Halogens, silver electrodes

Marquis and Lebel [534] precipitated potassium from seawater or marine sediment pore water using sodium tetraphenylborate, after first removing halogen ions with silver nitrate. Excess tetraphenylborate was then determined by silver nitrate titration using a silver electrode for endpoint detection. The content of potassium in the sample was obtained from the difference between the amount of tetraphenyl boron measured and the amount initially added. 

After precipitation of the halogen ions with AgN03, (>otassium is precipitated with sodium tetraphenylborate as described. The excess of tetraphenylborate is then determined by AgNO 3 titration, using a silver electrode for endpoint detection. The method has been applied to seawater samples as small as 1 mL as well as for interstitial waters, with a precision of better than 1 %. The procedure saves the tedious filtration and drying process but offers a somewhat lower precision than the gravimetric method. A detailed description is presented by Marquis and Lebel (1981). 

Halogen Electrodes.—The determination of the standard potentials of the halogens is simple in principle it involves measurement of the potential of a platinum electrode, coated with a thin layer of platinum or iridium black, dipping in a solution of the halogen acid or a halide, and surrounded by the free halogen. The uncertainty due to liquid junction can be avoided by employing the appropriate silver-silver halide or mercury-mercurous halide electrode as reference electrode. In practice, however, difficulties arise because of the possibility of the reactions... 

Other types of electrodes have been designed to measure the concentrations of ions other than H30. The simplest example of such an ion-selective electrode is a metal wire, which can be used to detect the concentration of the corresponding metal ion in solution. Silver and copper wires can be used reproducibly in this way to determine the concentrations of Ag and Cu, respectively. Still other electrodes have been developed to detect specific ions. For example, glasses of chemically modified composition are used to construct membrane electrodes to determine potassium and sodium ions or halogen ions. 

Trace amounts of cyanide are usually determined by flow injection spectrophotometric procedures. The target species is first halogenated with chlora-mine-T, after which it reacts with a mixture of pyrazolone or barbituric acid and isonicotinic acid or pyridine to form a bluish-violet polymethine dye. The implementation of gas-diffusion modules in the flow setup for hydrogen cyanide separation avoids matrix interferences and enables the adaptation of inherently nonselective detectors, such as metallic silver-wire electrodes for potentiometric measurements. Total inorganic cyanide, including free and complexed species, such as iron-cyanide complexes, may be determined by sample decomposition with UV irradiation and further photometric or amperometric analysis. 

Automatic titrimeter - with silver/glass electrode system. Wick lighter, small burner fed with sulphur and halogen free fuel. 

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