Carbon dioxide-sensitive electrode

Figure 4 (A) Combination pH-sensitive glass electrode with the reference electrode (Ag/AgCI). (B) Carbon dioxide gas electrode based on pH-sensitive glass electrode.

Mounting electrodes in a bioreactor is costly, and there is an additional contamination risk for sensitive cell cultures. Some other sensors of prac ticai importance are those for dissolved oxygen and for dissolved carbon dioxide. The analysis of gas exiting from a bioreactor with an infrared unit that detects carbon dioxide or a paramagnetic unit that detects oxygen (after carbon dioxide removal) has been replaced by mass spec trophotometry. Gas chromatographic procedures coupled with a mass spectrophotometer will detect 1 the volatile components. 

Since many new substances of interest are very poorly soluble in water, the assessment of the pKa in aqueous solution can be difficult and problematic. Potentiometry can be a quick technique for such assessment, provided the solubility of the substance is at least 100 pM. (Solutions as dilute as 10 pM can still be analyzed, but special attention must be given to electrode calibration, and ambient carbon dioxide must be excluded.) If the substance is soluble to only 1-10 pM and possesses a pH-sensitive UV chromophore, then spectrophotometry can be applied. CE methods may also be useful since very small sample quantities are required, and detection methods are generally quite sensitive. 

Being acidic, fluorocarbon ionomers can tolerate carbon dioxide in the fuel and air streams PEFCs, therefore, are compatible with hydrocarbon fuels. However, the platinum catalysts on the fuel and air electrodes are extremely sensitive to carbon monoxide only a few parts per million are acceptable. Catalysts that are more tolerant to carbon monoxide are being explored. Typical polarization curves for PEFCs are shown in Fig. 24-50. 

Volume 4 is dedicated to three important topics Catalysis (Part 4.1), Heterogeneous Systems (Part 4.2), and Gas Phase Systems (Part 4.3). The six chapters of Part 4.1 cover the most important aspects of electron transfer catalysis, from fundamental concepts to organic synthesis, from carbon dioxide fixation to protein catalysis, from redox modulation to biomimetic catalysis. Part 4.2 deals with the basic aspects and the latest developments in electron transfer on semiconductors, dye-sensitized electrodes, mono- and multilayers, intercalated compounds, zeolites, micelles and related systems. Part 4.3 covers gas phase systems, from atoms to small molecules, exciplexes, and supermolecules. 

Another important category of composite electrode sensors is that of enzyme electrodes. In these systems, the analyte is brought into contact with an enzyme immobilized on the surface of the sensor. The analyte then undergoes a catalytic reaction to yield a species for which an ISE is sensitive, for example, ammonia, carbon dioxide, or H" ions. 

Potentiometric gas sensors for the reaction products, NH3 and CO2, have also been employed. Since these measurements are based on gas diffusion through a hydrophobic membrane, no direct disturbances by sample constituents occur. As early as 1969, Guilbault et al. coupled immobilized urease with a carbon dioxide sensor. Anfalt et al. (1973) applied an ammonia gas sensitive electrode to urea assay. A major drawback of these sensors is their long response time which is due to the slow diffusion of the gases. Since it takes several additional minutes to reach a new baseline after each measurement, only a few samples can be processed per hour. Guilbault et al. (1985) therefore tried an NH3 electrode, the interned buffer of which was exchanged after each measurement (double injection electrode). This approach led to a substantial decrease of the washing time. 

In potentiometric enzyme electrodes lyases producing carbon dioxide or ammonia are used as terminal enzymes of sequences. In fact, the term enzyme sequence electrode was introduced on the occasion of the design of a potentiometric D-gluconate sensor containing gluconate kinase (EC 2.7.1.12) and 6-phosphogluconate dehydrogenase (EC 1.1.1.44) (Jensen and Rechnitz, 1979). The authors found that for such a sensor to function the optimal pH values of the enzymes and the transducer should be close to each other. Furthermore, cofactors, if necessary, must not react with one another nor with constituents of the sample. It was concluded that the rate of substance conversion in multiple steps cannot exceed that of the terminal enzyme reaction. A linear concentration dependence is obtained when an excess of all enzymes of the sequence is provided, i.e. complete conversion occurs of all substrates within the enzyme membrane. Different permeabilities of the different substrates results in different sensitivities. This is particularly important with combinations of disaccharidases and oxidases, where the substrate is cleaved to two monosaccharides of approximately the same molecular size. The above... 

Acid-base titrations can be performed in special hollow glass electrode cups which are available in different sizes. The increased sensitivity of modern pH meters allows the use of rather thick-walled electrodes which have a long life, even in routine laboratories. Sometimes it is desirable to exclude atmospheric carbon dioxide which necessitates special apparatus. See, for example, Sisco et al. (SIO) and Berret (B3). Most potentiometric methods are developed for special determinations,... 

Gas-sensing membrane electrodes use a gas-permeable membrane that allows measured species (as a dissolved gas) to pass through and be measured within the electrode. These probes are very selective and sensitive for gases such as ammonia and carbon dioxide. For carbon dioxide, the working electrode is usually a modified glass pH electrode covered in a teflon membrane. Carbon dioxide in solution forms carbonic acid which lowers the pH. 

Fig.l compares the in vivo performance of oxygen and carbon dioxide electrodes with a non-aqueous, solid polymer electrolyte with electrodes containing a conventional liquid electrolytes. These electrodes have been operating for several weeks with virtually unaltered sensitivity. 

Ion-selective electrodes (ISEs) are potentiometric sensors that include a selective membrane to minimize matrix interferences. The most common ISE is the pH electrode, which contains a thin glass membrane that responds to the H concentration in a solution. Other parameters that can be measured include fluoride, bromide, nitrate, and cadmium, and gases in solution such as ammonia, carbon dioxide, nitrogen oxide, and oxygen. ISEs do have their limitations including lack of selectivity and sensitivity and problems connected with conditioning of electrodes. Detection limits for nitrate-N, for example, are typically 0.098mgl for commercial field devices and have chloride as a major interferent. 

Figure 23-12 is a schematic showing details of a gassensing probe for carbon dioxide. The heart of the probe is a thin, porous membrane, which is easily replaceable. This membrane separates the analyte solution from an internal solution containing sodium bicarbonate and sodium chloride. A pH-sensitive glass electrode having a flat membrane is held in position so that a very thin film of the internal solution is sandwiched between it and the gas-permeable membrane. A silver-silver chloride reference electrode is also located in the internal solution. It is the pH of the film of liquid adjacent to the glass electrode that provides a measure of the carbon dioxide content of the analyte solution on the other side of the membrane. 

The potential obtained with the potential-sensitive barrel is called DC in the figure and the potential recorded with the ion-sensitive barrel is called pK + DC. The difference between these two potentials (called pK in the figure) is obtained with the differential amplifier and represents the potential that arises from the K" -activity. If electrodes were selected so that the tip potential of the reference barrel varied less than 2mV between 150mM KCl and 150 mil NaCl, and if we assume that the tip potential is the same in these solutions and in the brain then the resting level of in the brain corresponds to 3mM. This value was also obtained in the ventricle. The E.E.G. and E.C.G. are also shown. Before the rat was made anoxic at the electrical potential was stable but there are slow variations in pK (frequency of about 0.001 HZ, amplitude 5mV)., These variations disappered after the rat had been dead for a few minutes (heart stopped completely). A potential synchronous with the breathing was picked up equally well with the potential and the K" -sensitive barrel so that it cancelled out in the pK trace. The two small peaks, one at and one 1 minute before, show an electrostatic effect caused by the jet of gas mixture as it passed the head of the rat the first peak was caused by gas not directed at the tracheal cannula the second represents the onrush of gas that caused the anoxia. The immediate effect of the nitrous oxide + 5% carbon dioxide (start at ) is a decrease in pK of about 3mV (corresponding to a decrease in K" -activity of 16%). This decrease lasts for about 40 sec. and is followed by an increase in pK of about 5mV (which... 

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