Evaluation of a Microfluidic Device for the Electrochemical Determination of Halide Content in Ionic Liquids

Abstract

A microfluidic device designed for electrochemical studies on a microliter scale has been utilized for the examination of impurity levels in ionic liquids (ILs). Halide impurities are common following IL synthesis, and this study demonstrates the ability to quantify low concentrations of halide in a range of ILs to levels of ∼5 ppm, even in ILs not currently measurable using other methods such as ion chromatography. To validate the mixer device, the electrochemistry of ferrocene was also examined and compared with spectroscopic and bulk electrochemistry measurements. An automated "sample preparation, delivery, and calibration" method was developed, and the chip successfully used for linear sweep, cyclic voltammetry (under both quiescent and steady-state flowing conditions), square wave voltammetry, and differential pulse voltammetry. An effective method of electrochemically cleaning the electrodes is also presented.In the past decade, the use of ionic liquids (ILs) as alternative solvents and media for a wide range of applications has expanded significantly in both academia, as well as in industry. [1][2][3][4][5] The importance of impurities in ILs has been recognized by many researchers in this area with water, halide, and metal impurities from the synthesis of the ILs being the most prevalent. These affect both physical and chemical properties of the media, for example, drastically increasing viscosity 6 and thermal conductivity, 7 while also poisoning catalytic reactions. 8 Additionally, Jacquemin et al. have quantified the effect of halide and water content on the density of a wide range of ILs as a function of pressure and temperature, highlighting the importance of being able to measure their concentration accurately. 9,10 It should be noted that typical commercial samples contain between 10 and 1000 ppm of halide from their synthesis. 11 The purity strongly determines their cost because of the loss of IL during multistage purification.With respect to halide content, although commonly used, the technique of using the "silver nitrate" is only qualitative and is insufficient to examine ILs for many applications. 12 However, a number of other methods have been demonstrated for the quantification of halide in ILs, although it is important to note that many have only been investigated for halide in water-miscible ILs with few studies aimed at more hydrophobic ILs, such as those based on tetraalkylphosphonium cations or trispentafluoroethyltrifluorophosphate ([FAP] -) anions. These techniques include Volhard (silver nitrate) and chloride-selective electrodes, 6,13 spectrophotometry using fluorescent indicators, 14 ion chromatography (IC), [15][16][17][18][19] 23 In general, the electrochemistry techniques employed to date require a significant volume of IL (at least ∼1000 µL) for each sample, and multiple samples are required to produce calibration curves. In addition, each sample takes a significant amount of time to measure, including electrode preparation, multiple sample preparation, cleaning and reass...

The Kinetics of Ferrocene Volatilisation from an Ionic Liquid

“…Surface fouling of electrodes can occur in some circumstances. [5] To demonstrate that surface fouling was not occurring in the ionic liquid during repeated scanning over extended periods of time, the above experiment was repeated using a nonvolatile redox probe. The variation in ln(D) with 1/T is directly proportional to the activation energy of diffusion, E a , as expressed by Equation (4):…”

“…Ferrocene (Fc) has widespread applications, for example as an IUPAC recommended redox reference couple for non-aqueous electrochemical investigations, [1] as an electrochemical mediator, [2] reagent in organic synthesis [3] and probe (for viscosity changes, [4] mixing ability, [5] influence of electrode surface modification, [5] etc.). Fc sublimes relatively quickly at atmospheric temperature and pressure, [6,7] as well as in vacuo.…”

“…[13] As ILs are still relatively new materials, a number of fundamental parameters and techniques have not yet been developed for ionic liquid systems, such as well-defined reference electrodes. Therefore, there is currently a heavy reliance on in situ reference couples, such as cobaltocenium j cobaltocene, [1] TMPD j TMPD + · and Ag( 0 ) j Ag I , [14] as well as a significant number of iron-based compounds, [1,15,16] although the use of ferrocene j ferrocenium [1,4,5,7,8,[15][16][17][18][19][20] appears to be especially prevalent. It has previously been observed that placing ILs in vacuo or under a flow of inert gas can result in the volatilisation of dissolved Fc, [4,7,8] and this fact has been partially attributed [8] to discrepancies in the literature regarding diffusion coefficient values, D, and the erroneous assumption that D is strongly related to Fc concentration.…”

Integration and detection of biochemical assays in digital microfluidic LOC devices

“…Due to the ease of electrode fabrication, on-line electrochemical detection has been widely used for continuous-flow microfluidic systems. [115][116][117][118] However, there are not many reports on its integration with EWOD devices. 119 In a study by Dubois et al, electrochemical detection was used as an orthogonal on-line technique to the off-line HPLC-UV absorbance (see Section 3.2.1) for the detection of final products of solution-phase synthesis.…”

Small volume laboratory on a chip measurements incorporating the quartz crystal microbalance to measure the viscosity-density product of room temperature ionic liquids

“…Some techniques for characterizing the RTILs have already been developed on chip including solution phase synthesis 7 and halide content. 8 It has previously been shown that the quartz crystal microbalance ͑QCM͒ can be used to measure properties of RTILs, 9,10 which include the square root of the viscosity-density product ͑ ͱ ͒ using a small volume method requiring only 40 l. The viscosity of small volumes is often measured using cantilevers with a volume of only 1 ml. 11 The frequency response of a 5 MHz quartz crystal with a single sided liquid contact was characterized by the Kanazawa and Gordon equation ͑1͒, 12 which relates the change in frequency to the square root of the viscosity-density product,…”