DSpace at VNU: Fabrication of Solid Contact Ion Selective Electrode for Mercury (II) Using Conductive Polymer Membrane

Materials Transactions, Vol 56, No (2015) pp 1428 to 1430 Special Issue on Nanostructured Functional Materials and Their Applications © 2015 The Japan Institute of Metals and Materials Fabrication of Solid Contact Ion Selective Electrode for Mercury (II) Using Conductive Polymer Membrane Pham Thi Ngoc Mai+ and Phan Tri Hoa Faculty of Chemistry, Hanoi University of Science, Vietnam National University, 19 Le Thanh Tong, Hanoi, Vietnam A solid-contact ion-selective electrode for mercury (II) ions was fabricated to determine Mercury (II) in aqueous environment A conductive polymer membrane (polypyrrole-PPy) was synthesized electrochemically on paste carbon electrode, which is covered by the ionselective membrane (ISM) ® an important part of a complete solid contact ion selective electrode (SCISE) The electrode showed excellent potentiometric response over a wide concentration range from 10¹9 M to 10¹2 M and detection limit down to © 10¹10 M A good selectivity towards Hg2+ ions in comparison with other common ions in water has also been observed The electrode was employed for determination of Hg2+ in ballast water samples with high sensitivity and accuracy [doi:10.2320/matertrans.MA201562] (Received February 2, 2015; Accepted April 8, 2015; Published June 5, 2015) Keywords: solid contact ion selective electrode, mercury (II) determination, conductive polypyrrole Introduction Mercury is one of the most toxic elements impacting human and ecosystem health A wide range of mercury species exist within our environment, among which Mercury (II) is the most common oxidation state in nature.1) An understanding of mercury species transformations and accurate monitoring of mercury (II) compounds in the environment hence are essential for reliable risk assessment Common techniques to determine traces of mercury species in the environment include Atomic Absorption Spectrometry-AAS, Atomic Fluorescent Spectrometry-AFS and Anodic Stripping Voltammetry-ASV, however these techniques require expensive instrument and complicated analysis procedures.2) The use of Ion-selective electrode (ISE) is a better alternative due to its high selectivity, effective response simplicity and low cost.3,4) Usually, ISE consists of internal solution containing analyte ions with fixed concentration so that a potential can be developed between the two sides of the ion-membrane However, the internal solution is easy to get leaked, cannot stand of pressurized working environment, is too sophisticated to handle and difficult to integrate with continuous analysis system Recently, a solid-contact ion selective electrode (SC-ISE) has been studied, replacing internal solution by a solid conducting polymer layer.4,5) SC-ISEs have advantages of cheap fabrication, possibility of a pressurized working environment and small size, which allows application in any position such as in a Flow Injection Analysis (FIA) system, in chromatographic instruments (used as detector) and in Micro Total Analysis Systems (µ-TAS).4) Several authors have been successful at fabricating of SC-ISE for Hg(II) using polypyrrole, polyaniline as conducting polymer and several sulfur or nitrogen-containing ligands such as 1,3diphenylthiourea, 2-mercaptobenzimidazol as mercury ionophore.4­7) However the fairly narrow working concentration ranges from © 10¹2 to © 10¹7 M makes it difficult to determine trace Hg(II) in environmental samples + Corresponding author, E-mail: m.t.n.pham@gmail.com The present study aimed on the fabrication of a solidcontact ion selective electrode for mercury (II), in which we electrodeposited a conducting polypyrrole layer on the surface of paste carbon electrode, and finally covered by a mercury ion-selective membrane The main components of mercury ion-selective membrane include a mercury ionophore (Mercury Ionophore I), a plasticizer (DOS) and highmolecular weight PVC which creates a frame of the membrane to keep other components from being dissolved into solution Experimental Section 2.1 Chemical and reagents Pyrrole (Reagent grade, 98%), bis(2-etylhexyl) sebacate (DOS) Selectophore· (² 97.0%), 1,10-Dibenzyl-1,10-diaza18-crown-6 (Mercury Ionophore I), polyvinylchloride (PVC), Tetrahydrofuran, anhydrous ²99.9%; Hg(NO3)2·H2O ACS reagent, ²98.0% and HNO3 ACS reagent, 70% were from Fluka Chemie, Switzerland Aqueous solutions were prepared by deionized water Stock mercury (II) solution (1000 ppm) was freshly prepared The stock and sample solutions were kept at 4°C when not in use 2.2 Electrode preparation 2.2.1 Synthesis of polypyrrole Polypyrrole was synthesized electrochemically with constant potential at different voltages from 0.6 to 0.9 V in a solution of pyrrole 0.2 M and KNO3 0.2 M onto the surface of a paste carbon electrode with inner diameter of mm and outer diameter of mm Cyclic Voltammetry (CV) of PPy was carried out using VA 757 Computrace (Metrohm, Switzerland) in the solution of Hg(NO3)2 with different concentrations to dope Hg2+ ions into the PPy membrane and to examine the conductivity of PPy membrane via the shape and peak height of the signal curves 2.2.2 Fabrication of ion-selective membrane solution An ion-selective membrane (ISM) contains three main components: ionophore (Mercury Ionophore I) (6%), plasticizer (bis(2-etylhexyl) sebacate - DOS) (64%) and polymer Fabrication of Solid Contact Ion Selective Electrode for Mercury (II) Using Conductive Polymer Membrane (PVC) (30%) All components are dissolved in 0.5 mL of THF and stirred for at least 10 hours before being used The membrane mixture should be stored at 4°C A 50 µL microsyringe was used to cover ISM solution on the electrode surface which was covered with a thin layer of conductive polymer (PPy) The ISE was conditioned in the solution of mercury (II) ion before being utilized 2.3 Potential measurement The current-free potential measurement was carried out with a Martini mV/pH meter The electrode system consists of SC-ISE as working electrode and Ag/AgCl (KCl M) as reference electrode The response of SC-ISE was evaluated via steady-state measurement at constant ionic strength of 0.1 M KNO3 and pH = Selectivity towards other ions was measured in terms of the selectivity coefficients using separate solution method The environmental samples were filtered to remove suspended matters and adjust to suitable pH (3 ª 4) prior to analysis 1429 (a) (b) Results and Discussion 3.1 Deposition of conducting polypyrrole membrane The electrolysis potential has a significant effect on the formation of PPy thin film The PPy thin film was not formed at voltage of 0.6 V, while at 0.9 V the over-oxidation of PPy thin film easily occurs The optimum potential was found as 0.8 V, with the highest current density observed in CV diagrams The performance of PPy thin film is also affected by the electrolysis time The higher the deposition time, the thicker the film and the higher the current density is However, after 150 s the PPy film becomes saturated and no higher current density was observed This is in agreement with what observed by scanning electron microscopy (SEM) (see Fig 1) Electrolysis of 150 s allows PPy to form a fine membrane on the electrode surface Less time of deposition is not enough to form a continuous layer of PPy membrane while longer time causes cracks on the surface of membrane due to large amount of PPy synthesized CV diagrams of PPy synthesized on paste carbon electrode with deposition time of 150 s in Hg(NO3)2 solution with concentration varying from 0.001 M to 0.1 M are presented in Fig Since NO3¹ in the membrane can bind to Hg2+ in the solution to balance the membrane electrically, it can be an important stage to dope Hg2+ into the PPy membrane and lead to the improvement in the selectivity of SC-ISEs It is clear that different concentrations of Hg(II) significantly affect CV diagrams both in shape and peak height The concentration of Hg(NO3)2 0.1 M was chosen for the synthesis since the CV curve shows a smooth shape with highest peak value of about 500 µA 3.2 Potentiometric measurement of SC-ISE 3.2.1 Potentiometric response of SC-ISE towards Hg2+ ion The SC-ISE needs about 60 seconds to reach a constant value The response time is longer as compared to normal ISE (30 ª 45 s) since Hg2+ ions need more time to diffuse from the solution through the ISM and then reach the PPy membrane where the electron exchange process takes place Fig SEM image of PPy membrane after 150 s of deposition Fig CV diagram of PPy synthesized on CP electrode with deposition time of 150 s in Hg(NO3)2 solution (a) 0.001 M, (b) 0.01 M and (c) 0.1 M Linear potentiometric response was observed over a wide range of Hg2+ concentrations from 10¹9 M to 10ạ2 M with Nerstian slope of 23.7 ô 1.4 mV per decade (see Fig 3), a typical value for a divalent cation like Hg2+ The equation of the working curve is E = (23.6 « 1.4) log [Hg2+] + (195.6 « 14.6) while the LOD of the electrode determined by the meeting point between two lines is 5.9 â 10ạ10 M The obtained LOD are markedly lower than values reported by other studies: (6.1 ô 1.7) â 10ạ7 M,5) for a Hg-ISE electrode using a dithiophosphate-based ionophore and © 10¹6 M for a Hg-ISE electrode using 1,3-diphenylthiourea ionophore.6) This extremely LOD value allows the use of method for determination of Hg2+ in environmental samples 1430 P T N Mai and P T Hoa Table Comparison of results of analysis of mercury in water samples by the CV-AAS and SC-ISE method Sample Fig Potential response of PC-based SCISE with 6% of ionophore 3.2.2 Selectivity towards to other cations The measured selectivity coefficients of various cations including H+, K+, Na+, Cu2+, Ca2+, Fe2+, Mg2+, Pb2+, Cd2+, Mn2+ and Zn2+ are in the range of 0.4 to 10¹3, much lower than 1, indicating that these cations have little interference when present with Hg2+ ion in the solution The interference order is alkali ions > alkaline ions > transition metal ions > H+, which is in agreement with other studies on Mercury (II) ion selective electrode.6,7) Only Ag+ significantly affects the selectivity of SC-ISEs with high value of Kpot ij of 10 This significant influence is possibly due to the good complexation between Ag+ and 1,10-Dibenzyl1,10-diaza-18-crown-6 ionophore.8) However, the electrode can tolerate Ag+ ions at concentrations lower than 10¹6 M, which is a common observed concentration in water samples 3.2.3 Application in analysis of real sample To assess the applicability of the SC-ISE to real samples, we determined mercury in synthetic solutions and three of ballast waters Ballast samples were pre-treated with nitrous acid to avoid precipitation, centrifuged to isolate suspended matters and adjusted to pH ª using drops of concentrated nitric acid and ammonia The standard addition method was applied for the analysis of Hg in ballast water samples Results were compared with those measured by atomic absorption spectrometry (AAS) Results are summarized in Table and show the good agreement between the two methods The amount of Hg found in ballast water is below the danger level given by US-EPA Concentration of Hg2+ (ppb) SC-ISE AAS Synthetic 5.4 « 0.6 6.0 « 0.4 Synthetic 39.2 « 2.1 38.2 « 1.1 Ballast water 5.4 « 0.4 5.6 « 0.4 Ballast water 12.3 « 0.3 11.7 « 0.2 Ballast water 8.1 « 0.8 7.9 « 0.3 Conclusions This work demonstrates that a conducting polypyrrole layer can be used to replace the internal solution in the development of solid contact selective ion electrode for Hg2+ The electrode responds to Hg2+ ion in a Nerstian fashion and presents a good selectivity and low detection limit down to â 10ạ10 M The electrode is characterized by a relatively fast response, high selectivity and was successfully applied to the determination of Hg2+ in ballast water samples Our results suggest that in the future solid-contact ion selective electrode can become a vital part of environmental analysis with a wide range of application, particularly for field analysis Acknowledgements This study was supported by Project QG.13.08 funded by Vietnam National University, Hanoi (VNU-HUS) REFERENCES 1) L Friberg: Inorganic Mercury, Environmental Health Criteria 118, (World Health Organization, ISBN 92-4-157118-7, Geneva, 1991) 2) K Leopolda, M Foulkesb and P Worsfoldb: Anal Chim Acta 663 (2010) 127­138 3) M Mazloum, K M Amini and I Mohammadpoor-Baltork: Sens Actuators B 63 (2000) 80­85 4) X Yu, Z Zhou, Y Wang, Y Liu, Q Xie and D Xiao: Sens Actuators 123 (2007) 352­358 5) J J Gomez, F P Garcia, M T R Silva, A R Hernandez, C A G Vidal and M E P Hernandez: Talanta 114 (2013) 235­242 6) L P Marin, E O Sanchez, G M Miranda, A Perez, J A Chamaro and H L Valdivia: Analyst 125 (2000) 1787­1790 7) W S Han, K C Wi, W S Park and T K Hong: Russi J Electrochem 48 (2012) 525­531 8) G H Rounaghi, M Mohajeri, S Ashrafi, H Ghasemi, S Sedaghat and M Tayakoli: J Inclusion Phenomena Macrocyc Chem 58 (2007) 1­6  .. .Fabrication of Solid Contact Ion Selective Electrode for Mercury (II) Using Conductive Polymer Membrane (PVC) (30%) All components are dissolved in 0.5 mL of THF and stirred for at least... electrode using a dithiophosphate-based ionophore and â 10ạ6 M for a Hg-ISE electrode using 1,3-diphenylthiourea ionophore.6) This extremely LOD value allows the use of method for determination... Conclusions This work demonstrates that a conducting polypyrrole layer can be used to replace the internal solution in the development of solid contact selective ion electrode for Hg2+ The electrode