Cadmium flowing seawater

M. C. Yebra-Biurrun, A. Moreno-Cid and L. Puig, Minicolumn field preconcentration and flow-injection flame atomic absorption spectrometric determination of cadmium in seawater, Anal. Chim. Acta, 524(1-2), 2004, 73-77. 

A test piece of cadmium is placed (a) in flowing seawater, and (b) in stagnant seawater. Predict under which conditions cadmium would corrode. The following information is provided ... 

Olsen et al. [660] used a simple flow injection system, the FIAstar unit, to inject samples of seawater into a flame atomic absorption instrument, allowing the determination of cadmium, lead, copper, and zinc at the parts per million level at a rate of 180-250 samples per hour. Further, online flow injection analysis preconcentration methods were developed using a microcolumn of Chelex 100 resin, allowing the determination of lead at concentrations as low as 10 pg/1, and of cadmium and zinc at 1 pg/1. The sampling rate was between 30 and 60 samples per hour, and the readout was available within 60-100 seconds after sample injection. The sampling frequency depended on the preconcentration required. 

Fang et al. [661] have described a flow injection system with online ion exchange preconcentration on dual columns for the determination of trace amounts of heavy metal at pg/1 and sub-pg/1 levels by flame atomic absorption spectrometry (Fig. 5.17). The degree of preconcentration ranges from a factor of 50 to 105 for different elements, at a sampling frequency of 60 samples per hour. The detection limits for copper, zinc, lead, and cadmium are 0.07, 0.03, 0.5, and 0.05 pg/1, respectively. Relative standard deviations are 1.2-3.2% at pg/1 levels. The behaviour of the various chelating exchangers used was studied with respect to their preconcentration characteristics, with special emphasis on interferences encountered in the analysis of seawater. 

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). 

Anderson (11) was the first to report on the use of FIA for the analysis of seawater micronutrients. He developed a method for the simultaneous determination of nitrate and nitrite. The chemical reactions for the analysis of nitrate were based on the reduction of nitrate to nitrite by a copper-ized cadmium column placed in the flow path. The nitrite was then analyzed as an azo dye (11). This reaction sequence is conventionally used in both segmented CFA and manual analyses of nitrate and nitrite in seawater (2, 6, 7). The detection limits are 0.1 fxM for nitrate and 0.05 juM for nitrite. 

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. 

M.C. Yebra, A. Garcia, N. Cairo, A. Moreno-Cid, and L. Puig. Design of a field flow preconcentration system for cadmium determination in seawater by flow-injection-atomic absorption spectrometry. Talanta 56 777-785,2002. 

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