Nitrate electrodes

The following data were obtained using a nitrate electrode for a series of standard solutions of nitrate ... 

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. 

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. 

Paul, J.L. and Carlson, R.M. (1968) Nitrate determination in plant extracts by the nitrate electrode. Journal of Agricultural and Food Chemistry 16, 766-768. 

The values of [N03 ] are a function of mMT by regression analysis of nitrate electrode data. Similarly, regression analysis of the data of Krum-... 

Table VIII. Nitrate Electrode Measurements on Neodymium Nitrate—Nitiric Acid Solutions at Constant Total Ionic Strength (/)...

The nitrate ion activities shown in Figure 13 were measured using a special nitrate electrode standardization technique for samples with a high hydrogen ion concentration or acidity. Pure samples of known HN03 concentration and nitrate activity were analyzed with the nitrate... 

The percentage of sulfate ion interference for the nitrate electrode was calculated to be negligible using the empirical equation... 

Table I. Nitrate Electrode Data for Equilibrated Aqueous Phase...

The nitrate ion activities in the aqueous phase were measured with a nitrate ion selective electrode taking into account the presence of high hydrogen ion concentration by calibration of the nitrate electrode with nitric acid. The nitrate ion concentration in the organic phase owing to the extraction of neodymium complexes by HDEHP was determined by back-extraction of the organic phase with 3M sulfuric acid, dilution, and analysis with a nitrate ion electrode calibrated for different nitrate and sulfate concentrations. The amount of the nitrate species extracted into the organic phase increases as the initial neodymium nitrate concentration increases. 

ISEs designed for polyatomic anions such as N03 and CIO4 make use of strongly hydrophobic cations as ion exchangers. Typical examples of suitable cations are shown in table 9.7. The nitrate electrode responds to other anions, especially perchlorate. It is widely used to test for nitrates in soil samples and agricultural products. 

Sloan, C.H. and G.P. Morie Determination of nitrogen oxides in cigarette smoke with a nitrate electrode Tob. Sci. 18 (1974) 98-99. 

Shimizu Y., Okamoto Y., Yao S., MiuraN., and Yamazoe N., Solid electrolyte NO2 sensors fitted with sodium nitrate and/or barium nitrate electrodes, Denki Kagaku, 59, 465, 1991. 

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. 

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. 

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... 

Table 2. Typical selectivity coefficients of nitrate electrode interferants...

The importance of knowing selectivity coefficients can be seen by the example of the nitrate electrode given in Table 2. This electrode is much more selective to perchlorate than to nitrate anions, and another four anions also have to be present only in very low concentrations when nitrate is to be measured. Some other electrodes have higher selectivity, e.g., only OH ions interfere with F" ions when fluoride-selective electrodes are used. 

Until recently, nitrite could be assayed indirectly with ammonia gas or nitrate electrodes, respectively, following reduction to ammonium ion with nitrite reductase or oxidation to nitrate. A new PVC nitrite ISE, based on a lipophilic vitamin B12 derivative and bis(l-butylpentyl) adipate, permits nitrite assay in unmodified solutions of extracts from dried meat (69). The anion selectivity deviated from the classical selectivity sequence for anion exchangers, CIO4 >SCN >N03" >N02 >C1" x 10" and =... 

Operative lifetimes of the PVC entangled sensor systems so far described are considerably shorter than their solid-state counterparts, e.g. the lanthanum fluoride model. The principal cause is the loss of active component(s) from the polymer matrix. Thus, the deterioration of PVC potassium electrodes relates to the leaching of potassium membranes with an initial resistance of 5.8MD left in contact with deionized water for four days became paler and the resistance rose to 30MD compared with 50MQ for a PVC-only membrane. Similar, but slower, detrimental processes arise with PVC nitrate electrodes (74). 

Indicating electrode any of the electrode types described in section 3.6.2, and others, may be used. ASTM Method D 4251-89 [1] recommends the Orion 93-07 electrode, but an earlier version, D 4251-83, recommended the HNU ISE 20-31-00 nitrate electrode. Roth Scientific s RS 5000 electrode is also satisfactory. The Orion 93-05 fluoroborate electrode, which responds to cationics, can also be used. 

Specific ion electrodes have been used successfully in the determination of nitrate in fresh water Langmuir and Jacobson, 1970) but their application to the determination of nitrate in seawater is limited by interferences from most common anions, especially from chloride. For a detailed discussion of the nitrate electrode see Whitfield and Jagner (1981). 

Iren [63] measured the nitrate ion content of potato varieties after harvesting at different times. As sample preparation, the potatoes were sliced and dried at 65 °C to constant weight. One gram of the ground dried matter was shaken with 200 cm 0.04 M (NH4)2S04 for 30 min, and analyzed using commercial ion-selective nitrate electrode in a potentiomet-ric cell. 

The alcoholic content of the samples affects the potential of the ion-selective nitrate electrodes. Therefore, for analyzing the nitrate content of wine samples, the multiple standard addition method gives more accurate results than calibration curve-based potentiometric... 

The flow injection measuring technique combined with the detection with the ion-selective nitrate electrode proved very advantageous in the studies of Andac and coworkers [64]. 

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