Maintaining current efficiency

Maintaining Current Efficiency To illustrate why changing the working electrode s potential can lead to less than 100% current efficiency, let s consider the coulometric analysis for Fe + based on its oxidation to Fe + at a Pt working electrode in 1 M H2SO4. 

As anodes age their performance gradually deteriorates, and thus small changes in electrolyte composition must be made to maintain current efficiency and energy consumption. The EMOS system has been used by some operators to determine the optimum composition of the electrolyte in these circumstances and in some cases to control the composition selectively for each cell line. 

If the initial concentration of Cu + is 1.00 X 10 M, for example, then the cathode s potential must be more negative than -1-0.105 V versus the SHE (-0.139 V versus the SCE) to achieve a quantitative reduction of Cu + to Cu. Note that at this potential H3O+ is not reduced to H2, maintaining a 100% current efficiency. Many of the published procedures for the controlled-potential coulometric analysis of Cu + call for potentials that are more negative than that shown for the reduction of H3O+ in Figure 11.21. Such potentials can be used, however, because the slow kinetics for reducing H3O+ results in a significant overpotential that shifts the potential of the H3O+/H2 redox couple to more negative potentials. 

Since the current due to the oxidation of H3O+ does not contribute to the oxidation of Fe +, the current efficiency of the analysis is less than 100%. To maintain a 100% current efficiency the products of any competing oxidation reactions must react both rapidly and quantitatively with the remaining Fe +. This may be accomplished, for example, by adding an excess of Ce + to the analytical solution (Figure 11.24b). When the potential of the working electrode shifts to a more positive potential, the first species to be oxidized is Ce +. 

In this manner, a current efficiency of 100% is maintained. Furthermore, since the concentration of Ce + remains at its initial level, the potential of the working electrode remains constant as long as any Fe + is present. This prevents other oxidation reactions, such as that for liiO, from interfering with the analysis. A species, such as Ce +, which is used to maintain 100% current efficiency, is called a mediator. 

End Point Determination Adding a mediator solves the problem of maintaining 100% current efficiency, but does not solve the problem of determining when the analyte s electrolysis is complete. Using the same example, once all the Fe + has been oxidized current continues to flow as a result of the oxidation of Ce + and, eventually, the oxidation of 1T20. What is needed is a means of indicating when the oxidation of Fe + is complete. In this respect it is convenient to treat a controlled-current coulometric analysis as if electrolysis of the analyte occurs only as a result of its reaction with the mediator. A reaction between an analyte and a mediator, such as that shown in reaction 11.31, is identical to that encountered in a redox titration. Thus, the same end points that are used in redox titrimetry (see Chapter 9), such as visual indicators, and potentiometric and conductometric measurements, may be used to signal the end point of a controlled-current coulometric analysis. For example, ferroin may be used to provide a visual end point for the Ce -mediated coulometric analysis for Fe +. 

Salt that is substantially free of sulfate and other impurities is the cell feed. This grade may be purchased from commercial salt suppHers or made on site by purification of cmde sea or rock salt. Dried calcium chloride or cell bath from dismanded cells is added to the bath periodically as needed to replenish calcium coproduced with the sodium. The heat required to maintain the bath ia the molten condition is suppHed by the electrolysis current. Other electrolyte compositions have been proposed ia which part or all of the calcium chloride is replaced by other salts (61—64). Such baths offer improved current efficiencies and production of cmde sodium containing relatively Htde calcium. 

The anodes used were cast ferromanganese the electrolyte, KOH/K CO, and current efficiencies for this process were about 40%. Energy requirements for this process, about 15 kWh/kg of KMnO, plus the cooling requirement to maintain cells at 20°C, made this process uneconomical (70,71). 

Asahi s investigations showed that a Na+ concentration of 1.1 N was necessary in compartment II to maintain a current efficiency of 96% in the carboxylic membrane during operation with 3.5 N brine in compartment I and 32% caustic soda in compartment III. The Na+ concentration in compartment II was generally far lower than that in compartment I, and clearly indicates the tendency for depression of the Na+ concentration at the interface of the sulphonic and carboxylic layers in the normal... 

On a large scale, it is more difficult to maintain constant electrode potential and conditions of constant current are employed. Under these conditions, as the concentration of the substrate falls, the voltage across the cell rises in order to maintain the imposed reaction rate at the electrode surface. This causes a drop in current efficiency towards the end of the reaction, since as the working electrode potential rises, either oxygen or hydrogen evolution becomes significant. 

In a controlled-current coulometric titration, either the electrogenerated reagent reacts with an unknown substance in a stoichiometric or reproducible manner, or the unknown itself reacts directly at an electrode. The latter, called a primary coulometric titration, is rarely found in practice because the concentration of unknown continuously decreases during the electrolysis and it is difficult to maintain 100% current efficiency. The former approach, known as secondary coulometric titration, is used most of the time. 

The activity of the lead cathodes is maintained by periodic short-circuiting. The reaction has been intensively investigated by Standard Oil 508 511 over the past few years. When Hg and Pb/Hg cathodes are used, the yields and current efficiencies can be increased to above 90 %. Heteroaromatic carboxylic acids can likewise be converted into the corresponding alcohols ... 

The Hargreaves-Bird electrolyzer was loaded with 2600 A, current density on the surface of the diaphragm 2 A/sq. dm and voltage 3.6 to 4.0 V. 150 to 170 grams of sodium carbonate per one litre catholyte was obtained while currents efficiency was between 80 to 85 per cent. The temperature was maintained at about 80 °C. 

An acid brine electrolysis will give the best results if smooth platinum anodes are used. If chromate is added and the electrolyte temperature > maintained at 75 to 80 °C, a current efficiency of 95 per cent can be obtained. Nowadays, graphite or magnetite is substituted for platinum. 

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