Nitrate-sensing electrode

Example Nitrate-sensing electrode is employed to cater for a cell which will be sensitive exclusively to nitrogen dioxide (N02). The equilibrium of such a reaction may be represented as follows ... 

The nitrate-sensing electrode allows the determination of N02 in the presence of certain specific gases only, for instance, NH3, S02 and C02, that will also affect the change in pH of the internal electrolyte solution significantly. 

Figure 22. (A) The assembly of a nitrate-sensing electrode by the cross-linking of an affinity complex formed between nitrate reductase (cytochrome-dependent, EC 1.9.6.1), NR and an Fe(III)-protoporphyrin reconstituted de novo four-helix-bundle protein. (B) Cyclic voltammograms of the NR-two heme-reconstituted de novo protein-layered Au electrode at nitrate concentrations of (a) 0, (b) 12, (c) 24, (d) 46 and (e) 68 mM. Inset calibration curve for the amperometric response of the electrode at different nitrate concentrations (at E = —0.48 V vs. SCE). Potential scan rate, 5 mV s" 0.1 M phosphate buffer, pH 7.0, under argon electrode roughness factor, ca. 20.

It is possible to increase the selectivity of the gassensing probe by using an internal electrode sensitive to some species other than hydrogen ion for example, a nitrate-sensing electrode can be used to construct a probe that is sensitive to nitrogen dioxide. For this device, the equilibrium is... 

Fig. 26 (a) The assembly of a nitrate sensing electrode by the cross-linking of an affinity... 

Modern potentiometry with variety of classical and membrane sensing electrodes has, so far, rather occasionally been used in ion-chromatography. The first applications of poten-tiometric detection appeared in mid-seventies, when it was utilized for very sensitive detection of halide with silver/silver chloride electrode, and nitrate and nitrite with liquid-state membrane nitrate electrode.In later works number of indicating electrodes used for po-tentiometric detection has significantly increased. A list of reported applications is shown in Table 1. 

A nitrate sensor has been also reported by Kim et al.." This sensor uses a simple electrochemical system composed of a silver sensing electrode, a silver oxide reference electrode, and a platinum counter electrode in 0.01 M NaOH concentrically distributed electrolyte. These micro-electrodes are microfabricated on a silicon substrate, providing an electrochemical microcell ion which a single microfluidic channel is used to deliver the reagents. Nitrate concentration is detected using double-potential step chronocoulometry in which the current accrued from nitrate reduction to nitrite is integrated. 

Bacterial electrodes [11, 31, 33, 46, 48, 49, 60] In this type of electrode, a suspension of suitable bacteria is placed between the sensor proper and a dialysis membrane that prevents passage of high-molecular substances (see fig. 8.3). The sensor is usually a gas probe. In the simple types of bacterial electrode, the determinand is converted by a suitable strain of bacteria into a product sensed by the gas probe. Thus it is possible to determine arginine [46], glutamine [48],/.-aspartic acid [31],/.-histidine [60] and nitrate [33]. Hybrid bacterial - enzyme electrodes contain both a bacterial strain and a suitable enzyme. For example, an extract from ivingas Neurospora chossa can be used as a source of NAD nucleosidase and an Escherichia coli culture as a source of nicotinamide deaminase, so that the electrode responds to NAD [49] as a result of the series of reactions... 

Hadjidemietriou [25] used an Orion Model 93-07 nitrate ion-selective electrode with a 1 x 2 sensing module construction, and an Orion Model 90-02 double junction reference electrode fitted on a pH meter. The outer chamber was filled with 0.04 M ammonium sulfate solution and the inner chamber with Orion 90-00-02 solution. 

The apparatus used in this method consisted of a Corning liquid junction nitrate ion-selective electrode operating through a Pye Model 291 pH meter. This electrode has a flat end incorporating the sensing membrane. Also used... 

Measurements of nitrate in soils and waste waters using the TOAN-DBP electrode and the brucine method agreed closely. All these nitrate ISEs (66,67) are subject to serious interference from iodide, chlorate and perchlorate. This feature can in turn be exploited, e.g., the Corning nitrate exchanger may be readily converted to a viable chlorate-sensing cocktail using an extraction technique as described for the uranyl phosphate sensor (section 3.2.10). 

An example of the simplest (in the sense of the number of kinetic parameters) electrochemical reaction is reduction of silver ions (Ag+) from a dilute aqueous solution of a well soluble silver salt (e.g., nitrate) in the presence of excess of an indifferent salt (e.g., potassium nitrate) on a liquid silver-mercury alloy (also called amalgam) electrode. Besides the transfer of a single electron, only diffusion steps are involved in this process. The entire reaction can be very well modeled and the kinetic parameters are determined experimentally with high level of accuracy. The information gleaned while analyzing the mechanism of silver ion reduction can be used in elucidating more complex, multi-step, multiphase processes, such as the electrochemical reaction in a lithium-ion cell. 

Ion selective electrodes have historically been used in soil testing laboratories to conduct standard chemical soil tests, especially soil pH measurement. Many researchers in the 1970s and 1980s concentrated on the suitability of ISEs as an alternative to routine soil nitrate testing. More recently, researchers whose end goal was a mobile macronuttient sensing system have reported on laboratory tests of components of such systems. 

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