Potentiometric Determination using Ammonia Selective Electrode

3 Potentiometric determination using ammonia selective electrode 

If an ammonia selective electrode is available, it can be used for measuring [ammonium] 

Although ammonium can be determined gravimetrically either as the hexachlo-platinate or tetnq henyl borate, either of the above methods is suitable for quick and accurate measurements. 

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Figure 5.7 shows a typical application of gas-diffusion membranes isolation of the circulating sample from a voltammetric or potentiometric electrode for the electrochemical determination of gaseous species. The ion-selective electrode depicted in this Figure includes a polymer membrane containing nonactin that is used for the potentiometric determination of ammonia produced in biocatalytic reactions. Interferences from alkali metal ions are overcome by covering the nonactin membrane with an outer hydro-... 

The gas-sensing electrodes also are used for the potentiometric measurement of biologically important species. An enzyme is immobilized at or near the gas probe. The gas sensor measures the amount of characteristic gas produced by the reaction of the analyzed substance with the enzyme. For example, an enzyme electrode for urea [NH2C(0)NH2] determination is constructed by the immobilization of urease onto the surface of an ammonia-selective electrode. When the electrode is inserted into a solution that contains urea, the enzyme catalyzes its conversion to ammonia ... 

The colorimetric Nesslerization method uses alkaline phenol and hypochlorite to react with ammonia to form indophenol blue in an amount proportional to the ammonia concentration. The blue color is intensified with sodium nitroprusside, and the concentration is measured using a calibrated colorimeter. The titration of basic ammonia is accomplished with standard sulfuric acid using a mixed indicator. Determination of ammonia may also be based upon the indophenol reaction adapted to automated gas-segmented continuous flow analysis. Potentiometric determination of ammonia is performed by ion-selective ammonia electrodes. 

A gravimetric method was proposed for Ag and other precious metals, based on reduction with dipyrone (39)121. Determination of Ag(I) ions, strongly complexed with ammonia, sulfite, thiocyanate or thiosulphate, was carried out by potentiometric titration with sodium A.A-diethyldithiocarbamate (11). The titration end-point was better defined when using a sulfide ion-selective electrode (ISE)137. Polypyrrole-based ISEs are poisoned by Ag(I) cations leading to modified selectivities. Ag(I) ions can be implanted... 

Selective electrodes are of special use in potentiometric techniques as applied to kinetic analysis on account of their usually high sensitivity to concentration changes. The glass (pH) electrode is very frequently used to determine enzymes and their substrates, which take part in reactions that typically involve release or uptake of hydrogen ions. The ammonia-selective membrane electrode has also been used for enzymatic analyses. 

A specific-ion meter is capable of direct potentiometric determinations of electroactive species using pH, redox, or various ion-selective electrodes (i.e., chlorine, calcium, nitrate, ammonia/carbon dioxide, copper, and halides). Microprocessor control allows for instrument calculation of analyte levels by known additions, standard additions, or activity. An automatic temperature-compensation feature helps to reduce analytical errors. 

A urea sensor has conventionally consisted of a urease-immobilized membrane and electrode for the determination of the metabolites electrodes for ammonium ion (20), pH (27), ammonia gas (22), and carbon dioxide (23) have been used. These sensors were all potentiometric ones to give Nernstian responses, the linear concentration range of which was typically between 10" and 10 M of urea. If one wishes to detect urea with using the ion- or gas-selective electrodes, urease is indispensable not only for the specific recognition based on the bioaffinity interaction but also for chemical conversion of urea to detectable species, since urea is essentially electro-inactive. On the contrary, the polymer-modified electrode described here does not rely upon enzymes the concentration of urea can be determined directly. 

In 1981, Meyerhoff and Fraticelli [35] appear to be the first to report on the coupling of a gas-diffusion separation system to a flow-through potentiometric detector. Ammonia was isolated from the donor stream containing the sample by penetrating a microporous membrane and collection in an acceptor buffer stream, and then determined selectively using a tubular nonactin p>olymer membrane electrode. The precision (<7% r.s.d.) and sample throughput (30 h ) of this early application was rather low. 

To date, the majority of enzyme-based potentiometric sensors do not involve detection with an ionophore-doped selective membrane and fall outside of the scope of this chapter. The same is also true for most Severinghaus-type gas sensors, where a gas-permeable membrane covers an inner solution in which the gaseous analyte is determined with an ISE. Most Severinghaus-type electrodes use a pH-sensitive glass electrode to monitor the pH of this inner filling solution. However, ammonia has been detected indirectly with an ammonium-selective ionophore-based ISEs upon protonation in that inner solution, and the use of other ionophore-based ISEs for the more selective detection both in enzyme-based and Severinghaus-type ISEs is readily conceivable. 

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