Sensors and Actuators B 67 Ž2000. 96–100 www.elsevier.nlrlocatersensorb
A l-MnO 2-based graphite–epoxy electrode as lithium ion sensor Marcos Fernando de S. Teixeira a , Orlando Fatibello-Filho a , Luiz C. Ferracin b, Romeu C. Rocha-Filho b, Nerilso Bocchi b,) a b
Grupo de Quımica Analıtica, Departamento de Quımica, UniÕersidade Federal de Sao ´ ´ ´ ˜ Carlos, Caixa Postal 676, 13 560-970 Sao ˜ Carlos, SP, Brazil Laboratorio Departamento de Quımica, Cen. de Ciencias Exatas e de Tecnol., UniÕersidade Federal de Sao ´ de Pesquisas em Eletroquımica, ´ ´ ˆ ˜ Carlos, km. 235, Via Washington Luiz, Caixa Postal 676, 13 560-970 Sao ˜ Carlos, SP, Brazil Received 13 July 1999; received in revised form 6 December 1999; accepted 9 February 2000
Abstract The potentiometric response of a l-MnO 2-based graphite–epoxy electrode for the determination of lithium ions was examined. The electrode response to lithium ions was linear in the concentration range 10y6 –3.3 = 10y2 molrl with a slope of 58.2 mVrdecade Žat 258C. over a wide pH range Ž4–10.; no such linear response was found for the ions of other alkali metals or alkaline-earth metals. The logarithm of the selectivity coefficients for Liq against Naq, Kq, Csq, Mg 2q, Ca2q, Sr 2q and Ba2q was found to be y1.5, y2.0, y2.2, y2.0, y2.0, y2.1 and y2.1, respectively. The response time of the proposed electrode was lower than 30 s and its lifetime greater than 6 months. q 2000 Elsevier Science S.A. All rights reserved. Keywords: l-MnO 2 ; Graphite–epoxy electrode; Lithium ion sensor
1. Introduction The development of ion-selective electrodes for monitoring lithium ions in blood serum is very important in the field of clinical chemistry, as lithium salts are prescribed for the treatment of manic-depressive patients. Moreover, these electrodes are also promising for many other applications such as anti-inflammatory, antiviral, antifungal, antitumor etc. w1x. Membrane electrodes employing several neutral carriers with sufficient ability to complex lithium ions have been proposed as sensors for lithium ions w2–13x. Various solid-state electrodes based on ceramics w14,15x and different oxides w16–18x have also been investigated as lithium ion sensors. A few years ago, Kanoh et al. w16x proposed the use of l-MnO 2 , a spinel-type oxide, in a new lithium ion-selective electrode. Their Ptrl-MnO2 electrode, prepared by thermal decomposition, showed a near-nernstian response to lithium ion Žand not to other alkali metal and alkaline-earth metal ions. in the concentration range 3 = 10y6 –0.5 molrl. Kanoh et al.w16x attributed the electrode
) Corresponding author. Tel.: q55-16-260-8208; fax: q55-16-2608350. E-mail address:
[email protected] ŽN. Bocchi..
high selectivity for Liq to the presence in the l-MnO 2 spinel structure of vacant tetrahedral sites Žproton-free., which generally are protonated in other manganese oxides. The following redox mechanism for the Liq topotactic-extractionrinsertion reaction in the l-MnO 2 structure was previously proposed by Ooi et al. w19x: 2 l y MnO 2 Ž s . q j Liq Ž aq . q Ž jr2 . H 2 O Ž l .
™ Li Mn x
3q 4q x Mn 2yx O4
q Ž jr4 . O 2 Ž g .
Ž s . q j Hq Ž aq . Ž 0 j 1.
Ž 1.
More recently, LiMn 2 O4 thin films deposited on Pt by the laser ablation method were also investigated as lithium ion-selective electrodes w18x. Through cyclic voltammograms, it was shown that these films exhibit high reversible intercalation properties with very good cycling. Potential measurements of the PtrLiMn 2 O4 thin-film electrode as a function of Liq concentration showed that the electrode had a near-nernstian response to lithium ions Ž52 mVrdecade. in the concentration range 10y5 –3 = 10y2 molrl. Although these Ptrl-MnO2 and PtrLiMn 2 O4 thin-film electrodesw16,18x exhibited excellent performance as lithium ion sensors, their preparation methods are relatively complex and expensive. Moreover, the useful life-
0925-4005r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 Ž 0 0 . 0 0 3 8 9 - 0
M.F.d.S. Teixeira et al.r Sensors and Actuators B 67 (2000) 96–100
97
time of the electrodes was not mentioned by their respective proponents. In the present work, an alternative l-MnO 2-based graphite–epoxy electrode for lithium ion determination was investigated. The electrode was made from a mixture of : -MnO 2 and graphite powder incorporated in an epoxy resin. The effect of composition Ž: MnO 2, graphite and epoxy resin contents. on the potentiometric response, the pH dependence of the equilibrium potential, the selectivity for Liq against other alkali metal and alkaline-earth metal ions, and the response time of this : -MnO 2-based graphite–epoxy electrode were investigated.
2. Experimental Firstly, the spinel LiMn 2 O4 was prepared by heating a mixture of electrolytic MnO 2 and LiOH in the molar ratio 2Mn:1.05Li; the preparation of the electrolytic MnO 2 was carried out as described elsewhere w20x. This mixture was extensively ground and then calcined in static air at 7508C for 24 h. After, the product was quenched at room temperature in a desiccator and finally reground. For conversion to l-MnO 2 , a sample of LiMn 2 O4 was treated in an aqueous diluted sulphuric acid solution stirring for 45 min. When the pH of this mixture became stable, the solution was decanted and the remaining solid material washed by decantation with deionized water, filtered and dried in air at 908C w21x. The l-MnO 2 , graphite–epoxy composites were prepared by mixing l-MnO 2 , graphite powder ŽUltra Carbon, 10–20 mm particle size. and epoxy resin Ža mixture of resin P342 and catalyst from Reforplas, ´ Brazil, in a 5:1 mass ratio. in the mass proportions shown in Table 1. The electrodes were made by putting these different composites inside glass tubes Žexternal diameter 10 mm, internal diameter 8 mm and length 13 cm. so as to be 10 mm high, and then drying for 24 h. The other end of the electrodes was connected to a coaxial cable. All solutions were prepared using distilled deionized water. Chemicals of analytical-reagent grade were used without further purification. A lithium chloride Ž0.10 molrl.rborate buffer Ž5.0 = 10y3 molrl Na 2 B 4O 7 .10 H 2 O q 5.0 = 10y3 molrl H 3 BO 3 . stock solution was first prepared in water. After, the reference solutions of Liq in a borate buffer were obtained by suitable dilutions of the stock solution using a buffer solution of equal concentration to complete the final volume; when the pH was varied, HCl or NaOH solutions were used for the pH adjustment. Potential measurements were carried out at Table 1 Compositions of the different l-MnO 2 graphite–epoxy electrodes tested Žmass %. l-MnO 2 Graphite powder Epoxy resin
0 50 50
20 30 50
25 25 50
30 20 50
35 15 50
40 10 50
Fig. 1. Lithium ion activity dependence of the equilibrium potentials Žat 258C. of l-MnO 2-based graphite–epoxy electrodes of different compositions; Liq in a buffer solution Ž5.0=10y3 molrl Na 2 B 4 O 7 .10H 2 Oq5.0 =10y3 molrl H 3 BO 3 ; pH ;8..
25.0 " 0.28C in a thermostated glass cell using an AgrAgCl double-junction reference electrode ŽAnalion, Brazil, model R684. and a pHrion meter ŽOrion, USA, model EA940. with "0.1 mV precision. Thus, the electrochemical cell used for these potential measurements was: Ag
