Exhaustive electrolysis

The determination of the number of electrons involved in an electrochemical mechanism is crucial but not always as straightforward as the discussion above might suggest. This is because the electron transfer is not reversible. The number of electrons can be determined utilizing potential step experiments (see Section 2.8). Bulk electrolysis (exhaustive electrolysis of all the electroactive species present) is sometimes helpful, but here one must be careful in interpretation because time scales differ widely. Thin-layer bulk electrolysis brings the time scale of the electrolysis more in line with the CV time scale. 

Coulometric methods of analysis are based on an exhaustive electrolysis of the analyte. By exhaustive we mean that the analyte is quantitatively oxidized or reduced at the working electrode or reacts quantitatively with a reagent generated at the working electrode. There are two forms of coulometry controlled-potential coulometry, in which a constant potential is applied to the electrochemical cell, and controlled-current coulometry, in which a constant current is passed through the electrochemical cell. 

Minimizing Electrolysis Time The current-time curve for controlled-potential coulometry in Figure 11.20 shows that the current decreases continuously throughout electrolysis. An exhaustive electrolysis, therefore, may require a long time. Since time is an important consideration in choosing and designing analytical methods, the factors that determine the analysis time need to be considered. 

Substituting equation 11.30 into equation 11.29 and solving for fe gives the minimum time for an exhaustive electrolysis as... 

The titrant in a conventional titration is replaced in a coulometric titration by a constant-current source whose current is analogous to the titrant s molarity. The time needed for an exhaustive electrolysis takes the place of the volume of titrant, and the switch for starting and stopping the electrolysis serves the same function as a buret s stopcock. 

A 0.3619-g sample of tetrachloropicolinic acid, C6HNO2CI4, is dissolved in distilled water, transferred to a 1000-mL volumetric flask, and diluted to volume. An exhaustive controlled-potential electrolysis of a 10.00-mL portion of this solution at a spongy silver cathode requires 5.374 C of charge. What is the value of n for this reduction reaction ... 

Scale of Operation Coulometric methods of analysis can be used to analyze small absolute amounts of analyte. In controlled-current coulometry, for example, the moles of analyte consumed during an exhaustive electrolysis is given by equation 11.32. An electrolysis carried out with a constant current of 100 pA for 100 s, therefore, consumes only 1 X 10 mol of analyte if = 1. For an analyte with a molecular weight of 100 g/mol, 1 X 10 mol corresponds to only 10 pg. The concentration of analyte in the electrochemical cell, however, must be sufficient to allow an accurate determination of the end point. When using visual end points, coulometric titrations require solution concentrations greater than 10 M and, as with conventional titrations, are limited to major and minor analytes. A coulometric titration to a preset potentiometric end point is feasible even with solution concentrations of 10 M, making possible the analysis of trace analytes. 

In controlled-potential coulometry, accuracy is determined by current efficiency and the determination of charge. Provided that no interferents are present that are easier to oxidize or reduce than the analyte, current efficiencies of greater than 99.9% are easily obtained. When interferents are present, however, they can often be eliminated by applying a potential such that the exhaustive electrolysis of the interferents is possible without the simultaneous electrolysis of the analyte. Once the interferents have been removed the potential can be switched to a level at... 

As world deposits of petroleum and coal are exhausted, new sources of hydrogen will have to be developed for use as a fuel and in the production of ammonia for fertilizer. At present, most hydrogen gas is produced from hydrocarbons, but hydrogen gas can also be generated by the electrolysis of water. Figure 19-23 shows an electrolytic cell set up to decompose water. Two platinum electrodes are dipped in a dilute solution of sulfuric acid. The cell requires just one compartment because hydrogen and oxygen escape from the cell much more rapidly than they react with each other. 

During the deposition step, some fraction of the total analyte is deposited into the mercury electrode by electrolysis for a given length of time. An exhaustive electrolysis, in which all of the analyte is deposited into the electrode, is time consuming and generally unnecessary, since adequate concentrations can usually be deposited into... 

In the practice of electrolysis one mostly deals with altering and even exhausting redox concentrations at the electrode interface, so-called concentration polarization this has been considered already on pp. 100-102 for exhaustion counteracted by mere diffusion. The equations given for partial and full exhaustion (eqns. 3.3 and 3.4) can be extended to the current densities ... 

Shape of the polarographic curve. The kinetic theory of electrolysis (Section 3.2) for a redox system at a static inert electrode for partial and full exhaustion at the electrode under merely diffusion-controlled conditions leads, for ox + ne - red, to the relationship... 

Electrogravimetry is one of the oldest electroanalytical methods and generally consists in the selective cathodic deposition of the analyte metal on an electrode (usually platinum), followed by weighing. Although preferably high, the current efficiency does not need to be 100%, provided that the electrodeposition is complete, i.e., exhaustive electrolysis of the metal of interest this contrasts with coulometry, which in addition to exhaustive electrolysis requires 100% current efficiency. 

The condition of specific and complete conversion of the analyte means for alternative 1 an exclusive and complete electrolytic reaction of the analyte at the working electrode with 100% current efficiency (exhaustive electrolysis), and for alternative 2 preferential and detectable complete conversion of the... 

Iceland may start with methanol powered PEM vehicles and vessels. The University of Iceland is involved in research on the production of methanol (CH3OH) from hydrogen combined with carbon monoxide (CO) or C02 from the exhaust of aluminum and ferrosilicon smelters. This would capture hundreds of thousands of tons of CO and C02 released from these smelters. If this is combined with hydrogen generated from electrolysis using renewable power, Iceland could cut its greenhouse gas emissions in half. 

Several electrolysis regimes may be adopted. At the laboratory scale, exhaustive potential controlled electrolysis is usually preferred. When the electrode potential is poised such that the A concentration at the electrode is zero, the consumption of A and the production of B in the solution (see Section 6.2.8) are represented by the following exponential functions of time, t C° represents initial bulk concentration of the reactant A ... 

FIGURE 2.32. First-order reaction product (C) and second-order product (D) yields for Scheme 2.17 as a function of the competition parameter, a Constant concentration-constant potential and constant-current electrolyses, b Exhaustive constant-potential electrolysis. 

The variation in the yields with the competition parameter p ci f°r the two constant-concentration regimes has also a similar sigmoidal shape (Figure 2.33a). The yields in the constant-potential exhaustive electrolysis regime (Figure 2.33b) are obtained similarly by integration of the constant-concentration yield variations (see Section 6.2.8). 

The yields ensue (ECP for exhaustive constant potential electrolysis) ... 

As in the preceding case, yields at the end of a constant-potential exhaustive electrolysis are obtained by integration of the constant-concentration yields [see the establishment of equations (6.121) and (6.122)] ... 

In the exhaustive electrolysis regime, the same relationship applies as results from the combination of equations (6.195) and (6.201). The resulting variations are represented in Figure 2.38a. 

Figure 46 Cyclic voltammogram exhibited by a species, which undergoes reduction at E° = —0.67 V, before exhaustive electrolysis...

It should be kept in mind that controlled potential electrolysis is indispensable in ascertaining the stability of every electrogenerated species (i.e. to determine the chemical reversibility of a redox process). Such a determination simply requires the recording of a cyclic voltammogram on the exhaustively electrolysed solution. The chemical reversibility implies that such a response must be quite complementary to that initially recorded. For instance, if the voltammetric profile of Figure 46 had been obtained before electrolysis, one must obtain a complementary response of the type illustrated in Figure 47 after electrolysis. 

Figure 47 Cyclic voltammogram obtained under the experimental conditions of Figure 46 after exhaustive electrolysis at Ew = —0.9 V...

Cells used in exhaustive electrolysis present more problems of electrodes symmetry than those for cyclic voltammetry, due to the long experimental times and high currents involved. 

It is recommended to insert into the cell an electrode for cyclic voltammetry. This allows one to record a cyclic voltammogram of the species under study directly before and after exhaustive electrolysis,... 

Two successive, closely spaced oxidation processes are observed. By exhaustive electrolysis, the overall process proves to be a chemically reversible two-electron process. The separation of the two processes... 

This behaviour does not allow the catalytic processes of water oxidation to be detected on the time scale of cyclic voltammetry. However, during exhaustive electrolysis at the potential of the second process (Ew— +1.38 V), catalytic generation of gaseous oxygen is observed with restoration of the original complex.55... 

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