Oxygen potentiometric

Electrochemical analytical techniques are a class of titration methods which in turn can be subdivided into potentiometric titrations using ion-selective electrodes and polarographic methods. Polarographic methods are based on the suppression of the overpotential associated with oxygen or other species in the polarographic cell caused by surfactants or on the effect of surfactants on the capacitance of the electrode. One example of this latter case is the method based on the interference of anionic surfactants with cationic surfactants, or vice versa, on the capacitance of a mercury drop electrode. This interference can be used in the one-phase titration of sulfates without indicator to determine the endpoint... 

Almost all of the methods described in Chapter 23 can be used for in vivo analyses, both voltammetric and potentiometric ones. The former are used primarily in the analysis of organic substances, which, within certain ranges of potential, can be either oxidized or reduced. Another popular method is the amperometric determination of oxygen in different biological media with the Clark electrode (Section 23.3). 

The trityl radical (gold-coloured) is readily oxidized to peroxide (white) the comparable 2,4,6-tri-(tert-butyl)phenoxy radical (blue) in, e.g., cyclohexane was applied by Paris et al." to so-called free radical titration (either potentiometric or photometric) of oxygen or antioxidant (the latter by hydrogen abstraction). 

Trityl sodium is an extremely strong base, being soluble in ethers and aromatic hydrocarbons to give deep blood-red solutions so that in titration of an acid it can serve as its own visual indicator, although potentiometric detection is also possible a disadvantage is its high sensitivity to oxygen and moisture. 

Where R is the gas constant, T is the temperature, and F is the Faraday constant. Caused by the logarithmic correlation between the gas concentration and the voltage signal, the potentiometric measurement is best suited for measurements of small amounts of oxygen. A well-known application of this principle has been realized in the so called lambda-probe for automotive applications where they are used to control the lambda value within a small interval around 1 = 1. The lambda-value is defined by the relation between the existing air/fuel ratio and the theoretical air/fuel ratio for a stoichiometric mixture composition ... 

Potentiometric lambda-probes are not suitable for measuring the oxygen concentration at 1 3> 1. 

Zirconia cells similar to the ones employed in the present study, have been used i) by Mason et al (18) to electrochemically remove oxygen from Pt and Au catalysts used for NO decomposition. It was shown that electrochemical oxygen pumping causes a dramatic increase in the rate of NO decomposition (18,19), ii) by Farr and Vayenas to electrochemically oxidize ammonia and cogenerate NO and electrical energy (20,21), iii) by Vayenas et al (11,12,22,23) to study the mechanism of several metal catalyzed oxidations under open circuit (potentiometric) conditions. 

An amperometric technique relies on the current passing through a polarizable electrode. The magnitude of the current is in direct proportion to the concentration of the electroanalyte, with the most common amperometric techniques being polarography and voltammetry. The apparatus needed for amperometric measurement tends to be more expensive than those used for potentiometric measurements alone. It should also be noted that amperometric measurements can be overly sensitive to impurities such as gaseous oxygen dissolved in the solution, and to capacitance effects at the electrode. Nevertheless, amperometry is a much more versatile tool than potentiometry. 

L. C. Clark first suggested in 1956 that the test solution be separated from an amperometric oxygen sensor by a hydrophobic porous membrane, permeable only for gases (for a review of the Clark electrode see [88]). The first potentiometric sensor of this type was the Severinghaus CO2 electrode [150], with a glass electrode placed in a dilute solution of sodium hydrogenocarbonate as the internal sensor (see fig. 4.10). As an equilibrium pressure of CO2, corresponding to the CO2 concentration in the test solution, is established in the... 

The analytes typically determined by using this type of sensor are those usually addressed by gas-diffiision systems, viz. ammonia (or ammonium ion), carbon dioxide (or carbonates) and oxygen. The detection system used is most frequently photometric, fluorimetric or potentiometric, and can be integrated with or connected to the sensing microzone. The description below is based on the two choices shown in Fig. 5.4. 

An interesting effect of Ca2+ ions on the autoxidation of hydroxy-1,4-naphthoquinone, which may have a bactericidal function in sea urchins, has been reported. Autoxidation resulted in the detection of a semiquinone, which was accompanied by only minimal oxygen uptake. The presence of Ca2+ resulted in a massive enhancement in the rate of 02 uptake. Potentiometric titrations revealed that Ca2+ ions, by forming complexes with the compound, lowered the pKa values of its phenolic groups, thereby generating the oxidation-prone phenolate anions. Autoxidation was also facilitated by the spin stabilisation effect of the Ca2+ ions on the naphthosemiquinone.121 This effect may be of importance in mammalian cells, where quinones induce elevations in the free Ca2+ concentration. 

Figure 2 Schematic showing (a) potentiometric operation, (b) fuel cell operation and (c) electrochemical oxygen pumping.

If a cell is operated potentiometrically, then only two electrodes are required. However, if closed-circuit operation is to be employed it is necessary to measure electrode overpotentials. To measure electrode overpotentials it is necessary to use a three-electrode DC system. The system consists of a working electrode which is exposed to reaction conditions, a counter electrode (which is exposed to a constant partial pressure of oxygen in the case of an oxygen-ion conducting electrolyte), and a reference electrode. The reference electrode remains unpolarized during the measurements and may be exposed to either reaction conditions or the constant conditions of the counter electrode. A power supply is used to polarise the working and counter electrodes while the current is recorded (see Figure 3). 

An early use of oxygen-ion conducting solid electrolytes was in potentiometric devices for the measurement of oxygen partial pressure. In Section 2 it was shown that the e.m.f. of the cell relates the oxygen partial pressures on both sides through the Nemst equation,... 

Propene to acrolein. Hildenbrand and Lintz87,88 have used solid electrolyte potentiometry to study the effect of the phase composition of a copper oxide catalyst on the selectivity and yield of acrolein during the partial oxidation of propene in the temperature range of 420-510°C. Potentiometric techniques were used to determine the catalyst oxygen activity, and hence the stable copper phase, under working conditions. Hildenbrand and Lintz used kinetic measurements to confirm that the thermodynamically stable phase had been formed (it is known that propene is totally oxidised over CuO but partially oxidised over ). 

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