AglAgCl

With films based on a 2,2 -bipyridine ligand which is 4,4 -disubstituted with electron-with-drawing carboxyester groups the selectivity is drastically changed, since HCOO" is obtained with a high yield (p >90%) at a very low overvoltage (—0.75 V vs. AglAgCl).98 This difference was... 

Dopamine /S-hydroxylase adsorbed on gold electrode has been investigated elec-trochemically and applying quartz crystal microbalance [198]. In the neutral phosphate buffer solution, the adsorbed layer was stable and did not desorb within the potential range of 0.6 to —0.7 V (versus AglAgCl —1 M KCl). At potentials more positive than 0.8 V, the adsorbed compound was oxidized and, probably, residual tyrosine, tryptophan, and histidine participated in this process. 

The synthesis of the corresponding R2Dtc complexes also has been reported (107) and their electrochemical properties have been investigated (57). Two one-electron polarographic reduction waves between +0.75 and —2.2 V were observed on a DME in acetone (versus AglAgCl), the first of which was reversible. The following sequence of electrode reactions was suggested (57). 

As indicated in a previous review (627) comparison of voltametric data on the M(R2Dtc) complexes is difficult for many cases because different solvents and reference electrodes have been used. Major, extensive studies have been reported by two groups. The Nijmegen group have reported studies in CH2C12 versus a saturated calomel electrode (480,602,604,609). The Australian group, on the other hand, reported electrochemical studies in acetone versus a AglAgCl, 0.lAf LiCl electrode (115,116, 309, 311,312, 313). 

A potentiostatic, three-electrode circuit allows the separation of both functions physically for the reference potential, a non-polarisable electrode is used (a calomel or AglAgCl reference electrode), while the electrical-current conducting electrode is an inert metal electrode. With electrochemical, direct-current methods, the effect of this modification is limited to a reduction of the so-called IR-drop (or ohmic-drop), which is caused by... 

In Fig.3.6, current-potential curves are shown recorded in a buffer solution (pH=5.2 curve 1) and in the presence of hydrogen peroxide (curves 2-5) at room temperature on a glassy-carbon disc electrode, which was rotated at lOOOrpm. Two reduction waves can be observed, with a limiting-current plateau at around -0.5 V vs. AglAgCl, and a second one at around -IV vs. 

First, the electrochemical behaviour of the substrate material, which is platinum, was investigated as a function of applied potential by cyclic voltammetry. For that, the potential was swept in positive direction from -0.5 V vs. AglAgCl to a first vertex potential of 0.9V vs. AglAgCl, followed by a scan in reversed direction to a second vertex potential of -0.8 V and back to -0.5 V vs. AglAgCl. The curves obtained in solutions of different pH values are shown in Fig. 6.5. Similarly shaped curves were obtained by variation of... 

The second region is the mixed kinetic transport-controlled region, and the most negative part of it can also be used for kinetic and mechanistic studies of the electron-transfer reaction after the experimental currents have been compensated for transport limitations. Finally, a second wave is observed at potentials higher than 0.5 V vs. AglAgCl, which can be attributed to the oxidation of sulphite to sulphate. However, this wave is not further considered because the oxidation mechanism of sulphite showed poor reproducibility (see section 6.3), and sulphite detection in dyeing processes is not of great importance compared with dithionite detection. 

Figure 6.11 shows the result of such a chronoamperometric experiment, where a constant potential of 0.45V vs. AglAgCl is applied to the platinum electrode rotating at 6.67Hz in a pH of 12.5. The sodium dithionite concentration was increased in ten consecutive steps. It can be seen that the limiting-current corresponds to the values obtained with linear-sweep... 

Besides these main waves, three other waves are observed in Fig. 12.9. Waves I and II are attributed to oxidation of reaction products formed in wave V. This was proved by cycling the potential between -0.3 and 0.6 V vs. AglAgCl where waves I and II were absent. By addition of sodium dithion-ite to the solution, waves I and II again appeared, indicating that the reaction product formed in wave V is dithionite. Waves I and II were also observed at a bare-carbon-fibre electrode after addition of sodium dithionite. Therefore, it is not clear whether these reactions are electrocatalysed by [Fe(II)TSPc]4. However, when cycling between -0.3 and 0.6 V vs. AglAgCl, wave III still occurred under these conditions, indicating that this wave can be attributed to a species in solution. 

In a buffer of pH=8, completely different results were obtained for sulphur dioxide and sodium sulphite as starting species. Starting from sulphite, only one oxidation wave is obtained around 0.7 V vs. AglAgCl. From Table 12.1 it can be seen that sulphite itself is the main compound in solution, therefore it is clear that this wave can be attributed to Equation 12.17. Its slope (Fig. 12.10, curve 6), obtained by plotting the peak current versus concentration, is situated in the range that allows exchange of two electrons ... 

Starting from sulphur dioxide, only one wave around 0.7V vs. AglAgCl is observed at concentrations lower than lxlO moll-1. At concentrations higher than this value, this wave disappears and two new oxidation waves with =-0.2 and +1.1 V vs. AglAgCl and a reduction wave are observed. It is well known that S032- reacts to form I IS2( b in an excess of S0224. This... 

In Fig. 12.12, current-potential curves are shown of increasing S02 concentration, recorded in a pH=10 buffer at a carbon fibre modified with [Fe(II)TSPc]4+ It can be seen that in alkaline solution only one wave is obtained at 0.75 V vs. AglAgCl, which is identical to the one obtained when started from sodium sulphite. Therefore, it can be proposed that SO-,2 is the main (Table 12.1) electroactive compound in solution. As expected, the slope between peak current and S02 or Na2S03 concentrations corresponds to exchange of two electrons (Fig. 12.10, curve 7), and a detection limit of 7.3+0.1 xlO - molI 1 was obtained. 

The electrochemical behaviour of Ce(IV) at a gold-coated textile structure was investigated by cyclic voltammetry in a 1.0moll-1 H2S04 solution (Fig. 12.13). The potential is swept in positive direction from an initial potential of -0.2V vs. AglAgCl to a vertex potential of 1.7 V vs. AglAgCl and in the... 

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