Electrochimica Acta 56 (2011) 9910–9915 Contents lists available at SciVerse ScienceDirect Electrochimica Acta j ourna l ho me pag e: www.elsevier.com/locate/electacta Effect of oxygenation time on signal of a sensor based on ionic liquids Jacek G˛ ebicki a,∗ , Adam Kloskowski b , Wojciech Chrzanowski b a Department of Chemical and Process Engineering, Chemical Faculty, Gda´ nsk University of Technology, 11/12 Narutowicza Str., 80-233 Gda´ nsk, Poland b Department of Physical Chemistry, Chemical Faculty, Gda´ nsk University of Technology, 11/12 Narutowicza Str., 80-233 Gda´ nsk, Poland a r t i c l e i n f o Article history: Received 8 June 2011 Received in revised form 27 July 2011 Accepted 19 August 2011 Available online 8 September 2011 Keywords: Sensor Ionic liquids Reynolds number Overall cathodic coefficients of transition a b s t r a c t The paper presents an oxygen sensor based on ionic liquids and solid electrodes. The following ionic liquids have been employed: [BMIM][BF 4 ], [HMIM][Cl], [BMIM][N(CN 2 )]. Minimum time of the sensor exposure to analyte, after which the signal (current intensity) was stable, has been evaluated. An impact of volumetric flow rate of analyte on the sensor exposure time and signal has been determined. A prod- uct of permeability coefficient and solubility of oxygen in ionic liquids has been estimated. A mechanism of oxygen reduction on a surface of the solid electrodes, in the ionic liquid environment has been pre- sented. Overall cathodic coefficients of transition for the sensors with particular ionic liquids have been determined as a function of potential scan rate. © 2011 Elsevier Ltd. All rights reserved. 1. Introduction A substantial advantage of ionic liquids is the possibility of changing their structure. It is possible to influence on physico- chemical properties of ionic liquids by selecting a cation or an anion. That is why ionic liquids are often termed designable sol- vents. A big number of possible cation–anion combinations results in the fact that these compounds have found application in: organic synthesis, production of liquid membranes, liquid chromatogra- phy, capillary electrophoresis, biocatalytic reactions, liquid–liquid extraction, separation techniques, bactericidal and fungicidal sub- stances and first of all they are recognized as green solvents [1–6]. Ionic liquids are characterized by low vapour pressure, wide electrochemical window, high thermal stability, relatively high electrolytic conductivity and hence they constitute an attractive alternative to the solvents traditionally applied in electrochemistry. Moreover, they are promising in construction of stable and durable (resistant to exploitation conditions) gas sensors [7,8]. Ionic liq- uids have already been applied in the electrochemical sensors for measurement of CO 2 , O 2 , NH 3 , NO 2 , Cl 2 , SO 2 [9–12]. Application of ionic liquids in gas sensors excludes the membrane separating gas environment from internal electrolyte due to its low volatil- ity. Additionally, an outstanding thermal durability of ionic liquids in temperatures up to ca. 180 ◦ C would make it possible to use the sensor in conditions where conventional electrolytes cannot be employed. In certain cases lack of membrane is not advisable ∗ Corresponding author. E-mail address: jacek.gebicki@pg.gda.pl (J. G˛ ebicki). because of an interaction between water vapour and ionic liquid. Accordingly, the sensor-ionic liquid system must be cleaned with inert gas or vacuum evacuated [8] prior to measurements. In case of the sensors without a membrane their signal will be limited by the rate of analyte diffusion through the internal elec- trolyte (reversible reaction) or by the rate of electrode reaction (irreversible reaction). Comparable rates of diffusion and electrode reaction determine quasi-reversible reaction. Moreover, in case of the sensors where a measured analyte flows around the sensor’s surrounding one should take also an impact of the flow rate on sensor signal into account [13–16]. This effect is especially dis- tinct for low flow rates and for the flow direction parallel to the sensor orientation due to relatively significant thickness of limit- ing Prandtl layer [13,15–17]. During such type of liquid flow in a vicinity of wall there is a limiting layer ı established (Fig. 1), the velocity inside which, measured in a direction perpendicular to the wall, increases from zero (in direct vicinity of the wall) to a constant value characteristic for undisturbed flow outside the limiting layer. The flow in a vicinity of an interphase region is defined by: linear velocity u of liquid, kinematic viscosity  and length of the wall, along which the liquid flows x. These quantities are interrelated via a dimensionless parameter Re (Reynolds number): Re = ux  (1) Thickness of limiting (Prandtl) layer is given by the formula: ı = x √ Re (2) Viscosity of ionic liquids is usually 1–2 orders of magnitude higher than the one of the traditional organic solvents or basic 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.08.059