Ž . Sensors and Actuators B 56 1999 169–174 A novel l-MnO -based graphite–epoxy electrode for potentiometric 2 determination of acids and bases Marcos Fernando de S. Teixeira a , Fernando C. Moraes a , Orlando Fatibello-Filho a , Luiz C. Ferracin b , Romeu C. Rocha-Filho b , Nerilso Bocchi b, ) a Grupo de Quımica Analıtica, Departamento de Quımica, Centro de Ciencias Exatas e de Tecnologia, UniÕersidade Federal de Sao Carlos, Caixa Postal ´ ´ ´ ˆ ˜ 676, CEP 13.560-970, Sao Carlos, SP, Brazil ˜ b Laboratorio de Pesquisas em Eletroquımica, Departamento de Quımica, Centro de Ciencias Exatas e de Tecnologia, UniÕersidade Federal de Sao ´ ´ ´ ˆ ˜ Carlos, Caixa Postal 676, CEP 13.560-970, Sao Carlos, SP, Brazil ˜ Accepted 23 March 1999 Abstract The potentiometric response of a novel l-MnO -based graphite–epoxy electrode for the determination of acids and bases was 2 Ž . examined. The electrode was prepared by the incorporation of manganese IV oxide in a graphite–epoxy matrix. The data obtained in various acid–base titrations were compared with those using a glass electrode in the same conditions. The proposed electrode presented a Ž . linear response in the pH range 2 to 13 with a slope of y53.6 "0.5 mVrpH at 258C and a response time of ca. 10 s. The lifetime of Ž . this electrode was higher than 3 months ca. 1500 determinations and it showed an excellent performance for pH determination and end-point indication potentiometric titrations in the determination of acids and bases. The miniaturization of the proposed electrode is also possible. q 1999 Elsevier Science S.A. All rights reserved. Keywords: l-MnO ; Graphite–epoxy; Potentiometric titrations; Acid–base determination 2 1. Introduction Measurements of pH in routine laboratories, industrial processes, and alcoholic fermentation, as well as in medi- cal, agronomical and environmental sciences, are very relevant as they allow quality monitoring and process control. The glass electrode is usually the potentiometric sensor of choice in the pH control of solutions in a wide pH range, but it presents great deviations of the Nernstian behavior for very high and low pH values. Although the glass electrode demonstrates selectivity and sensitivity, it has some limitations such as high input resistance in aqueous and organic solutions, fragility, difficulties of wx miniaturization and contamination of the membrane 1. Various other types of electrodes have been shown to be useful in the construction of potentiometric sensors for pH w x measurements 2–15 . Such electrodes include metal–metal w x w x oxide 8–12 and substrate–metal oxide 13–15 . These electrodes are based on reversible redox systems, such as ) Corresponding author. Tel.: q55-16-260-8208; Fax: q55-16-260- 8350; E-mail: bocchi@power.ufscar.br CrO rCr 3q and PbO rPb 2q , whose redox equilibrium is 3 2 pH-dependent. Ž . Manganese IV oxide, MnO , presents various proper- 2 ties of technological interest. It is used as a powerful oxidizing agent, as a catalyst and as an electrode material, mainly in primary batteries. MnO is inexpensive, non- 2 toxic, abundant and exists in a wide variety of allotropic forms, natural or synthetic: ramsdellite, pyrolusite or b- MnO , nsutite or g-MnO , cryptomelane or a-MnO , 2 2 2 w x birnessite or d-MnO , ´-MnO and l-MnO 16 . Among 2 2 2 these, only the l form has a structure related to spinel, as it is obtained by an acid treatment of the spinel-type material LiMn O without changes in the original structure 2 4 Ž q 3q in LiMn O , Li is at the tetrahedral sites and Mn and 2 4 Mn 4q at the octahedral sites of a cubic closed-packed . oxygen framework ; since most of the lithium ions are removed from the tetrahedral sites, the resulting material is w x nearly pure MnO 17 . Thus, l-MnO can accommodate 2 2 H q andror Li q in its structure, which is of interest for solid-solution electrodes in electrochemical cells. Mecha- nisms for proton and lithium insertion in l-MnO have 2 w x been proposed 18,19 . 0925-4005r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. Ž . PII: S0925-4005 99 00173-2