Electrode techniques, partial pressures

Such an electrochemical arrangement can also be used to transport oxygen from one electrode to the other by the imposition of an externally applied potential. This technique, known as coulometric titration , has been used to prepare flowing gas mixtures of oxygen/argon with a controlled oxygen partial pressure, to vary the non-stoichiometry of oxides, to study the thermodynamics of dilute oxygen solutions in metals, and to measure the kinetics of metal oxidation, as examples. 

Quantifying Reactions 7.64 and 7.65 requires fixing the pH and partial pressure of 02 (p02) at some predetermined value and providing OH" upon demand. This is accomplished with a pH-stat technique. The technique utilizes a pH electrode as a sensor so that as OH" is consumed, (during Mn2+ oxidation), the instrument measures the rate of OH consumption and activates the autoburete to replace the consumed OH . It is assumed that for each OH" consumed, an equivalent amount of Mn2+ is oxidized. 

Although the basic principles of type III potentiometric sensors are apphcable for gaseous oxide detection, this should not obscure the fact that these sensors still require further development. This is especially true in view of the kinetics of equilibria and charged species transport across the solid electrolyte/electrode interfaces where auxiliary phases exist. Real life situations have shown that, in practice, gas sensors rarely work under ideal equilibrium conditions. The transient response of a sensor, after a change in the measured gas partial pressure, is in essence a non-equilibrium process at the working electrode. Consequently, although this kind of sensor has been studied for almost 20 years, practical problems still exist and prevent its commercialization. These problems include slow response, lack of sensitivity at low concentrations, and lack of long-term stability. " It has been reported " that the auxiliary phases were the main cause for sensor drift, and that preparation techniques for electrodes with auxiliary phases were very important to sensor performance. ... 

By chemically releasing the bound 02 in a closed chamber, the initial content was proportional to the final partial pressure as measured with an 02 electrode (24). This technique has been miniaturized to analyze blood samples down to 5 /J in volume (25, 26). A continuous flow system based on this measurement was used to monitor continuously the 02 content in dogs (27). 

Dynamic methods constitute the third category. Starting from de-saturated blood, Duvelleroy et al. (31) measured the increasing po2 as 02 was absorbed out of a closed gas phase. The partial pressure in the gas could be quantitatively related to the amount absorbed by the blood. Longmuir and Chow (32) further simplified this technique by generating 02 by electrolysis, thereby reducing the need for the gas phase electrode. 

Various techniques are used for the fabrication of semiconductor sensors. Conductance sensors from structurized sintered polycrystalline ceramics can be produced by thick- or thin-film technology. Chemically sensitive materials in the form of single crystals or whiskers can be attached to electrodes by thin- or thick-film techniques as well. Mass production of sensors requires that the resulting devices be characterized by a defined level of conductance. For example, the conductance of polycrystalline Sn02 can be adjusted by subsequent thermal treatment >800°C under a controlled partial pressure of oxygen. Another approach to defined conductance involves doping the semiconductor with antimony or fluorine. The reproducibility and stability of a sensor signal... 

The performance of a fuel cell is affected by many factors such as PEM conductivity, catalyst (Pt) loading level, membrane electrode assembly (MEA) fabrication techniques, fuel (hydrogen, methanol, etc.) and its feeding conditions (pressure, flow rate, humidification, etc.), oxygen/air feeding conditions (partial pressure, flow rate, humidification, etc.), and operation temperature. The fabrication techniques of MEA... 

In thermospray interfaces, the column effluent is rapidly heated in a narrow bore capillary to allow partial evaporation of the solvent. Ionisation occurs by ion-evaporation or solvent-mediated chemical ionisation initiated by electrons from a heated filament or discharge electrode. In the particle beam interface the column effluent is pneumatically nebulised in an atmospheric pressure desolvation chamber this is connected to a momentum separator where the analyte is transferred to the MS ion source and solvent molecules are pumped away. Magi and Ianni (1998) used LC-MS with a particle beam interface for the determination of tributyl tin in the marine environment. Florencio et al. (1997) compared a wide range of mass spectrometry techniques including ICP-MS for the identification of arsenic species in estuarine waters. Applications of HPLC-MS for speciation studies are given in Table 4.3. 

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