nickel(ii)-selective membrane electrode based on macrocyclic ligand

Nickel(II)-Selective Membrane Electrode Basedon Macrocyclic Ligand

Ashok Kumar Singh,* Chokhe Lal Sharma, Seema Baniwal, and Amit Panwar

Department of Chemistry, University of Roorkee, Roorkee-247667 India; e-mail: [email protected]

Received: November 1, 2000

Final version: January 30, 2001

Abstract

A polystyrene-based membrane of 3,4:11,12-dibenzo-2,5,10,13-tetraoxo-1,6,9,14-tetraazacyclohexadecane was prepared and investigatedas a Ni2þ-selective electrode. The best performance was observed with the membrane having polystyrene to ligand composition of 1:6,which worked well over a wide concentration range, 3.16610ÿ6–1.00610ÿ1 mol Lÿ1 with a near Nernstian slope of 30.7 mV per decadeof activity between pH 2.5 to 7.0. The electrode exhibits a fast response time of 10 s and was used over a period of 2 months with goodreproducibility (S¼ � 0.2 mV). The selectivity coefficient values for monovalent, bivalent and trivalent cations indicate excellent selectivityfor Ni2þ ions and it also works satisfactorily in a partially nonaqueous medium of methanol and ethanol. The practical utility of themembrane sensor was observed in solutions contaminated with detergents, cetyltrimethylammonium bromide and sodium dodecyl sulfateand the sensor was used successfully to determine nickel in electroplating waste.

Keywords: Membrane electrode, Nickel-selective electrode, Macrocycle

1. Introduction

Nickel is widely distributed in the occupational and general

environment from alloys and nickel-plated items. While acutenickel poisoning is very rare, occupational exposure to insolublecrystalline nickel compounds is a recognized nasal and

pulmonary carcinogenic hazard. Accordingly, the carcinogenicityof nickel governs occupational standards for airbone nickel, eventhough the human carcinogenicity of more soluble nickel species

is still under debate. The soluble nickel compounds are strongskin allergens, the Ni2þ ion acting as a hapten. The prevalence ofnickel allergy in women is about 10 % in western populations,placing nickel among the most important dermal allergen [1].

Oral nickel intake due to the use of nickel-containing kitchenutensils or intake of food items high in nickel may exacerbatedermatitis from skin contact with nickel.

Nickel, mainly present in red meat, chocolates, vegetable oilsand effluent discharged from electroplating industries, can bedetermined by various methods, viz., spectrophotometric,

coulometric or gravimetric. All of these methods are expensiveand time-consuming whereas ISEs provide a mean of determin-ing ions most conveniently, economically and without any

sample pretreatment. During the last few decades vigorous effortshave been made to develop ISEs for various ions, and a numberof electrodes are now commercially available for alkali andalkaline earth metals.

Since the first nickel(II)-ion-selective electrode reported byPungor [2] and co-workers, which was based on Ni-dimethyl-glyoxime complex, there have been a number of electrodes

reported for nickel ion. Later on, heterogeneous membranes ofnickel(II) phosphate [3] in paraffin and silicon rubber, bis(2-ethylhexyl)hydrogenphosphate [4] in PVC and a nickel complex

of 1,4,8,11-tetraazacyclotetradecane [5] in araldite were used forpreparing electrodes for this metal ion, but these electrodes hadsmaller working concentration range. In addition to these anumber of electrodes were prepared by using chelating ion-

exchange resins, porphyrins and macrocycles as ionophores [6–15]. These electrodes were not to the mark and suffered eitherfrom selectivity, reproducibility or showed non-Nernstian

response. Some of the recent electrodes have been compared tothe proposed electrode assembly which is based on macrocyclic

ionophore 3,4:11,12-dibenzo-2,5,10,13-tetraoxo-1,6,9,14-tetra-azacyclohexadecane bound by polystyrene and showed Nernstianresponse with a wide working concentration range and fast

response time than the reported electrodes (Table 1).

2. Experiments

2.1. Reagents

All the reagents were of analytical grade. Nickel(II) nitrate was

used as nickel salt for the studies of the membrane sensor.Polystyrene was obtained from G.S.C. (New Delhi, India). Metalsolutions of their respective nitrate salts were prepared in double

distilled water and standardized by appropriate methods [17].

2.2. Equipment

A Perkin-Elmer Model 3100 atomic absorption spectro-photometer (AAS) with a graphite furnace was used. The

potentials across the membranes were measured with an ECIL(Hyderabad, India) pH 5662 digital pH=potentiometer or acentury CVM (Chandigarh, India) 301 microvoltmeter, in

conjugation with saturated calomel electrodes (SCE) as referenceelectrodes, nickel(II) nitrate (1.00610ÿ1 mol Lÿ1) was used asinternal solution. All measurements were made at a constant

temperature of 25� 0.1 �C.

2.3. Synthesis of Macrocycle

The macrocycle 3,4:11,12-dibenzo-2,5,10,13-tetraoxo-

1,6,9,14-tetraazacyclohexadecane, [Bz2Oxo4(16)N4] (Scheme 1),was prepared as reported by Shakir [18] (C20H20N4O4 calculated(%): C, 63.2; H, 5.5; N, 14.7; found (%): C, 63.3; H, 5.4; N,

1209

Electroanalysis 2001, 13, No. 14 # WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2001 1040-0397/01/1410–1209 $17.50þ.50=0

14.5). The observed element analysis data for the ligand are in

good agreement with the calculated values.

2.4. Membrane Preparation

A number of membranes were prepared. Membranes of

adequate strength, which gave reproducible and stable potentialwith a fast response time and with no cracks developed on longusage were further used. To meet these requirements the varyingcompositions of ligand and binder (polystyrene) at 85 �C were

mount pressed under pressure (ca., 7000 psi) in a die. Afterpreparation of the membranes, they were investigated under amicroscope to observe for cracks and homogeneity of the surface.

Suitable membranes were then selected and explored for elec-trochemical examination. Further, only those membranes whichgave reproducible and stable potentials were sorted out for further

studies. Membrane to membrane (and batch to batch) reprodu-cibility was assured by carefully controlling the conditions offabrication. The membranes (2.5 cm diameter and 0.5 mm thick)

were affixed to one end of small (ca., 6 cm) Pyrex glass tubes,with Araldite, while the other end of each tube remained open.They were then equilibrated with Ni2þ solution [19]. Satisfactory

equilibration was achieved with 1.00 mol Lÿ1 Ni(NO3)2 solution

in contact time of three days.

2.5. Potential Measurements

All the potential measurements were carried out at 25� 0.1 �Cusing the assembly setup as reported earlier [20, 21], with the

following cell assembly.

Internalreference

Ni2þ

0.1 mol Lÿ1Membrane Test

solutionExternal

reference

electrode(SCE)

internalsolution

electrode(SCE)

Saturated calomel electrodes (SCE) were used as referenceelectrodes. All pH adjustments were made with diluted HNO3 or

ammonia.

3. Results and Discussion

3.1. Membrane Characteristics

The response characteristics of the membranes, namely the

working concentration range, response time, lifetime, and stabi-lity depend on the ratio of the membrane ingredients. Membraneswith different compositions were, therefore, prepared and the

effect of composition on the performance of the membranes wasalso studied (Table 2).

3.2. Working Concentration Range and Slope

A number of membranes were prepared and equilibrated with

various metal ions. A perusal of working concentration range ofthese cations presented in Figure 1 indicates that the electrode isbest suited for nickel as it exhibits a near Nernstian response for

Ni2þ while for other cations the response is either sub-Nernstianor non-Nernstian. The potential responses of the membranes(Nos. 1–4) are presented in Table 2 and Figure 2. It was found

Scheme 1.

Table 1. A comparison of the recently reported electrodes with the proposed electrode assembly. PVC, poly(vinyl)chloride; DOP, dioctylphthalate;(dien)2, iminobis(2-ethylamine); DBP, dibutylthalate; STB, sodiumtetraphenyl borate; TMPP, 5,10,15,20-tetra(4-methylphenyl)porphyrin.

References Membrane compositionWorking concentrationrange (mol Lÿ1)

Slope(mV=decade) pH

Responsetime (s) Lifetime

[5] 1,4,8,11-Tetraazacyclotetradecane,PVC

5.0610ÿ5–0.1 46 (Ist week)38 (IInd week)

3.5–8.0 40 8 weeks

[10] 1-Hydroxy-2-naphthaldoxime-formaldehyde polymer, PVC

4.0610ÿ5–0.1 28.5 3.0–7.5 10 2 months

[12] 5,7,12,14-Tetramethyl-1,4,8,11-tetrazacyclotetradeca-4,7,11,14-tetraene,DOP, PVC

5.0610ÿ6–0.1 27.0 3.0–7.0 10 3 months

[13] [Ni-(dien)2]2þ, BPh4–(tetraphenyl borate),DBP, PVC (coated C-rod)

5.6610ÿ5–0.1 29.5� 0.3 ca. 12 <5 1 year

[15] 1,3,6,10,12,15-Hexaazatricyclo[13.3.1.1]sicosane, DBP, DOP, STB, PVC,

1.0610ÿ5–0.1 29.0 2.8–5.5 20 4 months

[16] TMPP, STB, DBP, PVC 5.6610ÿ6–0.1 30.1 2.5–7.4 20 6 monthsProposed

electrodeassembly

3,4:11,12-Dibenzo-2,5,10,13-tetraoxo-1,6,9,14-tetrazacyclohexane,Polystyrene

3.16610ÿ6–0.1 29.6, (30.7) [a] 2.5–7.0 10 2 months

[a] slope in mV=decade of activity.

1210 A. K. Singh et al.

Electroanalysis 2001, 13, No. 14

that when the ratio of ligand to binder was 4:1 (membrane No. 1)

the working concentration range was 5.43610ÿ5–1.00610ÿ1 mol Lÿ1 and with a sub-Nernstian slope of 25.7� 0.2 mVper decade of activity. On the other hand, when the ligand to

binder ratio was above 6:1, as in membranes No. 3 and 4, theworking concentration range, slope and response time were poorand drift in potential was observed on long usage. The bestperformance was exhibited by membrane No. 2 with binder and

ligand in the ratio of 1:6, the working concentration range of3.16610ÿ6–1.00610ÿ1 mol Lÿ1 and a near Nernstian slope of30.7� 0.2 mV per decade of activity. Hence, membrane No. 2

was studied in detail as Ni2þ-selective electrode.

3.3. Response and Lifetime

The response time, i.e., the time taken by the electrode toachieve stable and low noise potentials for membrane No. 2 wasthe least (<10 s) over the whole working concentration range

(Table 2). Among membrane Nos. 1–4, No. 2 exhibits the best

results showing a wide concentration range of 3.16610ÿ6–1.00610ÿ1 mol Lÿ1 with a near Nernstian slope of 30.7 mV perdecade of activity. The response time was found to be <10 s and

it was used over a period of 2 months. During usage, membraneswere stored in 1.00610ÿ1 mol Lÿ1 Ni2þ solution and werereequilibrated with 1.00 mol Lÿ1 Ni2þ solution whenever anydrift in potentials was observed. The repeated monitoring of

potentials using this membrane (20 identical measurements)on any day of investigation with the same sample(1.00610ÿ3 mol Lÿ1) gave a standard deviation of � 0.2 mV.

3.4. pH and Nonaqueous Effect

The pH dependence of the electrodes potential was tested over

the pH range 1.0–8.0 at a fixed concentrations of Ni2þ ions(1.00610ÿ2 and 1.00610ÿ3 mol Lÿ1) (Fig. 3). The pH of thesolutions was adjusted by the addition of HNO3 or ammonia.

Table 2. Composition of polystyrene based membrane of Bz2Oxo4(16)N4 and performance characteristics of Ni2þ-selective electrode based on them.

Membrane No.

Ratio (w=w) of variouscomponents in membrane

Working concentration Slope, mV per decade of Response

or electrode No. Polystyrene Ligand range (mol Lÿ1) activity (� 0.2 mV) time(s)

1 1 4 5.43610ÿ5–1.00610ÿ1 25.7 302 1 6 3.16610ÿ6–1.00610ÿ1 30.7 103 1 8 1.00610ÿ5–1.00610ÿ1 28.9 254 1 10 1.32610ÿ5–1.00610ÿ1 31.9 28

Fig. 1. Potential response of various ion-selective membranes withpolystyrene and macrocyclic ligand in 1:6 (w=w).

Fig. 2. Variation of cell potential with Ni2þ concentration for electrodeNo. 1, 2, 3 and 4.

Nickel Selective Membrane Electrode 1211


A perusal of Figure 3 indicates that the potentials remain constantbetween pH 2.5 and 7.0. This range may be taken as working pH

range of the proposed membrane sensor. The sharp change in thepotentials at pH values >7.0 may be due to hydrolysis of Ni2þ,while at pH values <2.5 Hþ ions contribute to the chargetransport process of the membrane, thereby causing a strong

interference.The performance of the sensor was also investigated in

partially nonaqueous medium using methanol-water and ethanol-

water mixtures. The membrane works satisfactorily in nonaqu-eous medium up to 30 % (v=v) content of methanol and ethanolFigure 4a and b, as in these mixtures the working concentration

range and slope remain unchanged. However, above 30 %nonaqueous medium, the slope and working concentration rangewere reduced and drift in potential was also observed. It is worth

mentioning that the lifetime of the membranes was not altered innonaqueous solutions.

3.5. Potentiometric Selectivity

Selectivity is the single most important characteristic of any

ion-selective electrode. Several methods are proposed for theexperimental determination of this parameter, which includethe separate solution method, fixed interference method [22]and matched potential method [23]. The selectivity of the

proposed membrane sensor was examined in the presence ofa high concentration (1.00610ÿ2 mol Lÿ1) of interfering ions,by using the matched potential method (Fig. 5). In view of the

fact that the values obtained by different methods are quitedifferent and do not present a realistic picture of the selectivityaspect of the electrode system, matched potential method

is IUPAC recommended [24] and it gave practical KPotNi2þ;B

values.To be more precise and in order to assess the practical utilityof the sensor, some mixed runs were made in presence of varyinglevels of interference of Kþ and NHþ4 (Fig. 6a and b). It

is apparent from these curves that 1.00610ÿ5 mol Lÿ1 concen-trations of Kþ and NHþ4 cause no significant divergence inthe original potential vs. Ni2þ concentration plots. Thus, the

electrode can tolerate Kþ and NHþ4 ions at a concentration level41.00610ÿ5 mol Lÿ1 over the whole working concentration

range. Further, it is seen from these plots that higher concen-

trations of Kþ and NHþ4 ions cause divergence from the originalplot and thus cannot be tolerated over the whole concentrationrange. From the point of divergence, the corresponding reduced

working concentration ranges can be calculated. The values ofselectivity coefficients indicate that the sensor was selective overthe cations listed in Figure 5.

Fig. 4. Variation of cell potential with Ni2þ concentration in a) metha-nol-water mixture and b) ethanol-water mixture.

Fig. 3. Effect of pH on cell potential; a) [Ni2þ] ¼ 1.00610ÿ2 mol Lÿ1,b) 1.00610ÿ3 mol Lÿ1.



3.6. Analytical Application

The performance of the electrode assembly was also observedin solutions contaminated with detergent matter, cetyltrimethyl-ammonium bromide (CTAB) and soduim dodecyl sulfate (SDS).

It was observed that small amounts 51.00610ÿ5 mol Lÿ1 ofdetergent matter (CTAB and SDS) did not alter the membranefunctioning but at higher concentration 41.00610ÿ4 mol Lÿ1 of

CTAB and SDS do cause problem.

3.7. Water Analysis

The membrane sensor has been successfully used for deter-mining nickel in effluents discharged from electroplating works.Four samples of local electroplating wastes were collected and

stored in glass containers and analyzed within 12 h after collec-tion. The samples were analyzed by the proposed membranesensor and by AAS under identical conditions of temperature and

pH. As the samples contained particulate matter, they werecentrifuged and the potentials were measured after adjusting thepH with HNO3=ammonia if necessary. The data, given in Table 3,

indicate that the amount of nickel determined in effluent by thesensor is in close agreement with that determined by use ofatomic abosption spectrometry (AAS).

4. Conclusions

Membrane sensor No. 2 was found to show the best results andwith a lifetime of 2 months. A comparison of some recent Ni2þ-

selective electrodes reported in the literature and under investi-gation (Table 1) reveals that the proposed sensors is superior tothe reported ones in terms of working concentration range, slope

and response time. Further the utility of the sensor in wastemonitoring indicates practical applications of the sensor.

Table 3. Nickel content of electroplating waste as determined by themembrane sensor No. 2 and AAS.

pH Ni2þ in mg Lÿ1 determined

Sample No. Found After adjustment AAS Sensors

1 6.24 4.10 42.0 41.02 6.18 4.05 43.2 43.03 6.30 4.15 43.5 43.04 6.22 4.10 42.8 43.0

Fig. 6. Variation of cell potential as a function of Ni2þ concentration in presence of different concentrations of a) Kþ and b) NHþ4 ions.

Fig. 5. Comparison of selectivity coefficients with respect to Ni2þ ionsfor different cations.

Nickel Selective Membrane Electrode 1213


5. Acknowledgement

A. Panwar is thankful to the Council of Scientific and Indus-trial Research (CSIR), New Delhi, India for providing financialsupport to undertake this work.

6. References

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[3] E.B. Buchanan, J.L. Saego, Anal. Chem. 1968, 40, 517.[4] S.P. Awasthi, V.T. Kulkarni, M. Sundareson, J. Electrochem. Soc.

India, 1988, 37, 309.[5] U.S. Lal, M.C. Chattopadhyaya, A.K. Dey, J. Ind. Chem. Soc. 1988,

49, 1982.[6] E. Pungor, K. Toth, J. Havas, Microchim Acta 1966, 4–5, 689.[7] E.A. Materova, V.V. Muchovikov, M.G. Grigorieva, Anal. Lett.

1975, 8, 167.[8] E.V. Smironova, O.M. Petrukhin, S.L. Rogatinskaya, Zh. Anal.

Khim. 1982, 37, 2137.[9] M.D. Hampton, C.A. Peters, L.A. Wellington, Anal. Chim. Acta

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[10] V. Vickackaite, A. Abrutis, R. Kazlauskas, O.M. Petrukhin, Zh.Anal. Khim. 1987, 42, 2209.

[11] G.N. Rao, S. Srivastava, S.K. Srivastava, M. Singh, Talanta 1996,43, 1821.

[12] A.K. Singh, G. Bhattacharjee, M. Singh, S. Chandra, Bull. Chem.Soc. Jpn. 1997, 70, 2995.

[13] M.F. de Souza Teixeira, F. Filho, Orlando, J. Braz. Chem. Soc.1996, 7, 223.

[14] M. Pleniceanu, M. Preda, N. Muresan, L. Simoiu, Anal. Lett. 1996,29, 1485.

[15] L.P. Singh, H. Vardhan, Anal. Proc. Anal. Commun. 1995, 32, 193.[16] V.K. Gupta, A.K. Jain, L.P. Singh, U. Khurana, Anal. Chim. Acta

1997, 355, 33.[17] A.I. Vogel, in A Text Book of Quantitative Chemical Analysis, 5th

ed., ELBS Publication, London, 1989.[18] M. Shakir, S.P. Varkey, Ind. J. Chem. 1995, 34A, 355.[19] A. Craggs, G.J. Moody, J.D.R. Thomas, J. Chem. Educ. 1974, 51,

541.[20] A.K. Singh, A. Panwar, S. Kumar, S. Baniwal, Analyst 1999, 124,

521.[21] S. Baniwal, S. Chandra, A. Panwar, A.K. Singh, Talanta 1999, 50,

499.[22] F.J. Saez de Viteri, D. Diamond, Analyst 1994, 119, 749.[23] V.P.Y. Gadzekpo, G.D. Christian, Anal. Chim. Acta 1984, 164,

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nickel(ii)-selective membrane electrode based on macrocyclic ligand

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