Interferences working electrodes

Selecting a Constant Potential In controlled-potential coulometry, the potential is selected so that the desired oxidation or reduction reaction goes to completion without interference from redox reactions involving other components of the sample matrix. To see how an appropriate potential for the working electrode is selected, let s develop a constant-potential coulometric method for Cu + based on its reduction to copper metal at a Pt cathode working electrode. 

Coulometry. If it can be assumed that kinetic nuances in the solution are unimportant and that destmction of the sample is not a problem, then the simplest action may be to apply a potential to a working electrode having a surface area of several cm and wait until the current decays to zero. The potential should be sufficiently removed from the EP of the analyte, ie, about 200 mV, that the electrolysis of an interferent is avoided. The integral under the current vs time curve is a charge equal to nFCl, where n is the number of electrons needed to electrolyze the molecule, C is the concentration of the analyte, 1 is the volume of the solution, and F is the Faraday constant. 

Thus, if the adsorbate is the formyl species, oxidising it gives D+ which can be detected as HD by stepping the potential of the working electrode sufficiently negative to reduce the D +. For the experiment to be successful, the electrolyte must be replaced by pure protic solution after the chemisorption to prevent interference from the D+ produced via the reactions in equations (3.50) and (3.51). 

One problem associated with this design is that the Ru(bpy)32+ reservoir evaporates over time and the Ru(bpy)33+ concentration changes as the CE capillary effluent dilutes it, which affect both sensitivity and reproducibility of the CL response. To overcome this problem, recently a new in situ-generated Ru(bpy)33+ CL cell has been proposed [98], In this design, Ru(bpy)32+ is continuously delivered to the cell and Ru(bpy)33+ is then generated at the interface of the separation capillary and the working electrode. Electrochemical control of the production of Ru(bpy)33+ at the distal end of the separation capillary without interference from the CE current is provided and finally the ECL process is cou-... 

Suppose the sample containing the analyte also contains the electro-active substance Y that interferes with X in the chromatogram. When peak heights of Y and X are plotted versus corresponding working electrode potentials, voltammograms for X and Y are obtained simultaneously as shown in Figure 2-7. 

In principle, the auxiliary electrode can be of any material since its electrochemical reactivity does not affect the behaviour of the working electrode, which is our prime concern. To ensure that this is the case, the auxiliary electrode must be positioned in such a way that its activity does not generate electroactive substances that can reach the working electrode and interfere with the process under study. For this reason, in some techniques the auxiliary electrode is placed in a separate compartment, by means of sintered glass separators, from the working electrode. 

After the cell was assembled, the screws on the window holder were adjusted such that the window is parallel to the working electrode. Since the polished electrode surface was inevitably rounded to some extent, it was assumed that they were most parallel when the interference fringes were observed on the center of the electrode. 

Examination of the behaviour of a dilute solution of the substrate at a small electrode is a preliminary step towards electrochemical transformation of an organic compound. The electrode potential is swept in a linear fashion and the current recorded. This experiment shows the potential range where the substrate is electroactive and information about the mechanism of the electrochemical process can be deduced from the shape of the voltammetric response curve [44]. Substrate concentrations of the order of 10 molar are used with electrodes of area 0.2 cm or less and a supporting electrolyte concentration around 0.1 molar. As the electrode potential is swept through the electroactive region, a current response of the order of microamperes is seen. The response rises and eventually reaches a maximum value. At such low substrate concentration, the rate of the surface electron transfer process eventually becomes limited by the rate of diffusion of substrate towards the electrode. The counter electrode is placed in the same reaction vessel. At these low concentrations, products formed at the counter electrode do not interfere with the working electrode process. The potential of the working electrode is controlled relative to a reference electrode. For most work, even in aprotic solvents, the reference electrode is the aqueous saturated calomel electrode. Quoted reaction potentials then include the liquid junction potential. A reference electrode, which uses the same solvent as the main electrochemical cell, is used when mechanistic conclusions are to be drawn from the experimental results. 

One general method may always be used to reduce the effect of impurity adsorption on electrodes, and that is to work only for shorl times. Impurities take substantial times to adsorb. If the time in which the measurement is made is short enough, the adsorption aspect of impurity interference with electrode kinetic measurements can be reduced. Many of the techniques for doing this are described in Chapter 8 (transients). However, this approach does not eliminate the difficulty that at low current densities impurities in the solution may compete with electrons from the electrode. Further, although transient measurements may greatly reduce the adsorption of impurities during the measurement, it is difficult to arrange techniques so that the electrode is in contact with the solution for seconds only. 

The mediator lowers the required working electrode potential from 0.6 V to 0.2 V versus Ag AgCI. thereby improving the stability of the sensor and eliminating some interference by other species in the blood. 

One problem with glucose monitors is that species such as ascorbic acid (vitamin C), uric acid, and acetaminophen (Tylenol) found in blood can be oxidized at the same potential required to oxidize the mediator in Reaction 17-13. To correct for this interference, the test strip in Figure 17-10 has a second indicator electrode coated with mediator, but not with glucose oxidase. Interfering species that are reduced at electrode 1 are also reduced at electrode 2. The current due to glucose is the current at electrode 1 minus the current at electrode 2 (both measured with respect to the reference electrode). Now you see why the test strip has two working electrodes. 

Voltammetry is a collection of methods in which the dependence of current on the applied potential of the working electrode is observed. Polarography is voltammetry with a dropping-mercury working electrode. This electrode gives reproducible results because fresh surface is always exposed. Hg is useful for reductions because the high overpotential for H+ reduction on Hg prevents interference by H+ reduction. Oxidations are usually studied with other electrodes because Hg is readily oxidized. For quantitative analysis, the diffusion current is proportional to analyte concentration if there is a sufficient concentration of supporting electrolyte. The half-wave potential is characteristic of a particular analyte in a particular medium. 

The selective facilitation of the charge transfer of the species of interest is called electrocatalysis. In such a case, the species of interest are transformed at energies substantially lower than those of the interferants. The higher selectivity therefore implies a lower applied potential at the modified working electrode, which exhibits such selective electrocatalytic properties. In such a situation, the choice of the... 

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