Cadmium reduction method

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. 

C. Gal, W. Frenzel, J. Moeller, Re-examination of the cadmium reduction method and optimisation of conditions for the determination of nitrate by flow injection analysis, Microchim. Acta 146 (2004) 155-164. 

Nitrate can also be converted to N02 using cadmium reduction (columns or spongy cadmium) as described above. Once the NOa" is in the form of N02, the N02 can be isolated via organic extraction (e.g., Olson, 1981) or with SPE after the N02 is converted to an azo dye (Kator et al, 1992). Nitrate can be isolated by conversion to N2O via sodium azide in an acetic acid buffer solution (Mcllvin and Altabet, 2005). Another approaches uses a genetically engineered denitrifier to convert N03 to N2O (Sigman et al, 2001) the bacteria wiU denitrify NOa" in a sample to N2O, but lacking nitrous oxide reductase the bacteria cannot take the reaction to completion and form N2. The N2O produced by either approach can then be analyzed on a mass spectrometer. A more detailed discussion of these methods is presented in Chapter 31 by Lipschultz, this volume. 

The salts associated with cured meats, nitrates and nitrites, are assayed by aqueous extraction and colorimetric determination. The ISO methods involve extraction in hot water containing borax solution and deproteination of a portion of the extract with Carrez reagents. Nitrite (ISO 2918 1975) is determined directly by formation of an azo dye with sulfanilamide and N-(l-naphthyl)ethylenediamine dihydrochloride which is measured at 538 mn. Nitrate (including nitrite) (ISO 3091 1975) is determined by cadmium reduction to nitrite and azo dye formation. Reduction of nitrate to nitrite may be carried out on a special column (as in the ISO method) or using spongy cadmium. Care must be taken to prevent interference from ascorbic acid if it is present in the sample at relatively high levels (>20pgml ). 

This analytical instrument was successfully used in several cruises for real-time monitoring of nutrient concentrations in seawater, incorporating a sampling unit for the in situ collection and online determination of phosphate, nitrate, and nitrite. The method is based on the initial determination of phosphate as its phosphoantrmonylmolybdenum blue complex, which is then oxidized online by nitrite and the decrease in absorbance is monitored at 880 nm. Nitrate is determined as the difference between total and initial nitrite content in a separate flow after reduction to nitrite in a cadmium-reductive column. 

USEPA Method 353.3 Analysis of Nitrate-Nitrite by Cadmium Reduction and Colorimetry. http //www.caslab.com/EPA-Method-353 3/ (accessed September 16, 2015). 

Reductive Methods.—Aldehydes from Acid Derivatives. Acid chlorides are reduced to aldehydes by sodium borohydride in the presence of cadmium(il)... 

ISO Standard 14673-1 2004/IDF Standard 189-1 2004. Milk and Milk Products— Determination of Nitrate and Nitrite Contents. Method Using Cadmium Reduction and Spectrometry. Geneva, Switzerland International Organization for Standardization, Brussels, Belgium International Dairy Federation, 2004. 

Sulphate, chloride (Cl ), nitrate (NO ), and ammonium (NH ) in water were measured with a Technicon Autoanalyzer II system by the methylthymol blue, mercuric thiocyanate, cadmium reduction and sodium nitroprusside methods, respectively. Sulphate and Cl in highly colored samples were measured by means of ion chromatography. Cations were analyzed with an atomic-absorption spectrophotometer by means of flame-emission spectrophotometry for potassium (K ) and... 

Nitrite can be deterrnined by reaction with sulfanilamide to form the diazo compound, which couples with /V-(1-naphthyl)ethylenediamine dihydrochloride to form an intensely colored red azo dye. Nitrate can be deterrnined in a similar manner after reduction to nitrite. Suitable reducing agents are cadmium filings or hydrazine. This method is useful at a nitrogen concentration of 10 -lO " M. 

Docosanedioic acid has been prepared by Wolff-Kishner reduction of 6,17-diketodocosanedioic acid, formed by reaction of the half-ester acid chloride of adipic acid with the a,co-cadmium derivative of decane (%26 overall yield).3 Reduction of Wolff-Kishner method, followed by simultaneous reduction and desulfurization with Raney nickel of the 2,5-bis(co-carboxyoctyl)thiophene pro-... 

The highly reactive cadmium can be prepared by two different methods. One approach is a room temperature reduction of CdC with lithium naphthalide in THF or DME. The second approach allows the preparation of the reactive metal in a hydrocarbon solvent. First, lithium naphthalide is prepared in benzene addition of this solution to CdC produces a highly reactive cadmium powder. 

Spencer and Brewer [111] have reviewed methods for the determination of nitrate in seawater. Classical methods for determining low concentrations of nitrate in seawater use reduction to nitrite with cadmium/copper [ 112,116,117] or zinc powder [113] followed by conversion to an azo dye using N- 1-naphthyl-ethylenediamine dihydrochloride and spectrophotometric evaluation. Malho-tra and Zanoni [114] and Lambert and Du Bois [115] have discussed the interference by chloride in reduction-azo dye methods for the determination of nitrate. 

Spencer and Brewer [144] have reviewed methods for the determination of nitrite in seawater. Workers at WRc, UK [ 145] have described an automated procedure for the determination of oxidised nitrogen and nitrite in estuarine waters. The procedure determines nitrite by reaction with N-1 naphthyl-ethylene diamine hydrochloride under acidic conditions to form an azo dye which is measured spectrophotometrically. The reliability and precision of the procedure were tested and found to be satisfactory for routine analyses, provided that standards are prepared using water of an appropriate salinity. Samples taken at the mouth of an estuary require standards prepared in synthetic seawater, while samples taken at the tidal limit of the estuary require standards prepared using deionised water. At sampling points between these two extremes there will be an error of up to 10% unless the salinity of the standards is adjusted accordingly. In a modification of the method, nitrate is reduced to nitrite in a micro cadmium/copper reduction column and total nitrite estimated. The nitrate content is then obtained by difference. 

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. 

Guevremont et al. [117] studied the use of various matrix modifiers in the graphite furnace gas method of determination of cadmium in seawater. These included citric acid, lactic acid, aspartic acid, histidine, and EDTA. The addition of less than 1 mg of any of the compounds to 1 ml seawater significantly decreased matrix interference. Citric acid achieved the highest sensitivity and reduction of interference, with a detection limit of 0.01 pg cadmium per litre. 

For removing low levels of priority metal pollutants from wastewater, using ferric chloride has been shown to be an effective and economical method [41]. The ferric salt forms iron oxyhydroxide, an amorphous precipitate in the wastewater. Pollutants are adsorbed onto and trapped within this precipitate, which is then settled out, leaving a clear effluent. The equipment is identical to that for metal hydroxide precipitation. Trace elements such as arsenic, selenium, chromium, cadmium, and lead can be removed by this method at varying pH values. Alternative methods of metals removal include ion exchange, oxidation or reduction, reverse osmosis, and activated carbon. 

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. 

To prepare metal hexacyanoferrate films, very frequently the following procedure was followed first a film of the respective metal, for example, cadmium [79], copper [80], silver [81], or nickel [82, 83] was elec-trochemically plated on the surface of a platinum electrode, and that was followed by chemical oxidation of the metal film in a solution of K3[Fe(CN)6], leading to the formation of the metal hexacyanoferrates. The same method has been used to produce films of nickel hexacyanoruthen-ate and hexacyanomanganate using the appropriate anions [83]. It is also possible to perform the oxidation of the deposited metals in solutions containing hexacyano-ferrate(II) by cyclic oxidation/reduction of the latter. In a similar way, films of copper heptacyanonitrosylferrate have been deposited [84]. 

Recently, Darowicki [29, 30] has presented a new mode of electrochemical impedance measurements. This method employed a short time Fourier transformation to impedance evaluation. The digital harmonic analysis of cadmium-ion reduction on mercury electrode was presented [31]. A modern concept in nonstationary electrochemical impedance spectroscopy theory and experimental approach was described [32]. The new investigation method allows determination of the dependence of complex impedance versus potential [32] and time [33]. The reduction of cadmium on DM E was chosen to present the possibility of these techniques. Figure 2 illustrates the change of impedance for the Cd(II) reduction on the hanging drop mercury electrode obtained for the scan rate 10 mV s k... 

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