Electrode: auxiliary indicator

Figure 12.2 The electrochemical cell has a 25 p-m Teflon spacer sandwiched between the electrode and a window (Cap2 or Mgp2) to provide an electrolyte layer of known and controlled thickness. Working, reference, and auxiliary electrodes are indicated. Construction materials are glass and Teflon.

The way in which the three main processes (electrode reaction, doublelayer charging, and conduction) at one electrochemical interface concomitantly influence the relation between current and voltage is illustrated in Fig. 1. In the experiments, the total electrochemical cell contains two such interfaces. For kinetic studies, however, this complication is usually eliminated by making the surface area of the electrode of interest (the working electrode or indicator electrode ) much smaller than that of the second electrode (the auxiliary electrode or counter electrode ). 

Solution potentiometric sensors, such as ion-selective electrodes and ISFETs, rely on measurement of potential difference between two electrodes, the indicator (working) and the auxiliary (reference). This signal has its... 

An example of amperometric methods used for analytical purposes is the sensor proposed in 1953 by Leland C. Clark, Jr. for determining the concentration of dissolved molecular oxygen in aqueous solutions (chiefly biological fluids). A schematic of the sensor is shown in Fig. 23.1. A cylindrical cap (1) houses the platinum or other indicator electrode (2), the cylindrical auxiliary electrode (3), and an electrolyte (e.g., KCl) solution (4). The internal solution is separated by the polymer... 

In electrochemical kinetics, the concept of the electrode potential is employed in a more general sense, and designates the electrical potential difference between two identical metal leads, the first of which is connected to the electrode under study (test, working or indicator electrode) and the second to the reference electrode which is in a currentless state. Electric current flows, of course, between the test electrode and the third, auxiliary, electrode. The electric potential difference between these two electrodes includes the ohmic potential difference as discussed in Section 5.5.2. 

Fig. 5.15 Basic circuit for the electrode potential measurement during current flow A is working (indicator, test), B, auxiliary and R, reference electrode connected by means of the Luggin capillary (arrow) and P, potentiometer. 

Electrochemical oxidation-reduction of eluting mixture components is the basis for amperometric electrochemical detectors. The three electrodes needed for the detection, the working (indicator) electrode, reference electrode, and auxiliary electrode, are either inserted into the flow stream or imbedded in the wall of the flow stream. See Figure 13.13. The indicator electrode is typically glassy carbon, platinum, or gold, the reference electrode a silver-silver chloride electrode, and the auxiliary a stainless steel electrode. Most often, the indicator electrode is polarized to cause oxidation of the mixture components... 

I. 4-methoxyacetophenone (30 //moles) was added as an internal standard. The reaction was stopped after 2 hours by partitioning the mixture between methylene chloride and saturated sodium bicarbonate solution. The aqueous layer was twice extracted with methylene chloride and the extracts combined. The products were analyzed by GC after acetylation with excess 1 1 acetic anhydride/pyridine for 24 hours at room temperature. The oxidations of anisyl alcohol, in the presence of veratryl alcohol or 1,4-dimethoxybenzene, were performed as indicated in Table III and IV in 6 ml of phosphate buffer (pH 3.0). Other conditions were the same as for the oxidation of veratryl alcohol described above. TDCSPPFeCl remaining after the reaction was estimated from its Soret band absorption before and after the reaction. For the decolorization of Poly B-411 (IV) by TDCSPPFeCl and mCPBA, 25 //moles of mCPBA were added to 25 ml 0.05% Poly B-411 containing 0.01 //moles TDCSPPFeCl, 25 //moles of manganese sulfate and 1.5 mmoles of lactic acid buffered at pH 4.5. The decolorization of Poly B-411 was followed by the decrease in absorption at 596 nm. For the electrochemical decolorization of Poly B-411 in the presence of veratryl alcohol, a two-compartment cell was used. A glassy carbon plate was used as the anode, a platinum plate as the auxiliary electrode, and a silver wire as the reference electrode. The potential was controlled at 0.900 V. Poly B-411 (50 ml, 0.005%) in pH 3 buffer was added to the anode compartment and pH 3 buffer was added to the cathode compartment to the same level. The decolorization of Poly B-411 was followed by the change in absorbance at 596 nm and the simultaneous oxidation of veratryl alcohol was followed at 310 nm. The same electrochemical apparatus was used for the decolorization of Poly B-411 adsorbed onto filter paper. Tetrabutylammonium perchlorate (TBAP) was used as supporting electrolyte when methylene chloride was the solvent. 

The indicator or working electrode is given first then, after a solidus or virgule ("/") or On a second line, the reference electrode. In three-electrode configurations the nature of the auxiliary or counter electrode Is given in column 13-... 

This column is also used to give the identity of the auxiliary or counter electrode when a three-electrode configuration was used, and to indicate whether the indicator or working electrode was subjected to chemical, mechanical, or electrolytic pretreatment. 

Fig. 14.10. Transmembrane electron movement and redox reactions. Also shown schematically are electrodes and circuit diagram for cyclic voltammetry. WE, working electrode SCE, saturated calomel electrode AE, auxiliary electrode, p, and /7 are chemical and electrochemical potentials, respectively. Bulk concentrations of reduced (RED) and oxidized (OX) species on either side of the membrane as indicated by subscripts 1 and 2 interface concentrations are designated by a superscripts (Reprinted from H. T. Tien, Aspects of Membrane Chemistry,...

Figures 21a-e show SEM SE plan-view images of the first sample at five selected PEVD steps, indicated by a to e in Table 2. The auxiliary phase coverage at the working electrode of the sensor increased with PEVD processing time and PEVD flux from a to e. After 14 steps of auxiliary phase deposition and sensor response testing, the final thickness of the product was about 3 pm, which was estimated from an SEM SE image of a cleaved cross-section sample (Eigure 2If).

The results indicate that sensor response behavior is not only related to the thickness of the auxiliary phase, but is also controlled by other working electrode geometric factors - most likely the aspect ratio of the working electrode surface. Because of the high aspect ratio, the response and recovery times of the sensor are... 

This problem can be alleviated by the use of an indicator-type reference electrode. When this is not possible (because of chemical incompatibility), an "auxiliary reference electrode" can be used. This usually consists of a platinum wire placed near the working electrode. While the potential of such an electrode is not stable or well defined, it can be measured just before application of the transient, and it can be safely assumed to be constant during the transient. The transient is then performed with the platinum wire (which has a very small internal resistance) acting momentarily as the reference electrode. 

The setup for this experiment is indicated in the block diagram of Fig. 1. The working-electrode compartment and the auxiliary-electrode compartments are physically separated (by a frit or other such device) to avoid unwanted mixing of cathode- and anode-electrolysis products. During the experiment either the electrolysis current, i, the... 

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