AglAg+ reference electrodes

Table 7.3 Selected AglAg reference electrodes used in binary, ternary or higher IL media...

Experimentally, the AglAg reference electrode is constructed by dissolution of the silver salt into an IL. This IL solution is then placed into either a fritted tube or into a Luggin capillary tube. The electrical contact is then made via a silver wire immersed into the Ag /IL solution. In the case of the fritted tube, this should ideally be separated from the electrochemical cell by use of a second fritted tube or compartment depending on cell design, containing only neat IL which is the same for the electroactive species. This is done in order to minimise effects of any potential leakage of silver ions from the reference electrode. 

The electrochemical reaction for the AglAg" reference electrode for molten salt experiments is as follows [70] ... 

Even when this electrode was proven to be useful in different ILs, in situations where a large number of experiments are to be performed in the same IL, it is recommended to prepare the AglAg reference electrode using the IL under study as the solvent in the reference system and evaluate its potential against a standard redox compound. 

Saheb et al. described an AglAg+ reference electrode (for use in an imidazohum-based ionic hquid) in which the concentration of silver ion was estab-Ushed by addition of silver(I) nitrate in acetonitrile [27]. A problem noted with the use of this system is that the potential of the reference half cell varies significantly with the volume fraction of acetonitrile, which is likely to vary after extended... 

Coefficients of the E = E0 + kpm calibration dependences obtained against silver reference electrode (AglAg+, 0.2 mol kg-1) in the molten KCl-NaCl equimolar mixture at 700 °C... 

The temperature dependence of the potential of AglAg(I) reference electrode was found to be in the range from 0.54 to 0.57 mV Using this AglAg(I) reference... 

By far the most commonly reported reference electrode system for room temperature or low temperature binary, ternary or higher reference electrodes has been based on the AglAg" couple. Some selected examples of reference electrodes using the AglAg couple in selected ILs are presented in Table 7.3. All of these reported reference electrodes are based on use of a silver wire immersed into an IL containing a dissolved silver salt. 

As stated in Sect. 7.2.1 of this chapter, for electrodes of the first kind, the metal ions should not react with the ILs. Basile et al. have noted that the presence of water results in the formation of silver nanoparticles for solutions of Ag" ions dissolved into [Cqmpyr] [TFSI] [31]. These authors postulate that this occurs via chelation of Ag" by the [TFSI] anion and a subsequent disproportionation reaction in the presence of water to form a Ag -[TFSI] complex and Ag° [31]. Thus, the choice of a reference electrode based on AglAgOTfl[C4mpyr][TFSI] may be problematic. The authors did note that over a short period of time, when chemical reaction rates were low, the FcIFc" reversible potential was close to that reported by Snook et al. [27, 31]. According to Snook et al. [27] the AglAgOTfl[C4mpyr][TFSI] reference electrode is only stable for 3 weeks before the solution needs to be replaced and the electrode remade. Therefore, if the AglAg" couple is to be used in other ILs, then any potential chemical reactions of Ag irais within this IL should be explored prior to use, as well as the stability of the reference electrode over time. 

As can be seen from Table 7.5, similar to the case of the AglAg" couple, when the H2lH is referenced to an internal redox couple a shift in the occurs and is dependent on the IL studied. Additionally, Compton et al. have reported that the electrochemical reversibility of the H2lH couple is dependent on the identity of the IL [33]. This is most likely due to the difference in proton activity in the different ILs. As such it is important if the reader wishes to use the H2lH couple in a reference electrode then the reversibility of this reaction is checked prior to use. 

Some researchers have reported on use of a silver wire quasi-reference electrode (AglAg" couple) [85-87]. Experimentally this is accomplished by direct immersion of the silver wire into the PEL of interest. Bond and co-workers have reported the electrochemical response of cobaltocene and ferrocene in PlLs using this reference electrode [85]. They have shown that the CcICc couple is —1.34 V vs. FcIFc". However, they do note that the ferrocene can react chemically with the PIL moieties whereas the cobaltocinium cation does not appear to exhibit this reaction on the electrochemical timescale [85]. 

Snook GA, Best AS, Pandolfo AG, HoUenkamp AF (2006) Evaluation of a AglAg-t reference electrode for use in room temperature ionie liquids. Electrochem Commun 8(9) 1405-1411... 

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