Microelectrodes polarization

We synthesized a new series of Cgo-derivatized PTs obtained by electropolymerization of bithiophenic precursors 238 and 239 in which one or two polymerizable groups are attached to Cgo by alkyl spacers of variable length [158]. The precursor structure involves a 3,4-ethylenedioxythiophene associated with a 3-alkylsulfanylthiophene. In addition to the decrease of electropolymerization potential, the sulfide group represents a convenient way for the functionalization of thiophene by the facile deprotection of a thiolate function [159]. The analysis of the photoelectrochemical response of poly(239) (n=3) and poly(240) on platinum microelectrodes polarized at -0.10 V and irradiated with intermittent white light shows that for both the initial peak and the stabilized current, the values of photocurrent are more than twice in the case of poly(239) ( = 3). 

Hydrogen Oxidation and Evolution on Platinum in Acids, Fig. 6 Polarization curves with increasing Pt area ratio, Sp (symbols) calculated using the dualpath kinetic equation (parameters determined from microelectrode polarization curve in Fig. 3) and their corresponding linear approximation, j = SpJRk r) (lines)... 

The main classes of plasticizers for polymeric ISEs are defined by now and comprise lipophilic esters and ethers [90], The regular plasticizer content in polymeric membranes is up to 66% and its influence on the membrane properties cannot be neglected. Compatibility with the membrane polymer is an obvious prerequisite, but other plasticizer parameters must be taken into account, with polarity and lipophilicity as the most important ones. The nature of the plasticizer influences sensor selectivity and detection limits, but often the reasons are not straightforward. The specific solvation of ions by the plasticizer may influence the apparent ion-ionophore complex formation constants, as these may vary in different matrices. Ion-pair formation constants also depend on the solvent polarity, but in polymeric membranes such correlations are rather qualitative. Insufficient plasticizer lipophilicity may cause its leaching, which is especially undesired for in-vivo measurements, for microelectrodes and sensors working under flow conditions. Extension of plasticizer alkyl chains in order to enhance lipophilicity is only a partial problem solution, as it may lead to membrane component incompatibility. The concept of plasticizer-free membranes with active compounds, covalently attached to the polymer, has been intensively studied in recent years [91]. 

Amperometric titrations with twin-polarized microelectrodes (biamperometric... 

Hi) Amperometric titrations with twin-polarized microelectrodes (or Biamperometric Titrations or Dead-stop-end-point method). 

AMPEROMETRIC TITRATIONS WITH TWIN-POLARIZED MICROELECTRODES (BIAMPEROMETRIC TITRATIONS OR DEAD-STOP-END-POINT METHOD)... 

Figure 17.4 Diagram of Amperometric Titrations With Twin-Polarized Microelectrodes.

Part—III exclusively treats Electrochemical Methods invariably and extensively used in the analysis of pharmaceutical substances in the Official Compendia. Two important methods, namely potentiometric methods (Chapter 16) deal with various types of reference electrodes and indicator electrodes, automatic titrator besides typical examples of nitrazepam, allopurinol and clonidine hydrochloride. Amperometric methods (Chapter 17) comprise of titrations involving dropping-mercury electrode, rotating—platinum electrode and twin-polarized microelectrodes (i.e., dead-stop-end-point method). 

Amperometric Titrations with Twin-Polarized Microelectrodes (Biamperometric Titrations or Dead-Stop-End-Point Method)... 

A new approach is to assemble a large number of microelectrodes together. Studies and applications of such micro-arrays are a growth area at present. In these assemblies, if each electrode is polarized to a different potential, then (in principle, at least) each one could then monitor the amounts of different analytes. 

The results of this analysis are summarized in Figure 37. Like prior workers studying thin films, the authors conclude that dense films without a TPB under small or cathodic polarizations operate primarily by a bulk path since the surface path is blocked. (Interestingly, they found that dense films under anodic polarization appear to operate under a mixed regime, although it is not clear how much nucleation and transport of O2 along the solid—solid interface contributes to the apparent surface path current.) In contrast, as the porosity is increased (microelectrode diameter is decreased), the surface path be-... 

Equations (3.105)-(3.107) point out the existence of three different polarization causes. So, 7km is a kinetically controlled current which is independent of the diffusion coefficient and of the geometry of the diffusion field, i.e., it is a pure kinetic current. The other two currents have a diffusive character, and, therefore, depend on the geometry of the diffusion field. I((((s corresponds to the maximum current achieved for very negative potentials and I N is a current controlled by diffusion and by the applied potential which has no physical meaning since it exceeds the limiting diffusion current 7 ss when the applied potential is lower than the formal potential (E < Ef"). This behavior is indicated by Oldham in the case of spherical microelectrodes [15, 20, 25]. 

It can be a further advantage of microelectrodes that they often increase the electrode resistance to bulk resistance ratio Rei/Rbuik- This is so because Re 1 frequently scales with the inverse area of the electrode, whereas the bulk resistance between a circular microelectrode and a counter-electrode is proportional to the inverse microelectrode diameter dme (see Sec. 4.1). Hence Rei/Rb iik ocbulk resistance decreases with decreasing microelectrode diameter. This is particularly helpful in order to investigate electrode polarization phenomena below the detection limit in experiments using macroscopic electrodes. (The reduced importance of the electrolyte resistance is also one of the reasons for ultramicroelectrodes to be applied in liquid electrochemistry [33, 34].)... 

Fig. 43. Double-logarithmic plot of the electrode polarization resistance versus the microelectrode diameter measured with impedance spectroscopy (ca. 800 °C) at (a) a cathodic dc bias of -300 mV, and (b) at an anodic dc bias of +300 mV. In (b) the first data point of the 20-pm microelectrode is not included in the fit. (c) Sketch illustrating the path of the oxygen reduction reaction for cathodic bias, (d) Path of the electrochemical reaction under anodic bias the rate-determining step occurs close to the three-phase boundary.

Bias-dependent measurements were performed in order to check to what extent the mechanism depends on the electrical operation conditions. Fig. 43 shows double-logarithmic plots of the electrode polarization resistance (determined from the arc in the impedance spectrum) versus the microelectrode diameter observed at a cathodic bias of —300 mV and at an anodic bias of +300 mV respectively. In the cathodic case the electrode polarization resistance again scales with the inverse of the electrode area, whereas in the anodic case it scales with the inverse of the microelectrode diameter. These findings are supported by I-V measurements on LSM microelectrodes with diameters ranging from 30-80 pm the differential resistance is proportional to the inverse microelectrode area in the cathodic regime and comes close to an inverse linear relationship with the three-phase boundary (3PB) length in the anodic regime [161]. 

Fig. 44. (a) Optical microscope image of a 60-pm LSM microelectrode after dc polarization with a large anodic bias (0.6 V). (b) A 74 x 74 pm AFM image of a microelectrode after dc polarization with 1.25 V. 

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