An electrochemical characterization of thick-film electrodes based on RuO 2 -containing resistive pastes Roberto H. Labrador a , Juan Soto a, * , Ramo ´n Martı ´nez-Ma ´n ˜ez a , Carmen Coll a , Angel Benito a , Javier Iba ´n ˜ez b , Eduardo Garcı ´a-Breijo b , Luis Gil b a Instituto de Quı ´mica Molecular Aplicada, Departamento de Quı ´mica, Universidad Polite ´cnica de Valencia, Camino de Vera s/n, 46022 Valencia, Spain b Instituto de Quı ´mica Molecular Aplicada, Departamento de Ingenierı ´a Electro ´ nica, Universidad Polite ´cnica de Valencia, Camino de Vera s/n, 46022 Valencia, Spain Received 5 April 2007; received in revised form 13 August 2007; accepted 23 August 2007 Available online 4 September 2007 Abstract In the composition of commercial RuO 2 -containing resistive pastes there is also a significant amount of other electro-active metal oxides. Thus, when the pastes are used in the manufacture of thick-film electrodes, the electrochemical response is a complex contribution of all the electro-active metal oxides included in the paste composition. This is an attractive complex system whose electrochemical response has been studied and explained using a model based on surface activity changes of the solids involved in the redox processes. The developed model is able to describe the redox potential of the electrode as a function of the pH and explain the presence of certain anionic interferences. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Ruthenium (IV) oxide; Thick-film electrode; Electrode response; Modeling; Surface activity 1. Introduction The development of accurate, low cost and miniaturized electrodes is currently of great interest. Amongst the avail- able electrode fabrication technologies at present, one of the most suitable for use in industrial production is micro- electronic thick-film technology. A number of thick-film- based sensors have been reported, being pH sensors one of the most widely studied. Many pH sensors have been developed which utilize this technology, for example, glass electrodes [1–4], membrane electrodes [5,6] and electrodes based on the use of metal oxides [7–14]. A review which details the use of metal oxides in the determination of pH was presented by Fog and Buck in 1984 [12]. The study was focused on the PtO 2 , IrO 2 , RuO 2 , OsO 2 , Ta 2 O 5 and TiO 2 oxides, and concluded that RuO 2 was the most suit- able metal oxide for pH electrode fabrication. The authors also postulate five possible theories to explain the mecha- nisms of performance of these sensors, suggesting as the most probable the theory of oxygen intercalation. Since then, several studies have been reported on the use of ruthenium (IV) oxide as pH sensors in thick-film technol- ogy [9,13–15]. In these studies, RuO 2 was used alone or combined with borosilicates or polymers and, in all cases, a Nernstian or near-Nernstian response was observed. However, some of these papers have found significant response of the RuO 2 thick-film pH sensor toward others species. McMurray and Douglas [13] designed a thick-film sensor with a hand-made paste composed by RuO 2 · H 2 O mixed with lead borosilicate glass. This sensor suffered a maximum deviation of 20 mV between 2 and 12 unities of pH in front of anion chloride. In this way, Mihell and Atkinson [9] have reported that a planar thick-film pH sen- sor of ruthenium oxide stored into solutions of hydrochlo- ric acid showed a decline in a slope factor. On the other 0022-0728/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2007.08.019 * Corresponding author. Tel.: +34 963877343; fax: +34 963877349. E-mail address: jsotoca@qim.upv.es (J. Soto). www.elsevier.com/locate/jelechem Available online at www.sciencedirect.com Journal of Electroanalytical Chemistry 611 (2007) 175–180 Journal of Electroanalytical Chemistry