Electrode response rate

FIGURE 10. Voltammetric curves of fully aliphatic allylic sulphones (c = 3 x 10-3 M) in DMF/TBAP 0.1 m electrolyte, stationary mercury electrode, sweep rate 10 mV s—1 (a) and (b) curves in aprotic DMF (c) response of the sulphone, (b) with phenol 10-2 m (after Reference 26). 

Figure 16 Cyclic voltammetric responses recorded in MeCN solutions of tcnq ([NBu4J[BF4] 0.1 mol dm 3) without (a,c,e) and with the addition of 4.5% H2O (b,df). (a,b) YBa2Cui07-x electrode (c,d) La185Sr0 15Cu04 electrode (e,f) Ndj85Ce0j5CuO4 electrode. Scan rate 0.1 V s l...

It is further useful to measure ionic species in stirred or flowing solutions, because the electrode response is then faster, the determination limit is often better than in quiescent solutions and the measurement precision is also improved These improvements apparently result from the effect of solution movement on film diffusion at the electrode surface, which is assumed to be the response-rate determining step [92, 154], An obvious requirement is that the solution velocity and the cell geometry be constant. 

Fig. 1 A typical response of Ni to cyclic polarization in 0.1 mol dm NaOH. Potential measured against Hg HgO electrode. Scan rate 60 mV. Explanation see in the text [91].

Figure E7.3 shows the catalytic rate, r, and catalyst potential, (where WR is the catalyst s potential with respect to a reference electrode), response to step changes in current applied during C2H4 oxidation on a Pt catalyst electrode deposited on YSZ at T= 370 °C, = 4.6 kPa, and Pc h = 0.36 kPa As shown... 

Cyclic voltammetric studies show a strong pH dependence for the electrode response when doped with copper. Free amino groups protonate at low pH, causing swelling of the polymer and a more open structure. The detailed redox behaviour of the copper electrode is determined by the rate of charge diffusion through the film. 

Rates of ion exchange on kaolinite, smectite, and illite are usually quite rapid. Sawhney (1966) found that sorption of cesium on illite and smectite was rapid, while on vermiculite, sorption had not reached an equilibrium even after 500 h (Fig. 5.5). Sparks and Jardine (1984) found that potassium adsorption rates on kaolinite and montmorillonite were rapid, with an apparent equilibrium being reached in 40 and 120 min, respectively. However, the rate of potassium adsorption on vermiculite was very slow. Malcom and Kennedy (1969) studied Ba-K exchange rates on kaolinite, illite, and montmorillonite using a potassium ion-specific electrode to monitor the kinetics. They found >75% of the exchange occurred in 3 s, which represented the response time of the electrode. The rate of Ba-K exchange on vermiculite was characterized by a rapid and slow rate of exchange. 

The effect of pH on the rate of oxidation of H2S with H202 was determined from pH = 2 to 13 at 5, 25 and 45°C. These results are shown in Figure 18. Our results at 25°C from pH = 5 to 8 are in good agreement with the results of Hoffmann (46) (See Figure 19). At lower values of pH, his results are faster than ours. This may be due to problems with the emf technique he used. For the slower reactions of H2S with 02 or H202, the emf technique may yield unreliable results due to problems with the electrode response. 

A detailed examination of the mass transport effects of the HMRDE has been made. At low rotation speeds and for small amplitude modulations (as defined in Section 10.3.6.2) the response of the current is found to agree exactly with that predicted by the steady-state Levich theory (equations (10.15)-(10.17)) [27, 36, 37]. Theoretical and experimental application of the HMRDE, under these conditions, to cases where the electrode reaction rate constant was comparable to the mass-transfer coefficient has also been made [36]. At higher rotation speeds and/or larger amplitude modulations, the observed current response deviated from the expected Levich behaviour. 

The amount of enzyme, either in a pure or crude form, also affects the speed of response of the electrode. Yet, compromises must be made between the increase in enzyme activity and the concomitant increase in membrane thickness, which affects the rate of electrode response. As the amount of enzyme is increased, a shorter response time is observed, until an optimum level is reached. Further increase in the amount of enzyme tends to diminish the response time due to a thickening of the membrane layer and an increase in the time required for the substrate to diffuse through the membrane. Generally, using enzyme with the highest specific activity gives the thinnest membranes and the most rapid kinetics. 

Since electrode measurements involve low substrate concentrations, reactive impurities have to be held to a very low level. The physical data and purification methods for several organic solvents used in electrode measurements have been summarized (Mann, 1969). But even when careful procedures for solvent and electrolyte purification are employed, residual impurities can have profound effects upon the electrode response. For example, the voltam-metric observation of dications (Hammerich and Parker, 1973, 1976) and dianions (Jensen and Parker, 1974, 1975a) of aromatic hydrocarbons has only been achieved during the last ten years. The stability of radical anions (Peover, 1967) and radical cations (Peover and White, 1967 Phelps et al., 1967 Marcoux et al., 1967) of aromatic compounds was demonstrated by cyclic voltammetry much earlier but the corresponding doubly charged ions were believed to be inherently unstable because of facile reactions with the solvents and supporting electrolytes. However, the effective removal of impurities from the electrolyte solutions extended the life-times of the dianions and dications so that reversible cyclic voltammograms could be observed at ambient temperatures even at very low sweep rates. 

The manner in which kinetic data are treated in arriving at an electrode mechanism depends primarily upon whether the technique gives a direct measure of the response of the intermediate or an indirect measure, usually the effect of the chemical reaction on the electrode response of the substrate. In the former case, the conventional way of handling the data is to compare the experimental response with theoretical data in the form of a working curve and determine the mechanism from the best fit with theoretical data. The latter case usually involves the calculation of the electrode response to a particular mechanism and then comparing some measurable quantity, for example the sweep rate dependence of the peak potential, with the theoretical value. Which type of analysis is appropriate, direct or indirect, depends upon the... 

It has also been shown that the electrode response of some processes can appear to fit theoretical working curves in which the reaction order in the intermediate differs from the true value (Parker, 1981b). For example, the deprotonation of hexamethylbenzene radical cation studied by derivative cyclic voltammetry gave data which fitted theoretical data for a simple first order decomposition of the intermediate. However, the observed first order rate constants were found to vary significantly with the substrate concentration indicating a higher order reaction. A method was proposed to treat... 

The response and recovery times of both samples at each PEVD step are compared at a working electrode flow rate of 40 seem in Figures 25a and b, respectively. Because of the inability to fabricate a consistent Pt thick film at the working electrode of both sensors, the response and recovery curves do not exactly match. However, the curves from both samples follow the same trends. Taking into account the geometric factor for both samples, the results from this study can be considered to be fairly consistent. 

Fig. 25 Comparison of the response time (a) and recovery time (b) for the two same working electrode flow rate of 40 seem.

Fig. 7-42. Cyclic voltammetric response exhibited by [Fe(C5H4S)2]Rh(C5Mc5)(PMe3) in dichloro-methane solution. Platinum working electrode. Scan rate 0.1 V s .

The transport of charge by electron hopping is an attractive model for these systems. In the case mentioned above, the electrode response is better from the precomplexed polymer film than from one prepared by first coating with PVP, then dipping into a solution containing a source of [Fe(CN)5(H20)] thus the spatial distribution of redox centres is important as well as their number in determining electrode response. Data for the pentacyanoferrate system support charge transport via adjacent redox sites and the rate of this transport falls off rapidly below a critical concentration of centres. ... 

Modification of semiconductor electrode response with adsorbed or attached dye molecules is an attractive alternative to other photoelectrochemical systems (7-13). Metal oxides which are stable or have very low corrosion rates but are transparent to visible wavelength light can be used in light-assisted electrochemical reactions when modified with monolayers and multilayers of a wide variety of chromophores interposed between the electrode and electrolyte. With one exception, the initial reports of energy conversion efficiencies of electrodes with adsorbed dyes was disappointingly low. Recently however,... 

Also, third-generation biosensors for superoxide anion (O ) have been developed based on superoxide dismutase (SOD) immobilised by thin silica-PVA sol-gel film on a gold electrode surface [633]. The preparation of SOD electrode is easy and simple. The uniform porous structure of the silica-PVA sol-gel matrix results in a fast response rate of immobilised SOD and is very efficient for stabilising the enzyme activity. 

Figure 4-12. Catalytic voltammetry of Paracoccus pantotrophus nitrate reductase (NarGH) adsorbed as a film on a PGE electrode at pH 6. (A) Increasing the electrode rotation rate from 0 to 3000 rpm removes the mass transport limitation of the catalytic response in 50 pM NO3 . (B) The enzyme s greater rate of chlorate reduction compared to nitrate reduction is reflected in greater distortion of the waveform through dispersion of sluggish interfacial electron transfer rates (see also Fig. 4-4C). Scan rate 10 mV s. Adapted from ref. 64. with permission.

Umezawa et al., 1982), or with fluorescence markers (Ishimori et al., 1984). The latter system was shown to be most sensitive, the detection limit being 10-15 mol/L Double signal amplification can be obtained by inclusion of enzymes in the liposomes (Brahman et al., 1984). Thus, Haga et al. (1980) entrapped HRP in sensitized liposomes and used the liposomes to determine theophylline. The rate of HRP liberation was monitored by measuring the NADH oxidase activity of HRP with an oxygen electrode. The electrode response correlated with the theophylline concentration in the sample between 4 and 20 nmol/L... 

SECM SG/TC experiments were carried out to prove that the product of the initial two-electron oxidation process diffused into the solution, where it would react homogeneously and irreversibly. For these measurements, a 10 /xm diameter Au tip UME was stationed 1 /xm above a 100 /xm diameter Au substrate electrode. With the tip held at a potential of —1.3 V versus saturated mercurous sulfate electrode (SMSE), to collect substrategenerated species by reduction, the substrate electrode was scanned through the range of potentials to effect the oxidation of borohydride. The substrate and tip electrode responses for this experiment are shown in Figure 16. The fact that a cathodic current flowed at the tip, when the substrate was at a potential where borohydride oxidation occurred, proved that the intermediate formed in the initial two-electron transfer process (presumed to be mono-borane), diffused into the solution. An upper limit of 500 s 1 was estimated for the rate constant describing the reaction of this species (with water or OH ), based on the diffusion time in the experimental configuration. This was consistent with the results of the cyclic voltammetry experiments (11). 

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