Mercury Sulfide and Silver Sulfide Based Hg(II) Sensitive Solid State Ion Selective Electrode

Mustafa Ta§tekin* and Eren Qantay

Department of Chemistry, Science Faculty, Ankara University, Tandogan 06100 Ankara Turkey

ABSTRACT

This study is related to the development of a new solid state membrane ion-selective electrode for Hg(II) ions. The membrane was prepared by the amalgamation of metallic mercury with metallic silver powder and this solid amalgam was finely ground, mixed with Na2C03, K2C03 and elemental sulfur and was burned in nitrogen atmosphere in a glass container with a strong burner flame for six hours. The resulting mercury and silver sulfides were pressed into a 10 mm diameter and 0.2 mm thick membrane. The Hg (II) sensitive electrode prepared was found to have a slope of 29.5 ± 0.6 mV/pHg, detection range of 1.0 χ 10"1 -1.0 χ 10"6, response time of 15-30 s and electrode lifetime of 18 months. Among the ten cations studied (Ag+, Al3+, Cd2+, Cu2+, Co2+, Fe3+, K+, Ni2+, Ca2+, Mg2+, Ba2+, Pb2+), only Ag+ was observed to have a disruptive effect upon the performance of the electrode. The electrode was used in the determination of mercury in various dental filling materials titrating with EDTA and the results were compared with the spectral data.

Keywords: Ion-selective electrodes; Mercury(II); Solid-state;

1. INTRODUCTION

The literature related to Hg(II) sensitive ion selective electrodes reports mainly neutral carrier polymer- based membrane /1-15/, carbon paste /16-17/ and chalcogenide electrodes /18-20/. However, these electrodes have certain disadvantages such as relatively shorter electrode lifetime, leakage from the membrane and membrane rupture during the usage. Also, since they have internal filling solution, there are problems associated with surface contamination and cleaning when they are in continuous use /21/. There are relatively few studies related to the Hg(II)-selective metal sulfide based solid membrane electrodes. This is mainly due to the fact that Hg(II) sulfide is present as black colored cubic HgS

* Corresponding author. Tel.: +90-312-2126720/1281; Fax: +90-312-2232395 E-mail address: [email protected]

331 Vol. 31, No. 6, 2009 Mercury Sulfide and Silver Sulfide Based Hg(ll) Sensitive Solid State Ion Selective Electrode

(metacinnabar) and red colored hexagonal HgS (cinnabar) structures and cinnabar is difficult to press and has an inferior sensitivity to Hg(II) ions /21, 22/. Therefore, cinnabar must not be present in the structure of the membrane to be used in the construction of the solid-state membrane for ion-selective electrodes. That is why the precipitation of HgS in cubic structure, in other words its particle structure, is very important. There are various methods proposed in literature for the precipitation of Hg(II) ions /21, 22/.

This study involves the preparation of a new material formed by the mixture of Ag2S-HgS and its use in the development of an ion-selective electrode sensitive to Hg(II) ions. The performance of the electrode prepared with the use of this material was investigated.

2. EXPERIMENTAL

2.1. Apparatus

All Potentiometrie and pH measurements were made at 25±1°C using a Consort pH/mV meter (model C863) and an Ingold U402-S7/120 glass electrode was used for pH measurement .All the spectrometric measurements were performed with the use of a Digilab Hitachi U-2800 UV-VIS spectrophotometer.

2.2. Reagents and materials

All the chemicals used in the study were of analytical grade and were not subjected to any further purification. The stock solutions of the metal ions were prepared in 0.1 M. The experimental solutions were prepared by dilution of the stock solutions with deionized water obtained from Elgestat-prima 2 and Elgestat- maxima UF water purification apparatus. The conductivity of the deionized water was 2.5 χ 10"6 Siemens.

Hg(N03)2 (99.0 %), AgN03 (99.8 %), A1(N03)3 (99.8 %), Cr(N03)3 (99.0 %), Ni(N03)2 (97.0 %), Pb(N03)2

(98.5 %), CO(N03)2 (99.0 %), Cu(N03)2 (97.0 %), NaOH (97.0 %), HN03 (65 %; d = 1.42 kg/L), EDTA (98.0

%), KN03 (99.0 %), Ca(N03)2 (98.0 %), Fe(N03)3 (99.0 %), NaN03 (99.9 %), Mg(N03)2 (98.0 %) and standard mercury solution (0.05 M) were obtained from Merck, HCl (37 %; d = 1.19 kg/L) from Riedel-De- Haen, dithizone (99.0 %) from Fluka and mercury amalgam (=99.0 %) used in dentistry from Sdi ultracaps+ firms.

The pH values of the experimental solution were set to 2.5 with the use of HN03 and NaOH. The ionic

strengths of the solutions were adjusted to contain 0.1 Μ NaN03.

2.3. Preparation of the electrode

Metallic silver powder obtained by the electrolysis of silver nitrate was gradually added to metallic mercury by constant stirring until solid amalgam structure was obtained (there was 2.5 g of silver powder used for each 5.0 g of mercury). The mercury-silver amalgam was finely ground in a mortar. Then

approximately 100 g of this grounded amalgam was homogenously mixed with 15.0 g sulfur, 2.0 g Na2C03

and 10.0 g K2C03 and this mixture was placed in a fusion column and fused under nitrogen atmosphere at

332 Tastekin and Cantay Main Group Metal Chemistry

elevated temperatures. The fusion was started at low Bunsen flame until the material turned completely black then the temperature was increased to elevated values. Care was taken for the homogeneity of heating process. When the heating was not homogenous it was observed that HgS and elemental sulfur rapidly sublimated and recrystallized on the walls of the cooling system. The membrane material obtained after six hours of fusing process was treated with dilute HN03 to eliminate its alkalinity and washed with abundant amounts of water and alcohol in order to remove the residual sulfur. The resulting black Ag(I)-Hg(II) sulfide mixture had a very homogenous appearance with very small sized particles. 1 g of this finely powdered membrane material was taken, subjected to 9 tons of total pressure gradually applied in ten minutes, and kept under this pressure for 20 minutes to obtain 1 cm diameter and 0.3 mm thick membrane disks. The disc shape membrane was attached to a helical copper wire with silver adhesive. It was then mounted on the electrode with a suitable binder.

2.4. Potential measurements

The Potentiometrie cell was formed using Ag/AgCl reference electrode and Hg(II) sensitive electrode . The schematic representation of the cell is given below:

Ag/AgCl, KCl(saturated)//test solution /AgS.HgS (membrane)/ Cu

The cell potential was measured after stirring the solution with a magnetic stirrer and waiting for the potential to reach a steady value. To test the analytical application of the ion-selective electrode prepared, the determination of mercury in dental filling materials was tried. For this purpose, the sample of the material was dissolved in 6.0 Μ HN03 and diluted to an appropriate volume so as to adjust the mercury concentration to fall into a range suitable for the electrode working range. Any trace of mercury (I) ions was oxidized to mercury (II) by the addition of

KMn04; the excess of which was removed by H202, and the remaining H202 was expelled by boiling. The accompanying silver ions were removed by precipitation as AgCl with HCl and the final solution was diluted with water to the mark of volumetric flask. This solution was used for analysis.

3. RESULT AND DISCUSSION

Performance characteristics and optimum working condutions of the mercury (Il)-selective electrode were investigated. Working conditions involved were suitable pH-range, linear working range and response lifetime, and selectivity coefficients for the proposed electrode. The performance of the electrode for the analysis of real samples was also investigated by comparison with a well-established spectrophotometric method.

333 Vol. 31, No. 6, 2009 Mercury Sulfide and Silver Sulfide Based Hg(ll) Sensitive Solid State Ion Selective Electrode

3.1. Effect of pH

The accurate detection of Hg(II) ions was performed by measuring the potential of the ion-selective electrode at lower pH values /2, 8, 21/. This is due to the fact that at high pH values free Hg(II) ion forms the hydroxyl complex (HgOH+), which causes a decrease in the Potentiometrie response of the electrode. On the other hand, at extremely low pH values the interference of hydrogen ions partly affects the solubility of the membrane and results in variation in potential. In order to determine the optimum pH range at which the electrode is least affected, 50 mL of a ΙχΙΟ"1 Μ

Hg(N03)2 was taken and its pH value was adjusted to 1.5 with HN03 solution. Then the pH value was increased by 0.1 unit with 0.5 Μ NaOH solutions. The potential values recorded were plotted against pH values. This procedure was repeated till pH = 9.0. The pH value where the potential did not show a significant variation was found to be 1.5 - 3.0. Therefore all the measurements were taken at a pH value of 2.5 (Figure 1).

800

700

1 600 LU

500

1 400

0 2 4 6 8 10 pH Fig. 1: The effect of pH on the response of Hg(II)-selective electrode.

3.2. Determination of working range and lower detection limit

The calibration curve of Hg(II) ion selective electrode was constructed by the use of mercury(II) nitrate standard solutions with concentrations ranging from 1 χ 10"' Μ to 1 χ ΙΟ'7 Μ. The ionic strength of the solutions was adjusted to 0.1 Μ NaN03. The pH values of the solutions were adjusted to 2.5 by the use of

NaOH and HN03. The potential values of the calibration solutions prepared were measured against Ag/AgCl reference electrode. The calibration curve for Hg(II) was drawn by plotting the potential values against the concentration of Hg(II) solutions (Figure 2). Using this graph the linear working range of the electrode was determined from the linear region of the potential response. As seen from Figure 2 the working range of the electrode prepared was found to be 1.0 χ 10"6-1.0 χ 10"1 M. The average slope of five different electrodes prepared in the same manner was 29.5 ± 0.6 mV/pH 9 at 25 °C.

334 Tastekin and Cantay Main Group Metal Chemistry

8 6 4 2 0 pHg

Fig. 2: The calibration curve of the proposed solid state Hg(II)-selective electrode.

This value is very close to the Nernstian value of 29.6 mV/pH. This electrode has a moderate working range among the mercury selective electrodes reported in literature 12, 6, 23/. However, when compared with solid membrane electrodes, it is much superior to most of them /18, 22/. Using the data obtained from ten successive experiments made with the blank solutions the lower detection limit was computed at a 95 % confidence level /24/. The lower limit of Hg(II) selective electrode prepared was found to be 6.5 χ 10"7 M.

3.3. Response time and lifetime of the electrode

The response time of the electrode is greatly affected by the change in concentrations. When the electrode conditioned in pure water was placed in a solution with a Hg(II) concentration of 10"6 M, it took 40-60 seconds for the potential to reach a steady state value. This period was observed to be 5-10 seconds in the solutions having Hg(II) concentration above 10"5 M. The average response time was 15-30 s. The response time of the electrode was observed to be much shorter than most of the similar electrodes reported in literature/9, 16, 17,21/. There were no significant changes in the potential and Nernstian slope of the electrode after 18 months of routine use. This period is much shorter (in the range of a few months) for the neutral carrier polymer-based membrane electrodes/9, 12, 15/.

3.4. Determination of selectivity coefficients

The selectivity coefficients of the Hg(II)-selective electrode against Ag(I), Cu(II), Ca(II), Mg(II), Pb(II), Ni(II), K(I), Ba(II), Fe(III), Co(II), Cd(II) and Al(III) ions were determined by using fixed interfering method. A series of Hg (II) solutions in the range of 10"1 Μ to ΙΟ"7 Μ was prepared containing ΙΟ"3 Μ of each interfering ion. pH value of each calibration solution containing 0.1 Μ NaN03 was adjusted to 2.5. The

335 Vol. 31, No. 6, 2009 Mercury Sulfide and Silver Sulfide Based Hg(II) Sensitive Solid State Ion Selective Electrode

potentials of the solutions were measured and plotted against Hg(II) concentration and the selectivity coefficients were computed according to

^Hg,M = a Hg/ÖM

The selectivity coefficients (A:Hg,M) are given in Table 1. Almost all Hg(II) sensitive electrodes reported in literature are affected by the presence of Ag(I)ions. The present electrode is particularly prone to this effect

due to the Ag2S in its membrane structure. Apart from that the presence of CN", CI", I', Br", CH3COO" ions also affect the measurement due to their tendency to form strong complexes with Hg(II) and Ag(I) ions in the membrane.

Table 1 Selectivity coefficient of the Hg (II) electrode prepared

Interfering ion, (M) Selectivity coefficient, AHg;M

Ag(I) 3.16 Al(III) 1.26xl0"3 Cd(II) 3.31xl0"3 Cu(II) 4.68xl0"3 Co(II) 8.13x1ο"4 Fe(III) 1.00x10'3 K(I) 2.63xl0"3 Ni(II) 2.51xl0"3 Ca(II) 3.39xl0"3 Mg(II) 3.16xl0"3 Ba(II) 3.55xl0'3 Pb(II) 3.31xl0"3

Furthermore, Fig 3 shows the response of some metal ions of the proposed Hg(II) selective solid-state electrode.

3.6. The analytical applications of the electrode

Standard Hg(II) solutions were titrated with standard EDTA solution in order to determine whether the proposed electrode could be used as an indicator electrode in the titration of Hg(II) ions. An example of a titration curve thus obtained is given in Fig. 4. As it is seen, the amount of Hg(II) ions in solution can be accurately determined with this electrode. The Cu(II) and Zn(II) ions present in the amalgam sample were observed to have no interfering affect.

336 Tastekin and Cantay Main Group Metal Chemistry

mVHg mVNi mV Ca mV Cd mV Pb mV Zn mV Ba mV Ag

Fig. 3: Potentiometrie response of Hg(II) selective electrode against some ions

The mercury in amalgam was also determined spectroscopically /25/ using the same conditions and the results are also given in Table 2. As it can be seen from the data given in the table, there is no significant difference between the data obtained with these two different methods, at a confidence level of 95%.

750

740

730

^720

710

700 0123456789 10 mL EDTA

Fig. 4: The Potentiometrie titration curve of 10,0 mL 0,01 Μ Hg(II) with 0,0200 Μ EDTA.

337 Vol. 31, No. 6, 2009 Mercury Sulfide and Silver Sulfide Based Hg(Il) Sensitive Solid State Ion Selective Electrode

Table 2 Statistical comparison of Potentiometrie and spectroscopic data

Titrimetric analysis with EDTA using ion Spectrophotemetric Comparison of two methods selective electrode, Hg analysis, % Hg % 1 * 5Γ+ ts + 1 N xN VN S Ν S Ν 1 1 V i 2

Amalgam 53.14 ± 0.99 5 52.74 ± 2.75 2.21 5 0.40 <2.50 sample 1.23

4. CONCLUSION

A new method was developed to prepare a membrane material and this material containing black HgS

and Ag2S was used to construct a mercury (II)-selective solid-state electrode. The ion selective electrode developed gave satisfactory results with real samples. The relative error was found to be below 2 % in Potentiometrie titrations experiments carried out with different Hg concentrations. The results obtained indicated that the electrode might well be utilized for quantitative purposes. There were no interfering effects of Cu(II) and Zn(II) ions present in the medium. This is a further indication that the electrode could be used in real samples such as teeth filling amalgams..

ACKNOWLEDGEMENT

The financial support provided by the Ankara University Research Fund (BAP) is gratefully acknowledged (Project #:20050705106).

REFERENCES 1. R.K. Mahajan, R. Kaur, I. Kaur, V. Sharma and M. Kumar, Anal. Science, 20 (2004) 811-814. 2. RK. Mahajan, I. Kaur and T.S. Lobana, Talanta, 59 (2003), 101-105. 3. M.H. Mashhadizadeh and I. Sheikhshoaie, Talanta, 60 (2003), 73-80. 4. A.V. Kopytin, P.G. Klatsmanyi, V.P. Izvekov, E. Pungor and G.A. Yagodin, Anal. Chim. Acta, 148 (1983), 35-40.

338 Tastekin and Cantay Main Group Metal Chemistry

5. X. Yang, D.B. Hibbert and P.W. Alexander, Anal. Chim. Acta, 372 (1998), 387-398. 6. M. Mazloum, M.K. Amini and I. Mohammadpoor-Baltork, Sensors and Actuators B, 63 (2000), 80-85. 7. S.S.M. Hassan, M.B. Saleh, A.A. Abdel Gaber, R. A.H. Mekheimer and N.A. Abdel Kareem, Talanta, 53(2000), 285-293. 8. J. Lu, X. Tong and X.He, J. of Electroanal. Chem. 540 (2003), 111-117. 9. R.K. Mahajan, I. Kaur and T.S. Lobana, Talanta, 59 (2003), 101-105. 10. L. Perez-Marin, E. Otaza-Sanchez, G. Macedo-Miranda, P. Avila-Perez, J. Alonso Chamaro and H. Lopez-Valdivia, Analyst, 125 (2000), 1787-1790. 11. A.K. Singh, G. Bhattacharjeee and R. Singh, Sensors and Actuators B, 99 (2004), 36-41. 12. S. Han, M. Zhu, Z. Yuan and Xin Li, Biosensors & Bioelectronics, 16 (2001), 9-16. 13. A.A. Khan and M.M. Alam, Anal. Chim. Acta, 504 (2004), 253-264. 14. V.K. Gupta, S. Chandra and H. Lang, Talanta, 66 (2005), 575-580. 15. M.N. Abbas and G.A.E. Mostafa, Anal. Chim. Acta, 478 (2003), 329-335. 16. M.J. Gismera, D. Hueso, J.R. Procopio and M.T. Sevilla, Anal. Chim. Acta (2004), 347-353. 17. M. Neshkova and E. Pancheva, Anal. Chim. Acta, 300 (1995), 133-141. 18. M.T. Neshkova, Anal. Chim. Acta, 273 (1993), 255-265. 19. R. De Marco and J. Shackleton, Talanta, 49 (1999), 385-391. 20. G.A. East and A. Da Silva, Anal. Chim. Acta, 148 (1983), 41-50. 21. W.E. van der Linden and R. Oostervink, Anal Chim Acta, 108 (1979), 169-178. 22. W. Szczepaniak and J. Oleksy, Anal Chim Acta, 189 (1986), 231-243. 23. G.E Baiulescu and V.V. Cojofret, Talanta, 23, (1976), 677-678. 24. D.A. Skoog, F.J. Holler, and T.A. Nieman, Fundamentals of Analytical Chemistry, 1996, Saunders College Publishing, 7th Edition, p.59-60. 25. Z. Marczenko, Separation and Spectrophotometry Determination of Elements, 1986, John Wiley & Sons, NY10158, USA.

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