Pseudo-reference

Goodwin et ai. (1975) employed another variant on the Adams cell design but obtained rather different results to those of Kondrikov and colleagues. Direct oxidation of the TPA at 2.0 V vs. an Ag pseudo reference electrode produced no epr spectrum. However, if the potential was then stepped to potentials less than —0.35 V, an epr spectrum was observed, that could be assigned to the triphenylmcthyl radical. This was interpreted by the authors in terms of the following mechanism ... 

Cu, In, Ga, and Se are codeposited from the solution at room temperature in a three-electrode cell configuration, where the reference electrode is a platinum pseudo-reference, the counter electrode is platinum gauze, and the working electrode is the substrate. The substrates typically used are glass, DC-sputtered with about 1 pm of Mo. In all experiments, the applied potential is -1.0V versus the Pt pseudo-reference electrode. The corresponding current density range for the deposition is 5 to 7 mA/cm2. 

Most common reference electrodes are silver-silver chloride (SSC), and saturated calomel electrode (SSC, which contains mercury). The reference electrode should be placed near the working electrode so that the W-potential is accurately referred to the reference electrode. These reference electrodes contain concentrated NaCl or KC1 solution as the inner electrolyte to maintain a constant composition. Errors in electrode potentials are due to the loss of electrolytes or the plugging of the porous junction at the tip of the reference electrode. Most problems in practical voltammetry arise from poor reference electrodes. To work with non-aqueous solvents such as acetonitrile, dimethylsulfoxide, propylene carbonate, etc., the half-cell, Ag (s)/AgC104 (0.1M) in solvent//, is used. There are situations where a conventional reference electrode is not usable, then a silver wire can be used as a pseudo-reference electrode. 

Figure 18 Cyclic voltammetric responses at different temperatures of ferrocene in a 16 17 1 chloroethane tetrahydrofuran 2-methyl-tetrahydrofuran solution containing LiBF4 (0.6 mol dm 3) as supporting electrolyte, (a) Platinum electrode (b) Tl 1223 ( Tl0.sPbo.s Sr2Ca2Cu306). Scan rate 0.025 V s. Potential values are referred to a pseudo-reference silver wire...

Figure 4.18 — (A) Schematic diagram of a flow-injection system for potassium (1) carrier solution (2) injection valve (3,4) flow cell (5) pseudo-reference electrode (6) waste. (B) Detail of the flow-cell (3,4) CHEMFETs (5) pseudo-reference electrode. (Reproduced from [150] with permission of Elsevier Science Publishers).

Figure 14.16 Cyclic voltammetry study of rotaxane 10(4) +. (a), (b), and (c) potential range —0.4 V to 1.0 V, followed by 1.0 to —0.4 V. (d) Two consecutive scans. The electrochemical experiments have been performed at room temperature, in a 0.1 M solution of Bu4NBF4 in CH3CN CH2C12 (9 1), with a Pt working electrode, Ag wire as a pseudo-reference electrode, and Pt wire as a counterelectrode.

There are three types of reference electrodes discussed reference electrodes of the first kind, reference electrodes of the second kind, and redox reference electrodes. The first two are used with potentiometric chemical sensors, whereas the last one helps us to get around the difficult problem of comparing potentials in different solvents. There is also a pseudo-reference electrode that does not have a stable, defined, reproducible potential. It serves only as the signal return to satisfy the condition of closing the electrical circuit (see Section 5.2). Because the liquid junction always causes some leakage of the internal solution, electrodes of the first kind are particularly affected. 

Another remarkable feature of that probe is its complete independence of the reference electrode potential and nearly perfect temperature compensation. Thus, a pseudo-reference such as silver wire, functioning only as a signal return, can be used. 

All potentials vs. screen-printed Ag/AgCl pseudo-reference, except values marked with asterisk ( ), which are vs. Ag/3M AgCl double-junction reference electrode, and values marked with dagger CfO, which are vs. saturated calomel. Abbreviations CoPC cobalt phthalocyanine, SPCE screen-printed carbon electrode, GOD glucose oxidase, MWCNT multi-walled carbon nanotubes, NAD nicotinamide adenine dinucleotide, PQQ pyrroloquinoline quinone, FIA flow injection analysis. 

The different biosensors were obtained using thick-film SPEs produced by Biosensor Laboratory, University of Florence and commercialized by PalmSens. The electrochemical cell, consisting of a graphite working electrode and silver counter and pseudo-reference electrodes, was printed on a planar polyester substrate (Fig. 29.2). 

The electrochemical cells used are three electrode strips, based on a carbon working electrode, a carbon counter electrode and a silver pseudo-reference electrode [1],... 

Probe immobilisation The sequence (probe) was diluted in 2 x SSC at the final concentration of 40 gg/ml, and immobilised onto the working electrode surface applying +0.5V (vs. Ag/AgCl pseudo-reference electrode) for 5 min in stirring condition. 

The guanine redox activity of the target DNA was used to detect the duplex formation by carrying out a square-wave voltammetric measurement (SQW). The results of the parameters optimisation for the probe concentration (40 pg/ml), immobilisation time (5 min by applying a positive potential of +0.5V vs. Ag/AgCl pseudo reference electrode) and hybridisation time (10 min without any potential) were very similar to those obtained in previous work [3]. 

Fig. 16 Cyclic voltammetry curves recorded borate, 0.1 mol L 1, Ag wire pseudo-reference) using a Pt working electrode at a 100 mV s 1 (a) Compound 16 + (b) Chemically prepared...

Flow-injection analysis is a versatile technique to evaluate the performance of a detector system. CHEMFETs may have an advantage over ISEs because of their small size and fast response times. We have tested our K+-sensitive CHEMFETs in a wall-jet cell with a platinum (pseudo-)reference electrode. One CHEMFET was contineously exposed to 0.1 M NaCl and the other to a carrier stream of 0.1 M NaCl in which various KC1 concentrations in 0.1 M NaCl were injected. The linear response of 56 mV per decade was observed for concentrations of KC1 above 5 x 10"5 M (Figure 9). When we used this FIA cell (Figure 10) for determination of K+ activities in human serum and urine samples, excellent correlations between our results and activities determined by flame photometry were obtained (Figure 11). 

Fig. 7.8 Cyclic voltammograms of thiophene polymerization (0.2 M, 50mVs-1) onto a Pt working electrode (a) growth and (b) post-growth in [C2mim][NTf2], (c) growth and (d) post-growth in [C4impyr][NTf2], vs. a Ag pseudo-reference electrode. Arrows indicate the peak development with successive scans [27].

Pseudo-reference Electrodes and Internal Redox Reference Couples... 

Accordingly, much voltammetry in non-aqueous solvents has been conducted using a pseudo -reference electrode (alternatively labelled a quasi -reference electrode) constituting, quite simply, a metal wire, most often silver or platinum. It is then expected (hoped) that the potential of the wire remains constant throughout the voltammetric experiment. This may be a realistic hope if, as Bard and Faulkner [32] point out, the composition of the bulk solution is essentially constant during the period of experimentation, as may be realized during voltammetric studies but certainly not in electrosynthetic work. 

Table 11.1 presents the results of a literature survey to establish which reference and pseudo-reference electrodes have been and are being used in RTILs. The structures of the constituent anions and cations are shown in Figure 11.7. It is clear that the majority of researchers favor the use of pseudo-reference electrodes but that not all take the trouble to calibrate using internal standards such as Cp2Co+ or Cp2Fe. In the latter case, the philosophy is nicely and honestly summarized by Welton and colleagues [38] ... 

The electrochemistry was performed on the neat ionic liquid. In such a set-up, with no recognised background electrolyte or redox standard, the potential vs. the platinum pseudo-reference is difficult to compare with standard potentials, however, in such unusual conditions it is the qualitative nature of the electrochemistry that is important. ... 

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