Colorimetric detection selective oxidation

A number of composition analyzers used for process monitoring and control require chemical conversion of one or more sample components preceding quantitative measurement. These reactions include formation of suspended solids for turbidimetric measurement, formation of colored materials for colorimetric detection, selective oxidation or reduction for electrochemical measurement, and formation of electrolytes for measurement by electrical conductance. Some nonvolatile materials may be separated and measured by gas chromatography after conversion to volatile derivatives. 

Thin films of Pcs have shown to undergo reversible electrochromic changes depending upon their oxidation state [169,250,251], An example of ions detection by a Pc in aqueous solution has also been recently reported. In such a system, the RuPc 32 (Fig. 31a) experiences a dramatic color change due to the Cu(II)-promoted, one-electron oxidation of the macrocycle, thus providing a useful tool for the selective and highly sensitive colorimetric detection of copper(II) ions in neat aqueous solutions (Fig. 31b) [252],... 

A liquid emulsion membrane based on trioctylphosphine oxide extractant was applied for separation of uranium and thorium from Ce, Cu and Cd [2]. Highly selective U02 imprinted polymer (synthesized using uranium vinylbenzoate, divinylbenzene in styrene and 2,2 -azobisisobutyronitrile) was proposed for purification, preconcentration and determination of the uranyl ion [3]. Dibenzoylmethane was used as a colorimetric reagent. The detection limit of 160 ppt was achieved. 

EC sensors are relatively sensitive, as they react to chemical vapor concentrations at the low parts-per-million level. However, EC sensors are not as selective as colorimetric detectors (see Chapter 10). They may respond to various chemicals simultaneously without differentiation capability. This is because the oxidation-reduction reaction between the chemicals in the sample and the electrolyte controls the detection. Any chemicals contained in the sample that will react with the electrolyte on the working electrode surface will generate electrical current and are detected together with those from targeted chemicals. Using a chemical filter may reduce or eliminate some of the chemical interference potential. The nse of a second working electrode that responds to different sets of chemicals from the first working electrode within the same sensor may also lead to better selectivity. 

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