 13, 1100-Craiova, Romania (e-mail: spinu_cezar@yahoo.com) (Received 11 February, revised 24 June 2004) Abstract: This paper presents experimental and theoretical data regarding the preparation and characterization of three liquid-membrane electrodes, which have not been mentioned in the specialized literature so far. The active substances, the solutions of which in nitrobenzene formed the membranes on a graphite rod, are simple complex combinations of Cu(II) and Ni(II) ions with an organic ligand belonging to the Schiff base class: N- 2-thienylmethilidene -2-aminoethanol (TNAHE). The Cu 2+ -selective and Ni 2+ -selective electrodes were used to determine the copper and nickel ions in aqueous solutions, both by direct potentiometry and by potentiometric titration with EDTA. They were also used for the determination of Cu 2+ and Ni 2+ ions in industrial waters by direct potentiometry. Keywords: copper, nickel, liquid membrane electrodes, potentiometry, selectivity. INTRODUCTIONS In the past few years, the development of new ion-selective electrodes for copper and nickel metal ions has been reported in the literature. Thus, in a series of previous papers, 1 11 the possiblity of obtaining Cu 2+ and Ni 2+ selective electrodes with liquid membranes based on complex combinations of copper and nickel, having the property of being extracted in nitrobenzene, an organic solvent which is ummiscible with water. The reponse of the ion-selective electrodes to the concentration of Cu 2+ and Ni 2+ ions in solution was formally attributed to an exchange process of these metal ions between the to be analyzed aqueous solution and the membrane solution in nitrobenzene. The system is considered to be in equilibrium when the electrochemical potentials of the Cu 2+ and Ni 2+ metal ions are equal in the two phases, while the presence of copper (nickel) ions in the aqueous phase does not affect the activity of these ions in the organic phase, from which the membrane of the electrode is constituted. * Author for correpondence. 269

270 PLENICEANU, ISVORANU and SP NU Based on this hypothesis, an expression for the membrane potential has been thermodynamically deduced: E = E 0 + RT ln a (1) 2 2F M where a M 2+ represents the activity of the Cu 2+ and Ni 2+ ions in the aqueous solution. The above mentioned hypothesis does consider the mechanism of the functioning of the electrodes, which have a Nernstian character as is shown by expression (1), and has been successfully used for elaborating potentiometric methods of the determination of copper and nickel. EXPERIMENTAL Materials Analytical grade CuSO 4,Ni(NO 3 ) 2 and nitrobenzene were supplied by Merck with a purity higher than 99 %. Three Cu 2+ and Ni 2+ -selective electrodes with a liquid membrane were obtained and characterized. The electrodes have as a basis the simple complex combination of Ni(II) and Cu(II) with N- 2-thienylmethylidene -2-aminoethanol (TNAHE). The ligand used 12 has the structure shown below: The formula of the complex combinations of Cu(II) and Ni(II) with this ligand, the solutions of which in nitrobenzene constitute the membrane on a graphite rod, for the prepared electrodes are as follows: Electrode 1: Ni(TNAHE) 2 ; Electrode 2: Ni(TNAHE) 2 Cl 2 ; Electrode 3: Cu(TNAHE) 2 Cl 2. Equipment The electrodes used for the determination were made according to a previously described procedure. 10 For the potentiometric and ph measurements, a Jenway 3305 ph-meter has been used. RESULTS AND DISCUSSIONS The response of the electrodes to the concentration of Cu 2+ (Ni 2+ ) ions The variations of the electromotive force of the electrochemical cells of the type (2) obtained with the three ion-selective electrodes at 25 C and an ionic strength = 0.4 (provided with KNO 3, which does not influence the electrode potential), as a function of M 2+ concentration are shown in Fig. 1. The electrode potential measurements in acid solutions are greater than those in alkali solutions because in the alkali solutions, some of Cu 2+ and Ni 2+ cations are precipitated as hydroxide. Stainless steel thread Membrane supported on graphite rod Analyzed aqueous solution (M 2+ ) KCl saturated Calomel electrode (2)

ION SELECTIVE ELECTRODES 271 Fig. 1. The variation of electromotive force E for the electrodes et 25 C and ionic strenght = 0.4 TM KNO 3 as a function of log [M 2+ ] (curve a-electrode 1, curve b-electrode 2, curve c-electrode 3). The influence of ph The influence of ph on the response of the Cu 2+ and Ni 2+ -selective electrodes was studied. It was found that, unlike other types of electrodes, in the case of liquid ion-selective membrane electrodes, the membranes modify their compositions when the aqueous solutions are too acid or too alkaline. The ph of the aqueous solutions of Cu 2+ and Ni 2+ was cotrolled with a buffer solution, according to Walpole, Kolthoff and Vleeschhouwer, 13 while the determinations of the ph were made with a Jenway 3305 ph-meter, using a glass electrode and a saturated calomel electrode. a) b) c) Fig. 2. The influence of ph on the response of the ion-selective electrodes (a-electrode No. 1 with a membrane of Ni(TNAHE) 2 ), b-electrode No. 2 with a membrane of [Ni(THANE) 2 ]Cl 2, c-electrode No. 3 with a membrane of [Cu(THANE) 2 ]Cl 2.

272 PLENICEANU, ISVORANU and SP NU It was found experimentally that ph variations in the range 5 8 for Ni 2+ (for electrode 1), 4 8.5 for Ni 2+ (electrode 2) and 4.2 8.3 for Cu 2+ (electrode 3), do not influence the membrane potential and that the linear part of the E ph curves is a constant function of the concentration of the metal ions in the aqueous phase. The experimental results are presented in Fig. 2, curves a, b and c. Selectivity of the electrodes The electrodes were tested for a series of cations which can competitively participate in the ionic exchange equilibrium with the ions Cu 2+ (Ni 2+ ) in the membrane. It was found that a series of cations only interfere if they are present in concentrations approximately 1000 times higher than the concentrations of the Cu 2+ and Ni 2+ ions. For these cations, the selectivity constants were determined using the Eisenman method. 14 From Table I it can be seen that the studied electrodes have a cationic response of 29 mv at 25 C, which corresponds to a Nernstian slope of RT/2F. The Nerstian response was obeyed in the concentration range of 10 1 10 5 M for electrode 1 and of 10 1 10 6 M for electrode 2 and 3. As a result, the Cu 2+ (Ni 2+ ) selective electrodes can be used for the potentiometric determination of copper and nickel. The direct measurements of the potential were made in solution of CuSO 4 and Ni(NO 3 ) 2 at ph 6 (electrodes 2 and 3) and 6.6 (electrode 1), using a buffer solution of acetic acid sodium acetate 0.2 M. The experimental results are presented in Table I. TABLE I. The characteristics of the Cu 2+ and Ni 2+ -selective electrodes at 25 C and constant ionic strength = 0.4 M (KNO 3 atph6 6.6 Electrode E/ log c Range of linear Selectivity constants of the cations mv response Cu 2+ Ni 2+ Fe 2+ Co 2+ 1 29 10-1 10-5 M 1 10-3 5 10-4 3.2 10-3 2 29 10-1 10-6 M 4 10-4 2.5 10-4 1.3 10-3 3 29 10-1 10-6 M 5.6 10-4 3.2 10-4 1 10-3 The dynamic response and the reproducibility of the electrodes The response characteristics of the Cu 2+ and Ni 2+ -selective electrodes was evaluated by introducing the electrodes into solutions of different concentrations of CuSO 4 and Ni(NO 3 ) 2, usually ten times higher, and by recording the values of the potentials as a function of time. The response times of the electrodes in dilute solutions (10 4 10 6 M) were approximately two minutes, while in more concentrated solutions (10 1 10 3 M), the electrode potential reaches an equilibrium value in a few seconds. The three electrodes were tested over a period of 3 5 weeks, whereby the electrode potentials changed only by 2mV 3 mv, without any change of the slope of the electrodes (29 mv/decade of concentration).

ION SELECTIVE ELECTRODES 273 Analytical applications The obtained Cu 2+ and Ni 2+ - selective electrodes were used for the determination of copper and nickel ions in aqueous solutions, both by direct potentiometry and by potentiometric titration with EDTA. For the direct determination, calibration curves were used, which were obtained through the variation of the electromotive force of the ion selecitve electrodes 1, 2 and 3 as a function of log M 2+ versus the ECS at 25 Cand = 0.4 M KNO 3. The results are presented in Fig. 1, in which each point represents the average values of 5 measurements. Fig. 3. The curves of the potentiometric titration of the Ni 2+ ions with EDTA, using the Ni 2+ -selective electrodes No. 1 and 2 (curves a and b), and of the Cu 2+ ions with EDTA, using the Cu 2+ -selective electrodes No. 3 (curve c). The electrodes were also tested for potentiometric titration with EDTA. Potential rises of 122 mv for electrode 1, 146 mv for electrode 2 and 132 mv for electrode 3 were obtained. The experimental results are shown in Fig. 3, curves a, b and c. Determination of copper and nickel by direct potentiometry in industrial waters Samples of water from a water cleaning station were analyzed using electrodes 1, 2 and 3. The concentrations of copper and nickel were also determined for every sample by atomic absorption spectrometry (AAS). TABLE II. The results of the determination of Cu 2+ and Ni 2+ ions in industrial waters Sample No. with Ni 2+ -selective electrode No. 1 Ni 2+ /mg L -1 Cu 2+ /mg L -1 with Ni 2+ -selective electrode No. 2 AAS method with Cu 2+ -selective electrode No. 3 AAS method 1 0.33 0.34 0.35 0.58 0.60 2 0.42 0.44 0.45 0.86 0.89

274 PLENICEANU, ISVORANU and SP NU TABLE II. Contimued Sample No. with Ni 2+ -selective electrode No. 1 Ni 2+ /mg L -1 Cu 2+ /mg L -1 with Ni 2+ -selective electrode No. 2 AAS method with Cu 2+ -selective electrode No. 3 AAS method 3 0.16 0.18 0.20 1.00 1.03 4 0.22 0.24 0.27 1.78 1.80 5 0.57 0.59 0.61 1.25 1.31 6 0.27 0.30 0.31 0.80 0.83 7 0.13 0.15 0.17 2.10 2.15 8 0.25 0.26 0.27 1.11 1.16 9 0.19 0.21 0.23 1.42 1.46 10 0.11 0.13 0.15 1.57 1.60 The results of the experimental determinations are presented in Table II. Besides the value of concentration (expressed in mg. L 1 )incu 2+ and Ni 2+, potentiometrically obtained with electrodes 1, 2 and 3, the values obtained by the atomic absorption spectrometry method (AAS), both for copper and nickel, are presented. TABLE III. The results of the determination of Ni 2+ ions in industrial waters using the method of standard additions Sample No. Initial Ni 2+ (AAS) Addition of Ni 2+ Ni 2+ /mg L -1 Theoretical total Exp. Ni 2+ with electrode No. 1 Exp. Ni 2+ with electrode No. 2 1 0.45 0.60 1.05 1.03 1.03 2 0.35 1.75 2.10 2.05 2.07 3 0.31 1.50 1.81 1.77 1.78 4 0.15 1.00 1.15 1.13 1.14 5 0.61 2.00 2.61 2.58 2.60 It can be observed that the values obtained by the potentiometric method are in accordance with those obtained by the AAS method, for the analyzed industrial water samples. These results are in accordance with the values of the selectivity constants of electrodes 1, 2 and 3 Ni 2+ and Cu 2+ ions (see Table I above). To verify the advantage of using the potentiometric method with electrodes 1, 2 and 3, all the obtained experimental data were verified through the method of standard additions, the resultsbeingshownintablesiiiandiv.

ION SELECTIVE ELECTRODES 275 TABLE IV. The results of the determination of Cu 2+ ions in industrial waters using the method of standard additions Sample No. Initial Cu 2+ (AAS) Addition of Cu 2+ Cu 2+ /mg L -1 Theoretical total Exp. Cu 2+ with electrode No. 3 1 2.15 2.00 4.15 4.12 2 1.16 1.00 2.16 2.13 3 1.60 2.00 3.60 3.58 4 0.83 2.00 2.83 2.80 5 0.89 1.00 1.89 1.85 CONCLUSIONS Three selective electrodes were prepared and characterized for Cu 2+ and Ni 2+ ions. The electrodes were based on the complex combinations of copper and nickel with N- 2-thienylmethylidene -2-aminoethanol with the membranes being obtained in nitrobenzene on a graphite rod. The following characteristics were studied: the electrodes response to the Cu 2+ (Ni 2+ ) ion concentation; the influence of ph on the response of the Cu 2+ (Ni 2+ ) ion selective electrodes; the selectivity of the electrodes; the dynamic response and reproducibility of the electrodes. The study of these three electrodes proved they had primary and secondary characteristics favorable for their practical utilization in potentiometric titrations and in direct potentiometric determinations. The electrodes have a wide linear response range to the concentration of Cu 2+ (Ni 2+ ) ions. For this reason, they are suitable for the potentiometric determination of copper and nickel ions in dilute solutions (dilutions may go down to 10 5 M), as well as in the control of industrial waters. IZVOD JON-SELEKTIVNA ELEKTRODA SA TE^NOM MEMBRANOM ZA POTENCIOMETRIJSKO ODRE\IVAWE BAKRA I NIKLA MARIA PLENICEANY, MARIAN ISVORANU i CEZAR SPINU University of Craiova, Faculty of Chemistry, A. I. Cuza 13, 1100-Craiova, Romania U radu su prikazani eksperimentalni i teorijski podaci koji se odnose na pripremu i karakterizaciju tri elektrode sa te~nom membranom koje jo{ nisu navedene u odgovaraju}oj literaturi. Aktivna supstanca rastvorena je u nitrobenzenu i naneta na grafitni {tap formiraju}i membranu. Aktivnu supstancu ~ine proste kompleksne

276 PLENICEANU, ISVORANU and SP NU kombinacije Cu(II) i Ni(II) jona sa organskim ligandom koji spada i [ifove baze, N- 2-tienilmetiliden 2-aminoetanol (TNAHE). Cu 2+ -selektivne i Ni 2+ -selektivne elektrode kori{}ene su za odre ivawe bakrovih i niklovih jona u vodenim rastvorima direktno potenciometrijski ili potenciometrijskom titracijom sa EDTA. Kori{}ene su tako e za direktno potenciometrijsko odre ivawe Cu 2+ i Ni 2+ jona u industrijskim vodama. (Primqeno 11. februara, revidirano 24. juna 2004) REFERENCES 1. C. Luca, M. Pleniceanu, N. Muresan, Rev. Chim.-Bucharest 43 (1992) 64 2. M. Pleniceany, N. Muresan, V. Muresan, C. Luca, Rev. Chim.-Bucharest 43 (1992) 282 3. C. Luca, M. Pleniceany, N. Muresan, Rev. Chim.-Bucharest 37 (1992) 555 4. M. Pleniceany, Anal. Unive. Craiova Chim. XIX (1991) 44 5. M. Pleniceany, Rev. Chiom-Bucharest 45 (1994) 596 6. M. Pleniceany, Anal. Univ. Craiova Chim. XXI (1993) 45 7. M. Pleniceany, Al. Popescu, M. Preda, M. Baniceru, Rev. Chiom.-Bucharest 46 (1995) 929 8. M. Pleniceany, N. Muresan, M. Preda, Anal. Univ. Cariova Chim. XXIII (1995) 18 9. M. Pleniceany, M. Preda, N. Muresan, L. Simoiu, Analytical Letters 29 (1996) 1485 10. M. Pleniceany, M. Preda, N. Muresan, C. Spinu, Bull. Soc. Chim. France 134 (1997) 177 11. M. Pleniceany, L. Simoiu, M. Isvoranu, M. Baniceru, South. Brasilian J. Chem. 7 (1999) 67 12. C. Spinu, A. Kriza, Acta Chimica Slovenica 47 (2000) 179 13. I. M. Kolthoff, J. J. Vleeschhouwer, Biochem. Z. 179 (1926) 410 14. G. Eisenman, Anal. Chem. 44 (1972) 1545.