Electroactive impurity

It has been found50 that such a multielectron step does not exist with 58, which exhibits a classical two-electron scission. In general, allylic sulphones (59) without an unsaturated system in a suitable position are not reducible. Thus, they do not exhibit a cathodic step in protic solutions. However, in aprotic media the isomerization may be base catalyzed, since small amounts of electrogenerated bases from electroactive impurities, even at low concentration, may contribute to start the isomerization. Figure 10 shows the behaviour of t-butyl allylic sulphone which is readily transformed in the absence of proton donor. On the other hand, 60 is not isomerized but exhibits a specific step (Figure 10, curve a) at very negative potentials. 

If we measure a residual current-potential curve by adding an appropriate supporting electrolyte to the purified solvent, we can detect and determine the electroactive impurities contained in the solution. In Fig. 10.2, the peroxide fonned after the purification of HMPA was detected by polarography. Polarography and voltammetry are also used to determine the applicable potential ranges and how they are influenced by impurities (see Fig. 10.1). These methods are the most straightforward for testing solvents to be used in electrochemical measurements. 

The diffusion current is typically measured from a baseline that is obtained by extrapolating the residual current prior to the wave. Alternatively, the baseline can be recorded in a separate experiment on a solution of deoxygenated supporting electrolyte. The residual current arises from capacitive current required to maintain the expanding drop at the applied potential and from reduction of trace electroactive impurities. 

A number of companies supply reagent or spectroquality solvents that have been purified to remove UV-absorbing impurities. Some of them, particularly dimethyl sulfoxide, may be suitable for general electrochemical use as purchased. However, small quantities of electroactive impurities (particularly water) often are present in spectroquality solvents. Therefore, a particular batch of solvent always should be tested by measurement of the residual current with an appropriate supporting electrolyte and a platinum, gold, or carbon electrode (to test the anodic limits) and a platinum electrode (to test the cathodic limits). The voltage window or domain of electroactivity is a sensitive measure of the adequacy of the purification procedures. 

When very high solution purity is required, the last stage of purification should be preelectrolysis, with the use of a large-surface-area electrode, of the same metal as that selected for the experiment. The idea behind this procedure is simple if there is an electroactive impurity in solution, let it be consumed during preelectrolysis (which is typically conducted overnight or even for several days), so that none will be left to interfere with the reaction to be studied in the... 

The numbers just given should be taken as order-of-magnitude type estimates only, since as in any other measurement, it is the fluctuation in the background current, not its absolute value, that determines the lowest current that can be reliably measured. The importance of reducing the concentration of the electroactive impurity should, however, be clear. 

The preparation of the polarographic sample becomes more complicated in the analysis of drugs [211], e.g., of seeds or barks. A typical procedure can be exemplified with the determination of cinnamaldehyde in cinnamon bark. A quantity of about 2-5 g of this bark is pulverised and extracted with chloroform for 10 hr in a Soxhlet apparatus. The solvent is then distilled off and the residue dissolved in ethanol. An aliquot of this is mixed with an aqueous solution of the supporting electrolyte and polaro-graphed. A similar procedure proved successful in the determination of carvone in Semen carvi it differs only in the application of ethanol for the extraction. This extraction in the determination of cuminaldehyde in the Roman caraway seeds was a failure because electroactive impurities were extracted simultaneously. For this reason, it was necessary to make use of steam distillation. 

At first a current/voltage curve from the electrolyte is taken. High currents below the decomposition potential point to electroactive impurities in the electrolyte. These can often be removed by purging with nitrogen to remove oxygen or by preelectrolysis close to the decomposition potential. 

Figure 8. Influence of an electroactive impurity on the interfacial potential-dye concentration relationship. The x axis is the logarithm of the dye concentration, the y axis is the logarithm of the ratio of water volume to nitrobenzene volume, and the z axis is the difference between interfacial potential with and without the impurity. Standard potentials of transport for the impurity cation A (p — — 100 mV and for the anion A (p — +100 mV c = 1 X 10 5 mol/L.

A major contribution to decreased current efficiency is the presence of electroactive impurities in the sample. This can be a particular problem with acid-base titrations. In such cases, it may be necessary to generate the titrant externally (in the absence of sample), and then add it to the sample in increments. A suitable arrangement is illustrated in Figure 4.5. By the use of stopcocks the titrant can be generated and flushed into the cell in increments. 

A small wave of constant height at the base of the main wave sounds exactly like an adsorption prewave. But remove the main wave and the characteristics of this small wave fulfil the conditions for a diffusion-controlled wave for a substance at a fixed concentration. This small wave is probably due to an electroactive impurity in the supporting electrolyte. The running of a blank on the supporting electrolyte would show this, unless the impurity is rendered electroactive only by reaction with the analyte. This example shows that the mercury column height or time dependency of the wave height should always be investigated. 

The basic elements of a multilayered electrode are shown in Figure 12.3. Each part has its own design specification, depending on end use. First, electrode materials have to withstand both oxidizing and reducing conditions. Second, total electrode resistance should be below 10 Q to minimize errors associated with/S drop. Third, electrodes must have electroactive areas which are reproducible within 5% coefficient of variation. Fourth, during manufacture, all materials must withstand repetitive thermal cycling to T > 110 °C. Fifth, all the electrode materials must be mutually adhesive. Sixth, there has to be a complete absence of electroactive impurities. 

In the Figure 13 the construction of a differential amperometric detector is shown[41]. The detector consists of two identical thin-layer cells, which are in contact with a compartment containing the reference and auxiliary electrodes. A mobile phase is pumped through the reference cell so that the recorded differential signal is free of background noise, caused by the presence of electroactive impurities in the solvent. 

Direct comparison of the electrochemical window of different ILs is difficult for several reasons. Firstly, the reference electrode is different during the measurement of the electrochemical window since some reference electrodes such as Ft and Ag are only quasi reference electrodes and the standard redox potential is difficult to define. Secondly, the working electrodes are usually not the same and the decomposition potentials at different electrode surfaces are not the same. Thirdly, even when the same working and reference electrodes are used, the content of impurities in the IL is uncertain. Electroactive impurities such as halide ions or water significantly reduce the electrochemical stability of ILs. 

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