Measurement with oxygen electrodes

The response time of oxygen electrodes must be in the range of 2-3 s, if fetfl is to be measured to an accuracy of 10 s [560]. The concentration of oxygen in the air, which continuously decreases upon bubbling through and therefore for high values of k a (particularly for chemisorption ) has to be taken into consideration, can pose a problem. 

A corresponding correction must be applied, if there is a significant depletion of the gas phase (low gas pressure and high ki a values). In this case equations (4.17) and (4.15) are used and the time-dependent saturation concentration c (t) is taken into consideration as follows  

The value of kia can only be genuinely determined with the unsteady-state methods, if the time constant of the electrode tg l/fetfl- h x ki a 0.2 should apply in the case of low viscosity liquids (no additional reduction of the response time by boundary layer effects). 

After saturation with nitrogen at 2 bar, the pressure is reduced to 1 bar while stirring the tank and the measurement can be repeated under different stirring conditions. Since the stirrer used is a hollow stirrer, a change in the stirrer speed affects both the stirrer power and the gas throughput (see Section 4.12.1). 

This measuring method is, due to its adaptability, very good for investigating coalescence phenomena in any physical system and is suitable for clarifying the chemical kinetics in mass transfer-limited reactions in gas/liquid systems (e.g. hydrogenations, oxidations, phosgenations, etc.) (see also [215]). 

---

Table I. Glycine (Gly) oxidation in mitochondria from etiolated wheat leaves, green pea leaves, etiolated pea leaves and etiolated pea stalks in relation to oxidation of 10 mM malate (Mai), 10 mM succinate (Succ) and 1 mM NADH measured with oxygen electrode in respiration medium in the presence of ADP. For glycine, malate and succinate 1 mM NAD was present during assay and for succinate 100 nM ATP was also included. Values are based on 2 -3 separate preparations from each source of material.

Initial Rates of Oxygen Consumption of Isolated Mitochondria Measured with Clark Electrode or with the Hb02 Method0... 

The diffusion coefficient of oxygen in solid silver was measured with a rod of silver initially containing oxygen at a conceim ation cq placed end-on in contact with a calcia-zirconia electrolyte and an Fe/FeO electrode. A constant potential was applied across dre resulting cell... 

The concentration of dissolved oxygen in a fermenter is normally measured with a dissolved oxygen electrode, known as a DO probe. There are two types in common use galvanic... 

In addition to the method described above, there are at present, on the market, several glucose analyzers offered by several commercial companies, wherein the individual injects approximately 20 microliters of serum. This is then incubated with glucose oxidase and the peroxide or oxygen generated is measured with an oxygen sensing electrode. These instruments cannot perform the test on 1 xl nor are they designed for that purpose at the present time. 

At almost all electrodes, reaction (15.20) occurs with appreciable polarization in both the cathodic and anodic directions the exchange CD of this reaction is very low 10 ° to 10 mA/cm. For this reason the equilibrium potential of this reaction is not established at the electrodes. The OCP measured in oxygen is between 0.85 and 1.1 V (RHE) that is, it is 0.15 to 0.4 V more negative than the equilibrium potential. 

The first enzyme biosensor was a glucose sensor reported by Clark in 1962 [194], This biosensor measured the product of glucose oxidation by GOD using an electrode which was a remarkable achievement even though the enzyme was not immobilized on the electrode. Updark and Hicks have developed an improved enzyme sensor using enzyme immobilization [194], The sensor combined the membrane-immobilized GOD with an oxygen electrode, and oxygen measurements were carried out before and after the enzyme reaction. Their report showed the importance of biomaterial immobilization to enhance the stability of a biosensor. 

Amphipol A8-35 was then added and the sample treated as described in 5.1 ("AP-trapped PS2, pH8"). Oxygen evolution was measured with a Clark-type oxygen electrode under continuous, saturating light. 

Fig. 1. Amperometric monitoring of the autoxidation of epigallocatechin gallete in the presence of (A) 0, (B) 2.0, (C) 5.0, (D) 10, (E) 20, and (F) 50 pm CuCl2. The measurements were performed in 0.1 M Tris buffer (pH 9.0) with a Clark type oxygen electrode at 28 °C. The epigallocatechin gallate concentration was fixed at 50 pm. The catechin stock solution was injected into the test solution at t = 0. The inset shows the (initial) steady-state autoxidation rate as a function of Cu2+ concentration. Reprinted from Biochimica et Biophysica Acta, vol. 1569, Mochizuki, M. Yamazaki, S. Kano, K. Ikeda,T., Kinetic analysis and mechanistic aspects of autoxidation of catechins, p. 35, Copyright (2002), with permission from Elsevier Science.

In soil analysis, pH, specific ion, oxidation-reduction (redox), electrical conductivity (EC) cells, and oxygen electrodes are commonly used. For each of these measurements, a different specific electrode along with a separate or integral reference electrode is needed. In some cases, with extended use or long exposure to soil or soil-water suspensions, electrodes may become polarized. When this happens, erroneous results will be obtained and depolarization will need to be carried out using the electrode manufacturers directions [3],... 

---