Contact Galvani potential differences at liquidjliquid interfaces Part II. Contact diffusion potentials in microsystems Jacques Josserand a , Gre ´goire Lagger a , Henrik Jensen a , Rosaria Ferrigno b , Hubert H. Girault a, * a Laboratoire d’Electrochimie Physique et Analytique, Institut de Chimie Mole ´culaire et Biologique, Ecole Polytechnique Fe ´de ´rale de Lausanne, CH-1015 Lausanne, Switzerland b Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, MA 02138, USA Received 2 December 2002; received in revised form 28 January 2003; accepted 8 February 2003 Abstract The purpose of this work is to study diffusion potentials (i.e., liquid junction potentials) established between two static or flowing solutions in microsystems. One of the motivations of investigating the diffusion potential distribution is to be able to establish a potential gradient in a cell without introducing electrodes and using a potentiostat. By using finite element simulations, different geometries (i.e., microhole, Y-channel mixing and microtube injection) have been studied numerically. The calculations have allowed systematic studies of the influence of concentration ratio, flow rate and detector position. It is shown that the diffusion potential can be a useful way to quantify the degree of mixing or filling of solutions in microsystems. The theoretical part has been corroborated by experimental measurements of potential differences across a parallel flow channel, taking into account the diffusion potential and differences in the electrode potentials. It is apparent that the theoretical model gives a good fit to the experimental results. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Diffusion potential; Liquid junction potential; Micromixer; m-TAS; Simulation; Finite element method 1. Introduction When two liquids having different ionic composition are placed in contact, a diffusion potential (also called a liquid junction potential) is established as the ions partition between the two liquids to attain equilibrium. Basically, the diffusion potential is a result of the conservation of electroneutrality in the two phases, which leads to the establishment of a potential gradient that acts to speed up the transport of ions having a low mobility and slow down the transport of ions having opposite sign and a higher mobility. This phenomenon is well known and has been studied experimentally and theoretically for more than a century [1  /8]. Recently, new theoretical and experimental advances in the specific case of a single salt partitioning between two immiscible solutions have been reported [9]. When dealing with miscible solutions, the diffusion potential has been measured between two solutions flowing in parallel and in contact under laminar conditions [10], and for the injection of one solution into another [9]. Since the diffusion potential in a parallel constant flow is stable in time [10], it can be utilised to establish a potential difference and replace the use of a semi- permeable membrane often used in static systems [10]. Usually, efforts are made to minimize diffusion potentials, as they are often a source of error in electrochemical experiments. However, some studies have been reported that takes advantage of diffusion potentials as an easy and efficient way to induce potential differences in microsystems [11]. In traditional macrosystems, the mixing of two liquids is usually a fast process involving turbulent mixing whereas in micro- systems, such as for instance the so-called micro Total Analysis Systems (m-TAS), the flows of fluids are normally laminar. Consequently, efficient mixing of * Corresponding author. Tel.:  /41-21-693-31-45/51; fax:  /41-21- 693-36-67. E-mail address: hubert.girault@epfl.ch (H.H. Girault). Journal of Electroanalytical Chemistry 546 (2003) 1  /13 www.elsevier.com/locate/jelechem 0022-0728/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-0728(03)00160-8