Nitrite selective electrode

Although the ISEs based on cobyrinates have good selectivity for nitrite over several anions, they also respond to salicylate and thiocyanate. To eliminate this interference, the nitrite-selective electrode based on ionophore 2 was placed behind a microporous gas-permeable membrane (GPM) in a nitrogen oxide gas-sensor mode (75). NOx was generated from nitrite in the sample at pH 1.7 and, after crossing the GPM, was trapped as nitrite by an internal solution that was buffered at pH 5.5 (0.100 M MES-NaOH, pH 5.5, containing 0.100 M NaCl). The internal solution was "sandwiched" between the nitrite-selective electrode and the GPM. 

Figure 6 shows the selectivity behavior of this NOx gas sensor. The sensor had a sub-Nernstian response toward nitrite, with slopes in the range of -45 to -50 mV/decade. Further, the response observed with salicylate and thiocyanate was diminished substantially, as compared to that obtained with the original nitrite-selective electrode (Figure 3). In addition, the gas sensor described here does not suffer interferences from nitrate, bicarbonate, acetate, benzoate, or chloride. These excellent selectivity properties of the sensor are a combination of the selectivity characteristics of the nitrite-selective electrode and the additional discrimination provided by the GPM.

In order to improve the practical application of potentiometric sensors, much effort must be devoted to developing membrane materials whose selectivity deviates fi-om the Hofmeister series. Chloride- and nitrite-selective electrodes based on lipophylic vitamin B12 derivatives have been reported by Simon.xhese electrodes were the first anion-selective electrodes of which selectivity sequence did not obey the Hofmeister series. Since then, a number of anion-selective liquid-membrane electrodes have been developed using a variety of hosts metallocenes, diphosphonium dication salts, diquartemary ammonium dication salts, bisthiourea derivatives, metalloporphyrins, lipophylic macrocyclic polyamines, cytosine-dependent triamine and metallophtalocyamines. " " Among this large variety of compounds studied, the metalloporphyrins seem to be the most promising for the preparation of effective ion-selective potentiometric sensors. " - ... 

A potentiometric SIA method was developed to quantify the nitrite contained in cured meat products. The assessment of nitrite concentration was accomplished by using the Gran s plot approach using a nitrite-selective electrode. A silver sulfate solution was used to remove the salt present in the samples since chloride ion is one of the most important interferents (Zarate et al., 2009). 

P. Schulthess, D. Ammann, B. Krautler, C. Caderas, R. Stepanek, and W. Simon, Nitrite-selective liquid membrane-electrode. Anal. Chem. 57, 1397-1401 (1985). 

The Department of the Environment UK [155] has described a number of alternative methods for the determination of total oxidised nitrogen (nitrate and nitrite) in aqueous solution, while specific methods for nitrate and nitrite are also included. Among the methods for total oxidised nitrogen, one is based on the use of Devarda s alloy for reduction of nitrate to ammonia, and another uses copperised cadmium wire for reducing nitrate to nitrite, which is determined spectrophotometrically. Nitrate may also be determined spectrophotometrically after complex formation with sulfosalicylic acid or following reduction to ammonia, the ammonia is eliminated by distillation and determined titrimetrically. Other methods include direct nitrate determination by ultraviolet spectrophotometry, measurements being made at 210 nm, and the use of a nitrate-selective electrode. Details of the scope, limits of detection, and preferred applications of the methods are given in each case. 

Elemental composition Na 27.08%, N 16.48%, 0 56.47%. An aqueous solution of the salt is analyzed for sodium by various instrumental techniques (See Sodium). Nitrate ion in solution can readily be measured by ion chromatography, nitrate-ion selective electrode, or various colorimetric methods, such as its reduction with cadmium to nitrite followed by diazotization. 

Fig. 1. Simultaneous separation and detection of anions and cations on a latex agglomerate column. Column Dionex HPIC-CS5 cation exchange column (250X2 mm) with precolumn HPIC-CG5 (50 X 4 mm) eluent 0.5 mM copper sulfate, pH 5. 62 flow rate 0.5 ml/min sample volume 20 gl containing 0.1 m M of each ion detection two potentiomet-ric detectors equipped with different ion-selective electrodes in series. Peaks (1) chloroacetate, (2) chloride, (3) nitrite, (4) benzoate, (5) cyanate, (6) bromide, (7) nitrate, (8) sodium, (9) ammonium, (10) potassium, (11) rubidium, (12) cesium, (13) thallium. Reprinted with permission from [10].

To 1 g sample, add 100 mL of 10% caustic soda solution and stir for 12 h. (This treatment is required only if iron cyanides are suspected to be present in the sample.) After this, adjust the pH to less than 8.0 with 1 1 H2S04. Add about 0.2 g sulfamic acid to avoid nitrate/nitrite interference. This is followed by addition of 25 mg lead carbonate (to prevent interference from sulfur compounds). The mixture is distilled and collected over NaOH solution. This distillate is analyzed for CN by colorimetric, titrimetric, or ion-selective electrode method. 

Gas-selective electrodes are used to a hmited extent in food analysis although, in certain areas of food production their use is quite common. The oxygen-selective electrode is often used on the production hnes for fruit and vegetable juices, where a vacuum has been apphed to remove air before packaging, this in order to check the residual oxygen content. S02-selective electrodes are used to determine the SO2 content of wines and other S02-treated liquids either directly or after the acid conversion of a component such as NaHSOa to SO2. Other electrodes are available for similar measurements, such as nitrogen oxide (for nitrite contents) and carbon dioxide (for either CO2 directly or CO2 from acid-converted carbonate or bicarbonate salts). There are perhaps other more accurate ways of determining such analytes, but the gas-selective electrode is... 

Several authors have also reported the potential use of electropolyme-rized metalloporphyrin films as new electrode materials for anion detection and pH measurements. For example, Daunert et al reported that anion-selective membrane electrodes can be prepared by electropolymerizing aniline-substituted cobalt tetraphenylporphyrin complexes onto a glassy carbon surface. It was shown that the resulting electrodes are highly selective towards the detection of thiocyanate and nitrite anions. The mechanism by which these metalloporphyrin electrode sensors operate is probably related to the well-known axial complexation capability of the central metal cation of the porphyrin towards different kind of anions and bases. The same authors also reported a detection limit of 5 x 10 M for these ion-selective electrodes which have lifetimes of more than 2 months... 

Nitrate and Direct determination In electrochemical nitrite procedures, ion-selective electrodes are... 

A variety of methods has been described for the determination of nitrogen species (Table 4) but not all are routinely used. The cadmium reduction method is widely used in both batch and automated (continuous flow) spectrophotometric methods. In this procedure, nitrate is reduced to nitrite, which is then determined by diazotization with sulfanilamide and coupling with N-(l-naphthyl)ethylenediamine dihydrochloride (NED) to form an intensely pink-colored azo dye. This chemistry can be incorporated in a flow injection manifold to allow rapid, automated, in situ determinations in a robust and portable manner. Other common techniques for nitrogen determination are the nitrate ion-selective electrode and ion chromatography. 

Nitrate Reduction of nitrite followed by formation of a purple diazo complex Absorbance at 205 nm or ion-selective electrode... 

Ammonia (distillation, titrimetric, ammonia-selective electrode, phenate method) nitrite (ion chromatography, colorimetric) nitrate (ultraviolet spectrophotometric screening, nitrate electrode, reduction) organic (Kjeldahl, persulfate method) lodimetric and modifications, membrane electrode Acid digestion, colorimetric methods Gravimetric, atomic absorption spectrometry, colorimetric Colorimetry, iodometric, ion-selective electrode Gravimetric, turbidimetric, colorimetric... 

The graph shows the effect of pH on the response of a liquid-based nitrite (NO 2) ion-selective electrode. Ideally, the response would be flat—independent of pH. 

Response of nitrite ion-selective electrode. Shaded region is where response is nearly independent of pH. [From S. j. West and X. Wen, 4m. Environ. Lab., September 1997, p. 15.]... 

A NOx gas sensor constructed from a molecularly-imprinted nitrate-selective electrode has also been developed in our laboratory (30). This gas sensor is produced by placing the nitrate-selective electrode behind a gas-permeable membrane. Also, there is a buffer compartment present between the gas-permeable membrane and the ISE. As before, NOx species that pass through the gas-permeable membrane are trapped in the buffer compartment as NO2 and NOs . In this case, the electrode responds proportionately to the amount of nitrate in the buffer, rather than nitrite as in the NOx sensor described above. The gas sensor based on the N03 -selective electrode offers advantages over the Severinghaus arrangement similarly to the other aforementioned gas sensors (SO2 and NOx). Detection limits for this gas sensor were on the order of 1x10 M, with response characteristics being retained for over 80 days. This lifetime is consistent with lifetimes of the molecularly-imprinted nitrate-selective electrodes. 

Stepanek R, Krautler B, Schulthess P, Lindemann B, Ammann D, Simon W (1986) Aquocyanocobalt(ni)-hepta(2-phenylethyl)-cobyrinate as a cationic carrier for nitrite-selective liquid-membrane electrodes. Anal Chim Acta 182 83-90... 

Figure 2. Selectivity pattern of an ion-selective electrode based on electropolymerized [Co(o-NH2)TPP]. The electrode was exposed to (1) thiocyanate, (2) perchlorate, (3) iodide, (4) nitrite, (5) salicylate, (6) Iromide, (7) chloride, (8) bicarbonate, and (9) phosphate. (Repr uced with permission from ref. 37. Copyright 1991 American Chemical Society.)...

In potentiometric sensors, an electrical potential between the working electrode and a reference electrode is measured at zero current conditions in a solution containing ions that exchange with the surface. The first potentiometric MIP sensor was prepared in 1992 by Vinokurov (1992). The substrate-selective polyaniline electrode was electrosynthesized with polypyrrole, polyaniline, and aniline-p-aminophenol copolymers. The development of an MIP-based potentiometric sensor was reported in 1995 by Hutchins and Bachas (1995). This potentiometric sensor has high selectivity for nitrite with a low detection limit of (2 + l)x 10 M (Fig. 15.10). 

A tetraruthenated porphyrin was electropolymerised onto glassy carbon and used to catalyse the oxidation of nitrite to nitrate, with the resultant current giving a selective measure of the concentration of nitrite ion [81]. As an alternative method, soluble poly(3-octyl thiophene) [82] was cast along with tridodecylmethylammonium chloride onto glassy carbon, to give electrodes with superior selectivity over PVC-based membranes to lipophilic ions such as bromide or nitrate. 

A hydrophobic cobyrinate (Figure 2, structure 2) was used to prepare solvent polymeric membranes (10). The typical membrane composition was 1% (w/w) ionophore, 66% (w/w) plasticizer and 33% (w/w) polymer. Electrodes prepared with this ionophore, dioctyl sebacate (DOS) and poly(vinyl chloride) (PVC) presented, at pH 6.6, the selectivity pattern shown in Figure 3. The response of the electrodes was near-Nernstian for salicylate, thiocyanate, and nitrite. Their selectivity behavior clearly deviates from that of the Hofmeister series, with nitrite being the anion that presents the larger deviation. 

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