Nitrate determination

Rates of nitration determined over a range of temperatures in two-phase dispersions have been used to calculate energies of activation from 59—75 kj/mol (14—18 kcal/mol). Such energies of activation must be considered as only apparent, since the tme kinetic rate constants, NO2 concentrations, and interfacial area all change as temperature is increased. 

Discussion. The chloride solution is treated with excess of standard silver nitrate solution, and the residual silver nitrate determined by titration with standard thiocyanate solution. Now silver chloride is more soluble than silver thiocyanate, and would react with the thiocyanate thus ... 

Bromides can also be determined by the Volhard method, but as silver bromide is less soluble than silver thiocyanate it is not necessary to filter off the silver bromide (compare chloride). The bromide solution is acidified with dilute nitric acid, an excess of standard 0.1M silver nitrate added, the mixture thoroughly shaken, and the residual silver nitrate determined with standard 0.1 M ammonium or potassium thiocyanate, using ammonium iron(III) sulphate as indicator. 

Table 2.3 compares nitrate determinations in the presence of various interfering substances for this method and for two alternate methods - phenoldisulfonic acid and ion selective electrode methods. In general, the method proposed by Brown and Bellinger [123] is less subject to interference. 

The speed of the nitrate ion selective electrode makes its use potentially ideal for nitrate determinations on a large number of samples. However, the results from adding various interfering substances (Table 2.3) seem to cast some doubt upon the values obtained in the presence of chloride and bicarbonate, for although the results are precise, they are not accurate, being approximately 20-30% high. 

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. 

Durka W, Schulze ED, Gebauer G, VoerkeUus S (1994) Effects of forest decline on uptake and leaching of deposited nitrate determined from N and 0 measurements. Nature 372 765-767... 

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. 

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. 

Deters, B., J. P. Burrows, and J. Orphal, UV-Visible Absorption Cross Sections of Bromine Nitrate Determined by Photolysis of Br0N02/Br2 Mixtures, . /. Geophys. Res., 103, 3563-3570 (1998). 

One procedure that is widely used to circumvent these complications is to remove ambient ammonia from the sampled air without removing particles by inserting one of several types of diffusion denuders upstream from the filter(s). In fact, in a recent Environmental Protection Agency (EPA)-sponsored intercomparison of methods for determination of strong acid content of aerosols, all but one protocol utilized an ammonia denuder (63), and all used an impactor or cyclone to remove coarse particles. The presence of this denuder clearly prevents neutralization of acidic aerosols by ammonia but also disturbs the gas-aerosol equilibrium between sulfate-nitrate aerosols and gaseous species. Ammonia and nitric acid are released from the depositing particles (64, 65) and must be collected downstream if accurate particulate ammonium and nitrate determinations are to be made. If equal amounts of ammonia and nitric acid are released, then the absolute [H+] (neq/m3) will not be altered. No specific evidence is available in the literature to demonstrate alteration of the observed [H+] as the result of reequilibration, but this area deserves further study. 

Benzyl chloride and potassium selenocyanate give benzyl selenocyanate, which yields a para-compound on nitration, the latter also being obtained by the condensation of p-nitrobenzyl chloride with potassium selenocyanate. The strength of the nitric acid and the conditions of the nitration determine the resulting products in the case of chlorophenyl and tolyl compounds. Nitric acid (density 1 4) at 8° C. converts p-tolyl selenocyanate into di-2-nitro-p-tolyl diselenide and 2-nitro-p-tolylselenimc acid, whereas acid of density 1 5 at -10° C. yields 2-nitro-p-tolyl selenocyanate and a small amount of 3-nitro-p-tolyl selenocyanate. 

Other pollutants like atrazine, dichloroacetic acid, lindane, and trichloroethylene also undergo almost complete mineralization (X > 455 nm). The degradation of atrazine in general affords cyanuric acid as the final product when the photocatalyst is an unmodified titania material (30). The same was observed for TH. However, when 4.0% H2[PtCl6]/TH was employed, even cyanuric acid was mineralized by UV (A. > 320 nm) and visible (A, >455 nm) light as indicated by TOC and nitrate determinations (Fig. 7). After 6 h about 60% of the starting material was completely mineralized. 

Atmospheric NO2, SO2, and O3 were measured by various methods (1, 4, 5). light scattering was measured by integrating nephelo-meters. HNC g) and aerosol NC " were measured by the denuder difference method (6-8) using MgO-coated denuder tubes and nylon membrane filters, with ion chromatographic nitrate determination on alkaline filter extracts. Valid ammonia data were not obtained during any of the rain periods. 

Assay Dissolve about 250 mg of sample, dried at 105° for 2 h and accurately weighed, in 150 mL of water. Add 1 mL of nitric acid, and immediately titrate with 0.1 N silver nitrate, determining the endpoint potentiometrically, using silver-calomel electrodes and a salt bridge containing 4% agar in a saturated potassium nitrate solution. Perform a blank determination, and make any necessary correction (see General Provisions). Each milliliter of 0.1 N silver nitrate is equivalent to 7.455 mg of KCl. 

The quality control of pharmaceuticals is particularly important. Care must be taken to limit the levels of toxic metals in the final product. The acid dissolution. procedures described above (e.g. 6 M hydrochloric acid) are often equally applicable for the determination of impurities. Complete destruction of the matrix by wet oxidation or dry ashing may be necessary to obtain a completely independent method. Raw materials, catalysts, preparative equipment and containers are all possible sources of contamination. Lead, arsenic, mercury, copper, iron, zinc and several other metals may be subject to prescribed limits. Greater sensitivity is often required for lead and arsenic determinations and this can be achieved by electrothermal atomisation. Kovar etal. [112] brought samples into solution using 65% nitric acid under pressure at 170—180° C and, after adding ammonium and lanthanum nitrate, determined arsenic in the range 10—200 ng in a graphite... 

Calculate the potential of a zinc/silver cell in which the zinc electrode is immersed in a 0.0100 M zinc nitrate and the silver electrode is in 0.500 M silver nitrate. Determine which metal is reduced spontaneously. 

Braman, R. S., and Hendrix, S. A. (1989). Nanogram nitrite and nitrate determination in environmental and biological materials by Vanadium(III) reduction with chemiluminescence detection. Anal. Chem. 61, 2715—2718. 

Cresser, M. S. (1977). Nitrate determination by reduction to ammonia and gas-phase ultraviolet adsorption spectrometry. Analyst 102, 99—103. 

A metal will not always replace another metal in a compound dissolved in water. This is because metals differ in their reactivities. A metal s reactivity is its ability to react with another substance. In Figure 10-10 you see an activity series of some metals. This series orders metals by their reactivity with other metals. Single-replacement reactions like the one between copper and aqueous silver nitrate determine a metal s position on the list. The most active... 

Pentosans in wood cellulose, estimation of, >8 Perchlorate in potassium nitrate, determination of, 2... 

Sulfacetamide and other sulfonamides in pharmaceutical dosage forms have been determined with a silver sulfide electrode over the concentration range lO -lO 1 M. The sulfonamide is dissolved in dimethylformamide, 0.01 N silver nitrate is added, the solution filtered, and the resulting precipitate washed and the excess silver nitrate determined by potentiometric titration with 0.01 N sodium chloride (67,68). 

M. G. Mitrakas, et. al., Nitrate determination in sugar beet sap extracted with lead-acetate-lead monoxide using an ion-selective electrode, Commun. Soil. Sci. Plant Anal. 22 (5-6), 589-96 (1991). 

Table 1. Characteristics of the UV method for nitrate determination (without dilution)...

Nitrate determination in plant digests. With zone sampling one standard solution was sufficient to circumvent the influence of reducing agents in the extracts on the development of the colourforming reaction [350]. 

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