Nitrate ISE

Figure 9.6. A—temperature electrode B—reference electrode suitable for ISE measurements C—nitrate ISE electrode D—combination electrode E—conductivity electrode.

The dropping ISE described by Skobets and coworkers [160, 161 ], where the ion-exchanger solution drops from a glass capillary, permits regular renewal of the electrode surface. A nitrate ISE with a renewable surface [162a] is shown in fig. 4.4. 

Fig. 4.4. Nitrate ISE with renewable surface 1 - outer body of ISE ...

Nitrate ISEs can also be used for determination of nitrogen oxides after oxidation to the pentavalent form (for example with H2O2) [39, 41, 101, 109, 150]. The nitrate ISE system is also useful for 2,4-dichlorophenoxyacetate [73] and for tetrathionate [215]. 

The perchlorate ISE is based on a similar complex as the nitrate ISE, but with Fe(II) as the central atom (VI see [180]). This system is also useful for... 

Nitrate ISEs have been formulated using amino acid betaines with the general formula (CH2=CHCH2)3N+CHRCOOH CT within styrene-butadiene membranes. These have been shown to give a Nernstian response from 10 10 7 M with selectivity over chloride (selectivity coefficient 0.0034), representing a significant improvement on commercial sensors with response times <1 min [23]. 

The sulfide ISE is sensitive, reliable, and useful especially in analyses of the atmosphere and waters. The nitrate ISE does not exhibit particularly good analytical properties, but it enables very fast and simple ori-entative determinations of nitrate in, e.g., waters, vegetables, and foodstuffs, which is welcome with regard to public hygiene. ISEs for various inorganic anions have somewhat lost their importance in competition with ion chromatography and those for inorganic cations often cannot compete with spectral methods nevertheless, ISEs for copper(II), lead(II), and cadmium(II) ions are sometimes useful for endpoint detection in complexometric titrations. 

The content of nitrogen oxides (NO ) in the air usually varies from 1 to over 100pgm . The nitrogen dioxide (NO2) gas-sensing probe suffers from interferences and thus the nitrate ISE is used more often for determination, after suitable trapping and conversion of NO . 

Nitrogen oxides can be chemisorbed on lead dioxide heated to 190°C, with formation of lead(II) nitrate, which is extracted into hot water and then measured with a nitrate ISE. Carbon dioxide, water... 

A personal NO dosimeter employs trapping of the gases on a glass filter impregnated with sodium dichromate and sulfuric acid (NO is oxidized to NO2), followed by passage of the NO2 through a silicone rubber membrane into an acidic solution of hydrogen peroxide in which nitrate is formed and measured by a nitrate ISE. 

Liquid membrane anion ion-exchanger type sensors are best illustrated by the nitrate ISE (95-96). This has seen a role in the environmental screening of waters and effluents (97,) fo> nitrate, but such use has been somewhat diminished by the high sensitivity and facility for multi-ion analysis of the newer approach of ion-chromatography. 

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). 

Fig, 5.12. Continuous concentration gradient titration of sulphide with mercuric nitrate, using an S ISE [40]. 

Examples of the use of FIA with ISE detection involve the determination of nitrate and total nitrogen in environmental samples [48, 49, 125, 166], potassium, sodium [125], calcium [51] and urea [124] in serum or major nutrients in fertilizers [73]. An interesting combination of an ISFET sensor with the FIA principle [52] is shown in fig. 5.17. This is a simultaneous determination of potassium, calcium and pH in serum during dialysis on an artificial kidney. 

Consequently, an ISE for nitrate for example, a strongly hydrophilic ion, must have a strongly hydrophobic ion-exchanger ion. This conclusion has been demonstrated experimentally for a series of NO3 ISEs based on tetra-alkyl-ammonium salts with long alkyl chains [161] (see fig. 7.2). It was found that, in the studied series of substances, the tetradodecylammonium ion which is... 

As a rather strongly hydrophilic anion, nitrate requires an ISE membrane containing a strongly hydrophobic cation, as described on p. 169. This function was fulfilled in the first nitrate electrode from Orion Research by cation V [180] in nitro-p-cymene 5. The electrode can be used in the pH range 4-7. In other commercial electrodes, the ion-exchanger ion is a tetra-alkylammonium salt, for example in the electrode from Coming Co., substance XIII in solvent 6 [27]. An ISE with a renewable membrane surface was found to be very useful (see section 4.1 and fig. 4.4), in which the ion-exchanger solution contains the nitrate of crystal violet VII dissolved in nitrobenzene [191]. The NOj ISE also responds to nitrites that can be removed by addition of aminosulphonic acid. 

Similar as for nitrate electrodes, various tetra-alkylammonium ions as ion-exchanger ions (see [77, 87, 170]) are useful for perchlorate and chlorate ISEs as CIOJ and CIO3 are less hydrophilic than NO3, requirements on the hydro-phobicity of the ion-exchanger ions are not as stringent. 

New polymer membrane-based ISEs for nitrate and carbonate exhibit detection limits and selectivities that may be applicable for ocean measurements. In addition, a number of these ISEs can be used as internal transducers for the design of useful potentiometric gas sensors. For example, dissolved C02 can be detected potentiometrically by using either a glass membrane electrode or a polymer-based carbonate ISE, in conjunction with an appropriate reference electrode, behind an outer gas permeable membrane. Novel differential pC02 sensors based on two polymer membrane-type pH sensors have also been developed recently. 

A nitrate-selective potentiometric MIP chemosensor has been devised [197, 198]. For preparation of this chemosensor, a polypyrrole film was deposited by pyrrole electropolymerization on a glassy carbon electrode (GCE) in aqueous solution of the nitrate template. Potentiostatic conditions of electropolymerization used were optimized for enhanced affinity of the resulting MIP film towards this template. In effect, selectivity of the chemosensor towards nitrate was much higher than that to the interfering perchlorate ( o3 cio4 = 5.7 x 10-2) or iodide ( N03, r = x 10 2) anion. Moreover, with the use of this MIP chemosensor the selectivity of the nitrate detection has been improved, as compared to those of commercial ISEs, by four orders of magnitude at the linear concentration range of 50 pM to 0.5 M and LOD for nitrate of (20 10) pM [197]. 

The sample is titrated with standardized silver nitrate (0.0141 M) in a continuously mixed beaker with an Ag electrode, an Ag/ AgCl electrode, or a chloride—ISE. The reference can be a glass mercuric sulfate, calomel, or Ag/ AgCl electrode. 

Many of the UV-VIS spectrophotometric methods (shown in Tables 12.3 and 12.6) have been automated by using flow analyzers. Thus, nitrite and nitrate,50,82 ammonium,50,83 orthophosphate,50,84,85 silicates,50,86 chloride,50,87 cyanide,50,88 and sulfate50,89 are measured by CFA and FIA. Oxygen is measured by iodometric titration51,90 and electrochemical methods91 (Table 12.7). Other dissolved gasses (Table 12.2) are measured by ISE-based gas sensors. 

Figure 3. Selectivity pattern of an ISE based on ionophore 2. The electrode was exposed to the following anions salicylate (1), thiocyanate (2), nitrite (3), perchlorate (4), iodide (5), benzoate (6), bromide (7), bicarbonate (8), hydrogen phosphate (9), nitrate (10), chloride (11), sulfate (12). (Reproduced with permission from ref. 10. Copyright 1989 Alan R. Liss.)...

Nitraminopropanol Nitrate, l NitriBnino-2 nitioxy-propane, 1 Nitramino 2 proponol Nitrate or N-(/3 Nitroxypropyl)nitr-amine called by Blomquist Fiedorek Ise-Me-NENA,... 

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