Sensors and Actuators B 188 (2013) 433–439 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal h om epage: www.elsevier.com/locat e/snb A chronopotentiometric approach for measuring chloride ion concentration Yawar Abbas ∗ , Wouter Olthuis, Albert van den Berg BIOS-Lab on a Chip Group, MESA+ Institute of Nanotechnology, University of Twente, 7500 AE Enschede, The Netherlands a r t i c l e i n f o Article history: Received 28 March 2013 Received in revised form 27 May 2013 Accepted 14 July 2013 Available online xxx Keywords: Chloride ion detection Chronopotentiometry Stimulus/response approach Ag/AgCl pseudo-reference electrode Concrete structures a b s t r a c t In this paper, a novel approach is reported for the electrochemical measurement of chloride ions in aqueous solution. This sensor is based on the stimulus/response principle of chronopotentiometry. A current pulse is applied at the Ag/AgCl working electrode and the potential change is measured with respect to another identical Ag/AgCl electrode in the bulk electrolyte. The potential difference is related to the Cl - ion concentration via the Nernst equation and follows an inverse logarithmic trend. By varying the applied current pulse, the sensitivity of the sensor is tunable to different concentration ranges. The potential response is also influenced by the pH of the electrolyte, this effect is pronounced at lower concentration of Cl - ions (<1 mM KCl) and at high pH values (>12 pH). The advantage of this approach is the use of a bare Ag/AgCl electrode as a pseudo-reference electrode, which enables this system for long term application such as the in situ measurement of Cl - ions in concrete. © 2013 Elsevier B.V. All rights reserved. 1. Introduction The major cause of deterioration in reinforcement concrete is due to the presence of chloride ions in the concrete itself [1,2]. When the amount of chloride ions increases beyond a certain threshold, pitting corrosion of the steel reinforcements initiates and ultimately results in localized structural failure [3]. This is particu- larly a problem in structures near salt water or which are exposed to de-icing salt. In 2011, the U.S.A. Federal Highway Administration stated that 11% of the national bridges are structurally defective [4] mainly due to degradation from chloride ions. Delayed mainte- nance could result in the collapse of these structures and human casualties, but unnecessary upkeep increases both costs and CO 2 emissions. To precisely predict the optimal time for maintenance of these structures, a service-life model of the concrete is required [5] and the chloride ion concentration is an essential parameter. The well-established method for chloride ion detection in concrete requires destructive sampling of structures and time- intensive laboratory processing [6,7]. In contrast, a non-destructive in situ monitoring provides fast measurements, data reliability and Abbreviations: WE, working electrode; RE, reference electrode; CE, counter elec- trode. ∗ Corresponding author at: University of Twente, MESA+Institute of Nanotech- nology, BIOS chair, Carré 2247, P.O. Box 217, 7500 AE Enschede, The Netherlands. Tel.: +31 053 489 2604. E-mail addresses: y.abbas@utwente.nl (Y. Abbas), w.olthuis@utwente.nl (W. Olthuis), a.vandenberg@utwente.nl (A. van den Berg). real time ingress profile without destroying the structures [8]. In recent years many groups have investigated in situ measurement of chloride in concrete using mainly electrochemical [9–11] and opti- cal methods [12–14]. However, optical methods have bulky setups, are difficult to integrate as a standalone system and have other disadvantages such as the photo bleaching of dyes (optical trans- ducers) and the leaching of transducer (polymer matrix) due to the high pH environment inside concrete. While electrochemical sensing is the preferred and most researched method, many devices are only suitable for in situ measurements in a laboratory setting and no solution for long- term measurements in concrete structures currently exists (with a required sensing period of 20 years or more). Most of the reported electrochemical sensors use potentiometry [8,11,15–17] where the half-cell potential of a silver/silver chloride (Ag/AgCl) electrode is measured with respect to a reference electrode at equilibrium. This method requires the reference electrode to be embedded inside concrete (near the Ag/AgCl working electrode) to reduce errors due to diffusion potential [18]. The limiting factor of this approach (for in situ measurement in concrete) is the long term stability of the ref- erence electrode and the drift in potential [19]. Therefore, to design an electrochemical sensor capable of measuring a real-time Cl - ion concentration in concrete over the span of decades, a system that is not dependent on a stable reference electrode is desired. Potentiometry is a static (zero current) approach and the alternative to this is a dynamic measurement approach (e.g. chronopotentiometry) which involves measuring the response of an equilibrated system to an applied stimulus [20,21]. In this tech- nique, a controlled, chemical stimulus defines the condition at the 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.07.046