Potentiometric sensors hydrogen electrodes

More recendy, two different types of nonglass pH electrodes have been described which have shown excellent pH-response behavior. In the neutral-carrier, ion-selective electrode type of potentiometric sensor, synthetic organic ionophores, selective for hydrogen ions, are immobilized in polymeric membranes (see Membrane technology) (9). These membranes are then used in more-or-less classical glass pH electrode configurations. 

Figure 2 shows the structure of this sensor which is similar to that of the potentiometric sensor reported earlier (10). The only difference is that in this sensor a short circuit current between the sensing electrode and the counter electrode is measured with an ammeter. The proton conductor, antimonic acid (Sb205 2H20), was prepared from antimony trioxide and hydrogen peroxide according to a method described elsewhere (7,14). The sample powder was mixed with... 

The key component of a potentiometric sensor is the ion-selective electrode (ISE), an electrode or electrode assembly with a potential that is dependent on the concentration of an ionic species in the test solution and is used for electroanalysis. This interfacial potential at the electrode surface is caused by the selective ion exchange reaction. Ion-selective electrodes are often membrane type electrodes. The well-known glass pH electrode, which is selectively sensitive to hydrogen ions, is a typical ISE and has been used for years for the measurement of acidity or basicity of aqueous solutions in... 

Since the potential of selective potentiometric sensors is in a well-defined relationship with the activity of the detected ion, it is obvious that potentiometric electrodes can be used for pH measurements. The Nemst equation for the hydrogen ion is... 

Redox-based biosensors. Noble metals (platinum and gold) and carbon electrodes may be functionalized by oxidation procedures leaving oxidized surfaces. In fact, the potentiometric response of solid electrodes is strongly determined by the surface state [147]. Various enzymes have been attached (whether physically or chemically) to these pretreated electrodes and the biocatalytic reaction that takes place at the sensor tip may create potential shifts proportional to the amount of reactant present. Some products of the enzyme reaction that may alter the redox state of the surface e.g. hydrogen peroxide and protons) are suspected to play a major role in the observed potential shifts [147]. 

The change of electrode potential (E) of the catalase reaction with time was measured by a voltmeter. pH and E values for aqueous hydrogen peroxide were determined simultaneously for possible correlations between pH metric and potentiometric results of enzymatic activity of catalase-biomimetic sensors. The electrochemical unit was also equipped with a magnetic mixer. 

For the purpose of determining low hydrogen peroxide concentrations, the authors have designed the most cost-effective and simple to use potentiometric-biomimetic sensors based on immobilized catalase mimics. These sensors possess high hydrodynamic properties and the fastest speed of response. Figure 8.3 shows experimental data on catalase activity of biomimetic electrode in 0.03% aqueous H202. For the sake of comparison, catalase activities of aluminum electrode and aluminum electrode with applied adhesive are also shown. 

It is evident from the equation that potentiometric CO2 electrodes as well as amperometric O2 or H2O2 electrodes can be used as transducers. Both potentiometric and amperometric sensors have been covered by a layer of oxalate oxidase protected by a dialysis membrane (Bradley and Rechnitz, 1986 Rahni et al.f 1986a). The sensors had a pH optimum at pH 3.5-4. Diffusion control was reached at 1 U oxalate oxidase per electrode. Oxalate determination was not affected by ascorbic acid or amino acids. The hydrogen peroxide-detecting sensor (Rahni et al., 1986a) has been used to measure oxalate in urine diluted 1 40. 

The above authors coimmobilized choline oxidase and AChE on a nylon net which was fixed to a hydrogen peroxide probe so that the esterase was adjacent to the solution. The apparent activities were 200-400 mU/cm2 for choline oxidase and 50-100 mU/cm2 for AChE. The sensitivity of the sequence electrode for ACh was about 90% of that for choline, resulting in a detection limit of 1 pmol/l ACh. The response time was 1-2 min. The parameters of this amperometric sensor surpass those of potentiometric enzyme electrodes for ACh (see Section 3.1.25). Application to brain extract analysis has been announced. 

But the most smdied electropolymerizable porphyrins are the tetra-amino-and tetrahydroxyphenyl-substituted ones. Indeed, both of them were developed to elaborate electrodes having potentiometric responses to several kinds of anions, such as iodide for example . Co(II) tetrakis(p-hydroxyphenyl)porphyrin 53-based films were used for the elaboration of potentiometric and fibre optic pH sensors with minimal interferences from anions. Zn(II)-53-based films were also used and characterized for kinetics studies of hydrogen evolution at their surface . Zn(II)-53 as well as Pd(II)-53 and free base tetra(4-methyl pyridinium) porphyrin (H2-54) were electropolymerized on indium tin oxide substrates, leading thus to starting materials for the making of donor/acceptor... 

Glass electrodes sensitive to hydrogen ions are the most commonly used sensors in chemistry and related disciplines. They belong to the group of potentiometric... 

Typical electrochemically detectable (co-) substrates and products include oxygen, hydrogen peroxide, hydrogen ion, ammonia, carbon dioxide, reduced or oxidized cofactors, and redox-active (oxidized or reduced) prosthetic groups in oxi-doreductases, all of which can easily be converted into electrical signals by suitable transducers (e.g., amperometric or potentiometric electrodes or conductometric sensors. Table 19). 

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