Cadmium reductor

The flow injection technique is based on three main principles sample injection, reproducible timing, and controlled dispersion [128]. The dispersion can be described as limited, medium, or large in a colorimetric system based on a reaction between the sample and a suitable reagent, a medium dispersion is preferred. Thus in the flow injection determination of nitrate, the reductor column should not excessively increase the dispersion. In a copperised cadmium reductor, more than 90% of the total nitrate is reduced within 1 - 2 s with minimum risk of further reduction of nitrite [167]. Consequently, the reductor can be made very small, which results in a minimal increase of dispersion. 

Henrickson and Selmer-Olson [18] applied an autoanalyser to the determination of nitrate and nitrite in soil extracts. In an autoanalyser, the water sample, buffered to pH 8.6 with aqueous ammonia-ammonium chloride, is passed through a copperised cadmium reductor column. The nitrite formed is reacted with sulfuric acid and N-l-naphthylethylenediamine, and the extinction of the azo dye is measured at 520 nm. For soil extracts, the range and standard deviation are 0.5-1.0 and 0.007mg/1, respectively. 

Garcia Gutierrez [19] has described an azo coupling spectrophotometric method for the determination of nitrite and nitrate in soils. Nitrite is determined spectrophotometrically at 550 nm after treatment with sulfuric acid and N-1 -naphlhylclhylcnediamine to form an azo dye. In another portion of the sample, nitrate is reduced to nitrite by passing a pH 9.6 buffered solution through a cadmium reductor and proceeding as above. Soils were boiled with water and calcium carbonate, treated with freshly precipitated aluminium hydroxide and active carbon, and filtered prior to analysis by the above procedure. 

Tecator [20] has described a flow injection system for the determination of nitrate and nitrite in 2 mol/1 potassium chloride extracts of soil samples. Nitrate is reduced to nitrite with a copperised cadmium reductor and this nitrite is determined by a standard spectrophotometric procedure in which the soil sample extract containing nitrate is injected into a carrier stream. Upon the addition of acidic sulfanilamide a diazo compound is formed which then reacts with N-(l-naphthyl)ethylcncdiamine dihydrochloride provided from a second merging stream. A purple azo dye is formed, the intensity of which is proportional to the sum of the nitrate and the nitrite concentration. Nitrite in the original sample is determined by direct spectrophotometry of the soil extract without cadmium reduction. 

The cadmium reductor has been used by Treadwell as a substitute for the zinc reductor. One application was the reduction of chlorate to chloride. Perchlorate was reduced to chloride only in the presence of a small amount of titanium ion as a catalyst. 

A reaction need not go to completion before the sample enters the detector in FIA (15-20). The extent of reaction will be the same in all samples and standards if constant flow rates and sample volumes are maintained. Successful FIA systems have been used in which the extent of reaction was less than 10%. However, the extent of reaction is surprisingly high for many reactions with residence times less than 30 s. The reaction used for the determination of phosphate is greater than 90% complete in less than 15 s (13) even though the manual method calls for at least 5 min for full color development (2). This extent of reaction was accomplished by heating a portion of the manifold to 50 °C. Greater than 90% of the nitrate in seawater is reduced to nitrite in a cadmium reductor in less than 2 s (11, 12). The reaction of nitrite to form an azo dye is complete in less than 15 s (15). 

P.H. Petsul, G.M. Greenway, S.J. Haswell, The development of an on-chip micro-flow injection analysis of nitrate with a cadmium reductor, Anal. Chim. Acta 428 (2001) 155. 

There is no salt effect and no interference from the normal constituents of seawaters. Hydrogen sulphide, however, will form some cadmium or copper sulphide precipitations on the cadmium reductor. Unless high amounts of sulphide are passed through the column repeatedly, the reductor will stay active. 

A common source of error in N03 measurements is caused by a decrease in reductor efficiency over time, which has been reported as 1% per 4—6 h of autoanalyzer run-time (Garside, 1993). Reduction efficiency can decrease due to a decrease in surface area, precipitation of hydroxides on the cadmium, and loss of the copper coating. It is recommended that N02 and standards be analyzed... 

Reductor columns have been prepared from zinc, silver, lead, cadmium, bismuth, antimony, nickel, copper, tin, and iron. 

Other reductor systems can be used, which will yield equally satisfactory results [25]. These can be the Jones, lead, cadmium, iron, nickel, or bismuth reactors, with the Jones reactor being chosen for use in the compendial assay method. Liquid mercury amalgams can also be used as redactors, being prepared with zinc, cadmium, bismuth, lead, or tin. While the liquid amalgams are easier to handle, and are more rapid than are column reactors, none of these is as simple as the aluminum foil reductor. 

Nitrite may be determined by its absorbance at 522 nm after extraction from foods such as cheese or flour and reaction with a color reagent comprising sulfanilic acid and 1-naphthylamine in aqueous acetic acid. Nitrate is determined by this method after reduction to nitrites using cadmium in a modified Jones Reductor as described in AOAC method 976.14. 

In the case of nitrate, the majority of applications for its determination are based on copperized-cad-mium or cadmium-coated zinc reductor columns, which are generally used for reducing nitrate to nitrite. After formation of color, specific reagents are added and nitrate is determined from the difference between the total nitrite and nitrate concentrations. 

Nitrate prevails as an ion in seawater and is neither bound nor complexed (see Section 10.1.2). For the determination, it is reduced in a reductor (see Fig. 10-8) filled with copper-coated cadmium granules. The conditions of the reduction are adjusted so that nitrate is almost quantitatively converted into nitrite and not reduced further ... 

The determination of nitrate in seawater is not subject to interferences. It has been claimed that nitrate might occur together with small amounts of hydrogen sulphide. For thermodynamic reasons this can only be true for waters from the transition layers between oxic and anoxic environments where intense vertical turbulent mixing processes occur. In this case the hydrogen sulphide is precipitated on top of the reductor as copper or cadmium sulphide and does not interfere in the nitrate analysis. Nitrate values observed together with hydrogen sulphide should be interpreted with care. 

The required reactor path length (contact path of cadmium surface and sample) depends on the sample volumes and allowed reduction times. For example, 20cm of a 4 mm i.d. (internal diameter) reductor allow flow rates of about 6-8mL/min, Le., 50mL of sample are reduced in 6-8 min. The same reductor may be used for manual and flow-analysis. In flow-analysis with flow rates of about 2mL/min reduction is close to 100 % with a reductor 10 cm long (4 mm i.d.). Excessive length increases the flow resistance and the risk of blocking and... 

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