Dual Electrode Systems

Assuming the redox reactions (12.116) and (12.117) occurring at the generator and collector electrode, respectively, the index k presents A and B. The dimensionless diffusion coefficient d = Dk/D becomes unity if identical D s are assumed for all 

This is the MWA transformation mentioned in Sect. i2.3.3.i, but appiied to bands. Inserting into Eq. (i2. i28) gives the time-dependent diffusion equation in conformai coordinates for species k, 

The computational domain in (9, F) coordinates is presented in Fig. 12.23. For diffusion limited redox reactions at the generator and collector, the boundary conditions in conformal coordinates are 

A maximum value of F has to be estimated from the expansion of the diffusion layer during the experiment. A value of 6 is sufficient, see (5.5) in Sect. 5.2, where T ax is the duration of the experiment to be simulated. Taking into account the position of the outer electrode edges in (X, Z) space, Fig. 12.22b, the maximum values of X are 

The current in units of Ampere is obtained by multiplication of these equations with nJFDAC l, where / is the length of the UMBE. 

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Other dual electrode systems that operate at steady state and show similar shielding and collection effects include microelectrode arrays and the scanning electrochemical microscope (SECM). With microelectrode arrays (Section 5.9.3), one monitors diffusion between two neighboring electrodes. In a similar way, one can use the SECM (Section 16.4) to study diffusion between an ultramicroelectrode tip and substrate electrode. In both of these systems, convective effects are absent and the time for interelectrode transit is governed by the distance between the electrodes. 

Like most of the older standard methods, BS2067 employs dual electrode systems intimate electrodes in contact with the specimen, and secondary rigid electrodes carrying the connections to the measuring device, Such methods suffer from the dangers of poor contact and or contamination through the use of adhesive materials. 

A simple and sensitive liquid chromatographic method coupled with ECL was developed for the separation and quantification of naproxen (a nonsteroidal antiinflammatory drug) in human urine. The method was based on the ECL of naproxen in basic NaN03 solution with a dual-electrode system. The detection limit was 1.6 x 10 g mL (S/N = 3) [58]. Furthermore, MEKC chromatography was used with ECL of Ru(bpy)3 as a fast and sensitive approach to detect an antipsychotic and antihypertensive drug, i.e., reserpine in urine. Field-amplified injection was used to minimize the effect of ionic strength in the sample and to achieve high sensitivity. In this way, the sample was analyzed directly without any pre-treatment with LOD (S/N = 3) to be 7.0 x 10 mol L [59]. 

Haroon et al. (38) developed a dual electrode system for the detection of vitamin K compounds. The device utilized two sequential generator/detector electrodes. Vitamin K was electrolyzed at the first electrode and the reaction products were then detected electrochemically at a second electrode. The minimum mass of vitamin that they claim could be detected was 100 pg. They applied their system to the analysis of rat liver extracts and claimed it was superior in both sensitivity and specificity to the UV detector. 

Brister MC (2015) Dual Electrode system for a continuous analyte sensor. US 7,831,287... 

In a similar way, electrochemistry may provide an atomic level control over the deposit, using electric potential (rather than temperature) to restrict deposition of elements. A surface electrochemical reaction limited in this manner is merely underpotential deposition (UPD see Sect. 4.3 for a detailed discussion). In ECALE, thin films of chemical compounds are formed, an atomic layer at a time, by using UPD, in a cycle thus, the formation of a binary compound involves the oxidative UPD of one element and the reductive UPD of another. The potential for the former should be negative of that used for the latter in order for the deposit to remain stable while the other component elements are being deposited. Practically, this sequential deposition is implemented by using a dual bath system or a flow cell, so as to alternately expose an electrode surface to different electrolytes. When conditions are well defined, the electrolytic layers are prone to grow two dimensionally rather than three dimensionally. ECALE requires the definition of precise experimental conditions, such as potentials, reactants, concentration, pH, charge-time, which are strictly dependent on the particular compound one wants to form, and the substrate as well. The problems with this technique are that the electrode is required to be rinsed after each UPD deposition, which may result in loss of potential control, deposit reproducibility problems, and waste of time and solution. Automated deposition systems have been developed as an attempt to overcome these problems. 

Aniline, methyl aniline, 1-naphthylamine, and diphenylamine at trace levels were determined using this technique and electrochemical detection. Two electrochemical detectors (a thin-layer, dual glassy-carbon electrode cell and a dual porous electrode system) were compared. The electrochemical behavior of the compounds was investigated using hydrodynamic and cyclic voltammetry. Detection limits of 15 and 1.5nmol/l were achieved using colourimetric and amperometric cells, respectively, when using an in-line preconcentration step. 

Again it seems not necessary to discuss the considerations of the chemical versus electrochemical reaction mechanism. It is clear from the extremely negative standard potential of silicon, from Eqs. (2) and (6), that the Si electrode is in all aqueous solutions a dual redox system, characterized by its OCP, which is the resultant of an anodic Si dissolution current and a simultaneous reduction of oxidizing species in solution. The oxidation of silicon gives four electrons that are consumed in the reduction reaction. Experimental results show clearly that the steady value of the OCP is narrowly dependent on the redox potential of the solution components. In solutions containing only HF, alternatively alkaline species, the oxidizing component is simply the proton H+ or the H2O molecule respectively. 

A dual-electrode liquid chromatography-electrochemistry (LCEC) system used in the detection and identification of flavanols and procyanidins in wines and grape seeds is a valuable tool (30). Voltammetric behavior of phenolic compounds by LCEC could provide information that cannot be obtained using HPLC with UV detection, for which the identification is usually based on a comparison of the retention time with that of standard compounds, especially for the identification of catechins and procyanidins with a small amount of sample available (30). Figure 10 shows the procyanidins commonly found in wines. 

Figure 5.15 Circuit for dual-rcference-electrode system.

Even before the introduction of miniaturized biosensor arrays, however, some systems able to simultaneously measure glucose and lactate had been reported in the literature. One example is provided by Osborne et al. [157], who described plastic film carbon electrodes fabricated in a split-disk configuration and then modified to obtain a dual biosensor. They achieved a continuous monitoring of these metabolites by placing the dual electrode in a thin-layer radial flow cell coupled to a microdialysis probe. The stability of the sensors was sufficient for short-term in vivo experiments in which the crosstalk, i.e., the percentage of current measured by one biosensor but due to product generated by the partner biosensor, was acceptable for an in vivo application. 

Phenol and the three dihydroxybenzenes (20, 42, 66) in water were determined by LLE with a hydrophilic solvent followed by amperometric titration. LOD was in the ppm range . A dual electrode in a FIA system has been used as detector for total phenols in wastewater. The upstream coulometric electrode has a large surface area and is used to eliminate compounds that cause interference and the second one is an amperometric electrode for oxidative detection of all phenols. Optimal results were found working with a phosphate buffer at pH 6.8, at potentials of +0.35 V and +0.78 V for the coulometric and amperometric electrodes, respectively. A high sample throughput of 60 per hour can be attained with RSD of 0.1-4%. This method is more reliable than the colorimetric method . The concentration of fenobucarb (142) in drinking water was determined after a short alkaline hydrolysis, and oxidation of the resulting 2-s-butylphenol with a GCE at 750 mV, pH 3.5 LOD was 3.6 x 1Q- M, RSD 3.74% for 1 x IQ- M (n = 11, p = 0.05)37 . 

This principle has been expanded to a dual electrode arrangement in which pH differences in the samples could be compensated (Durand et al., 1984). With a BuChE loading of 7.5 U/cm and under substrate saturation conditions the system was sensitive to micromolar inhibitor concentrations. The inhibition was markedly different with different pesticides. Such sensors are superior to physicochemical assays in that they detect the effectiveness of the inhibition. 

The simplest treatments of convective systems are based on a diffusion layer approach. In this model, it is assumed that convection maintains the concentrations of all species uniform and equal to the bulk values beyond a certain distance from the electrode, 8. Within the layer 0 x < 5, no solution movement occurs, and mass transfer takes place by diffusion. Thus, the convection problem is converted to a diffusional one, in which the adjustable parameter 8 is introduced. This is basically the approach that was used in Chapter 1 to deal with the steady-state mass transport problem. However, it does not yield equations that show how currents are related to flow rates, rotation rates, solution viscosity, and electrode dimensions. Nor can it be employed for dual-electrode techniques or for predicting relative mass-transfer rates of different substances. A more rigorous approach begins with the convective-diffusion equation and the velocity profiles in the solution. They are solved either analytically or, more frequently, numerically. In most cases, only the steady-state solution is desired. 

Dual working electrode systems have been developed which can be operated with the electrodes in parallel or in series. In the later one working electrode is located upstream of the other and is complementary in function. This system can be used with analytes which undergo reversible oxidation or reduction and in use enhances selectivity. In the former configuration the electrodes are mounted at 90° to one another in the flow-cell. Different fixed voltages can be applied to the electrodes and the response monitored and ratioed to check peak purity. Alternatively, one electrode can be operated as a cathode the other as an anode and oxidants and reductants determined simultaneously. 

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