Nitrate-selective electrode

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. 

Of special interest is the nitrate electrode, which has found many applications in quantitative analysis of nitrates in biological fluids and agricultural products. A nitrate-selective electrode [132] based on a tributyl-octadecylphosphonium ligand is also selective for perrhenate and perchlorate ions. 

Another method used for nitrate determination on dried and milled herbage employs the nitrate selective electrode. One of the first published methods was that of Paul and Carlson (1968). Other anions, especially chloride, can interfere. These authors removed chloride with silver resin, but Barker ef al. (1971) omitted the resin because it tended to foul the electrode and cause excessive drift. Normally the Cl N03 ratio is so low as not to interfere, but saline precipitation from coastal plots could affect this. The method was further modified to allow storage of extracts for up to 64 h by adding a preservative of phenyl-mercuric acetate and dioxane, both very toxic (Baker and Smith, 1969). This paper mentions the need to change the electrode s membrane, filling solution and liquid ion exchanger every 2 months to minimize chloride interference. It is easy to overlook electrode maintenance between batches of nitrate analyses, and this can lead to errors and sluggish performance. 

Hutchins RS, Bachas LG. Nitrate-selective electrode developed by electrochemically mediated imprinting doping of polypyrrole. Anal Chem 1995 67 1654-1660. 

Therefore, to prepare a selective anion sensor for less lipophilic anions in the series, the selective interactions between the ionophores and the targeted anion should be considered. Truly selective nitrate electrodes were developed by the electrochemical deposition of PPy onto glassy carbon electrodes in the presence of NaNOa [131]. As seen in Figure 8.79, the selectivity pattern of PPy(N03 ) electrodes exhibited a significant deviation firom the Hofineister series. The electrodes showed selectivity especially for nitrate and thiocyanate, while the commercially available nitrate-selective electrodes were affected by the more lipophilic anions including iodide and perchlorate. 

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. 

One kilogram of each species was crushed and homogenized after washing. Ten grams from the obtained puree was extracted with the 50 cm and lOmM CUSO4 solution after intensive shaking for 30 min. After that, the extracts were filtered, filled up to 100 cm, and analyzed with direct potentiometry using the commercial nitrate-selective electrode. The simple method was found sufficiently accurate and reliable. 

Kneebone, B.N. and Freiser, H., 1973. Determination of nitrogen oxides in ambient air using a coated-wire nitrate selective electrode. Anal. Chem. 45 449-452. 

Sutton PG, Braven J, Ebdon L et al (1999) Development of a sensitive nitrate-selective electrode for on-site use in fresh waters. Analyst 124 877-882... 

Volumetric methods used for hydrocarbon-type surfactants [1] are applicable to fluorinated surfactants, unless the solubility of the fluorinated surfactant imposes some limitations. Anionic fluorinated surfactants can be titrated potentiometri-cally with benzethonium chloride (Hyamine 1622), using a surfactant-selective [36] or a nitrate-selective electrode (Fig. 9.2). Cationic surfactants can be titrated... 

Add to a 150-mL beaker a sample consisting of about 0.15 mmoles anionic surfactant in a volume of about 50 mL H2O. Titrate potentiometrically with benzethonium chloride solution at a rate of 0.5 mL/min until well past the inflection point of the curve. An automatic titrator with 5-mL buret assembly is used, equipped with a nitrate-selective electrode (HNU Systems model lSE-20-31-00, Orion model 93-07, or equivalent) and a Ag/AgCl reference electrode with ground-glass sleeve. 

Since commercial surfactant electrodes are available, there is no reason for the analyst to prepare membranes and construct electrodes. Considerable know-how is involved in producing durable electrodes which behave reliably, so we should leave this to the instrument manufacturers. To illustrate this point, we may mention a study in which six electrodes were compared for use in end point detection of a titration of a cationic with an anionic. Two commercial surfactant-selective electrodes, two tetrafluoroborate selective electrodes, a nitrate selective electrode, and a homemade PVC-membrane electrode incorporating a tetraphenylborate salt were tested (135). Only one electrode, one of the commercial models, was traly suited for routine use, giving smooth potentiometric curves without reconditioning for over 100 titrations. The relative standard deviation of the end point was about 0.5%, while that for the other electrodes was 1.3-2.3%. The standard deviation of the end point potential was 3 mV for this electrode, compared to 8 mV or more for the other electrodes. Besides this, those electrodes not designed as surfactant electrodes required reconditioning (i.e., soaking in a dispersion of a surfactant ion pair) after 25 titrations in order to remain usable. 

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