Dissolved Oxygen Measurement Techniques

There are several techniques nsed to determine the dissolved oxygen content in a fluid. In practice, five general methods exist chemical, volumetric, tubing, optodes, and the electrochemical electrode (Carroll, 1991 van Dam-Mieras et al., 1992). This section will discuss these methods and some of their limitations and uses the emphasis, however, will be on electrochemical electrodes as they are the most common dissolved O2 sensors. 

1 Chemical Method. In the chemical method, a sample is taken from the reactor and the dissolved oxygen concentration is determined off-line using a titri-metric method. The use of chemical methods for systems that have rapidly changing dissolved oxygen content is limited because these methods are laborious, slow, and prone to error if not done correctly. 

Other chemical methods such as the NADH oxidation and phenylhydrazine oxidation have been employed to determine dissolved oxygen content (van Dam-Mieras et al., 1992), but are not frequently used. 

2 Volumetric Method. The volumetric method is simple and robust in principle, but rather inaccurate in practice. This method relies on the conversion of dissolved oxygen to carbon dioxide which is then driven out of solution. As the carbon dioxide is driven out of solution, it is collected and its volume is determined at a known pressure and temperature. Then, using the ideal gas law and an elemental balance for the oxygen to carbon dioxide reaction, the oxygen concentration is determined. While simple in theory and nearly unaffected by other compounds that might be in the sample, this method, similar to the chemical method, is slow and lacks the sensitivity needed for dynamic biological appUcations (van Dam-Mieras etal., 1992). 

4 Optode Method. A photometric transducer or optode can be used to measure gaseous and dissolved oxygen concentrations (Koeneke et al., 1999). Many types of optodes exist, of these the fluorescence quenching optode is most widely used for oxygen measurements (Turner and White, 1999). 

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Another technique for organics measurement that overcomes the long period required for the BOD test is the use of continuous respirometry. Here the waste (full-strength rather than diluted as in the standard BOD test) is contacted with biomass in an apparatus that continuously measures the dissolved oxygen consumption. This test determines the ultimate BOD in a few hours if a high level of biomass is used. The test can also yield information on toxicity, the need to... 

Substantial loss in sensitivity is expected for analytes with slow electron-transfer kinetics. This may be advantageous for measurements of species with fast electron-transfer kinetics in the presence of a species (e.g., dissolved oxygen) that is irreversible. (For the same reason, the technique is very useful for the study of electron processes.) Theoretical discussions on AC voltammetry are available in the literature (16-18). 

Jensen and Hvitved-Jacobsen (1991) developed a direct method for the determination of the air-water oxygen transfer coefficient in gravity sewers. This method is based on the use of krypton-85 for the air-water mass transfer and tritium for dispersion followed by a dual counting technique with a liquid scintillation counter (Tsivoglou et al 1965,1968 Tsivoglou andNeal, 1976). A constant ratio between the air-water mass transfer coefficients for dissolved oxygen and krypton-85 makes it possible to determine reaeration by a direct method. Sulfur hexafluoride, SF6, is another example of an inert substance that has been used as a tracer for reaeration measurements in sewers (Huisman et al., 1999). 

An indication of the effectiveness of the deaeration technique selected can be obtained by measurement of the final oxygen level in the medium. Deaeration should occur immediately prior to the use of the medium to prevent reaeration. However, to perform deaeration just prior to the point of use is not always practical. Therefore, one should generate data that support the use of media that have undergone some level of reaeration and still show acceptable levels of dissolved oxygen. 

Trick J. K., Stuart M., and Reeder S. describe the tools available to the field sampler for the collection of groundwater samples, methods of on-site water quality analysis, and the appropriate preservation and handling ofsamples. The authors discuss the merits of different purge methodologies and show how on-site measurements such as pH, specific electrical conductance (SEC), oxidation—reduction potential (ORP), dissolved oxygen (DO), temperature, and alkalinity can be used to provide a check on subsequent laboratory analyses. Techniques for the preservation and analysis of samples and quality assurance and quality control are also presented. 

Whilst most of these standardised methods must be performed within laboratories, some of them (mainly probe techniques such as those for measuring for instance pH, conductivity, temperature, turbidity, and dissolved oxygen) may be readily adapted for use in the field and can be used for making in-situ measurements. Others methods have been automated with standardized systems, based on FIA or CFA systems. This... 

Recent years have seen the development and refinement of chemical analytical techniques for the determination of trace amounts of dissolved oxygen (DO). For example, Kent et al. (1994) reported DO measurements with a precision of 20% below 30 yuM and a detection limit of 0.03 /jM. They found I >uM DO in suboxic zone groundwaters down gradient from a sewage disposal site. 

This leads to the concept of mass transfer calculation techniques in scaleup. Figure 36 shows the concept of mass transfer from the gas-liquid step as well as the mass transfer step to liquid-solid and/or a chemical reaction. Inherent in all these mass transfer calculations is the concept of dissolved oxygen level and the driving force between the phases. In aerobic fermentation, it is normally the case that the gas-liquid mass transfer step from gas to liquid is the most important. Usually the gas-liquid mass transfer rate is measured, a driving force between the gas and the liquid calculated, and the mass transfer coefficient, KqO or t a obtained. Correlation techniques use the data shown in Fig. 37 as typical in which KqO is correlated versus power level and gas rate for the particular system studied. 

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