Thallium, elemental Subject

The reaction is a sensitive one, but is subject to a number of interferences. The solution must be free from large amounts of lead, thallium (I), copper, tin, arsenic, antimony, gold, silver, platinum, and palladium, and from elements in sufficient quantity to colour the solution, e.g. nickel. Metals giving insoluble iodides must be absent, or present in amounts not yielding a precipitate. Substances which liberate iodine from potassium iodide interfere, for example iron(III) the latter should be reduced with sulphurous acid and the excess of gas boiled off, or by a 30 per cent solution of hypophosphorous acid. Chloride ion reduces the intensity of the bismuth colour. Separation of bismuth from copper can be effected by extraction of the bismuth as dithizonate by treatment in ammoniacal potassium cyanide solution with a 0.1 per cent solution of dithizone in chloroform if lead is present, shaking of the chloroform solution of lead and bismuth dithizonates with a buffer solution of pH 3.4 results in the lead alone passing into the aqueous phase. The bismuth complex is soluble in a pentan-l-ol-ethyl acetate mixture, and this fact can be utilised for the determination in the presence of coloured ions, such as nickel, cobalt, chromium, and uranium. 

If the final determination is subject to interference by concomitant elements, or the detection limits are insufficient for the samples to be analyzed, separation methods are necessary, increasing the time of analysis per sample, and thus the costs. As low amounts of thallium have to be expected on the one hand, and many determination methods suffer from interferences on the other, separation steps have to be considered sometimes. According to the author s experience, losses because of incomplete recovery of thallium are more probable than the introduction of blanks. 

No attempt has been made to give more than a short introduction to the subject to economize on space, most of the simple equations have been omitted. The elements have been classified, in so far as is possible, in the simple groups with which the reader is already familiar, and methods of separation have been briefly indicated. Thus thallium and tungsten are in Group I molybdenum, gold, platinum, selenium, tellurium, and vanadium in Group II and beryllium, titanium, uranium, thorium, and cerium in Group III. It is hoped that the subject-matter of this chapter will suffice to enable the reader to detect the presence of one or two of the less common ions. 

Philips and Osram spectral discharge lamps have been used as spectral sources for analytical atomic fluorescence. These lamps have internal electrodes and produce intense spectral lines. The spectral lines, however, are subject to line reversal and the lamps are available only for a limited number of elements. Use of Philips and/or Osram lamps require careful control of input energy to produce maximum intensity without line reversal. Under these conditions they have produced satisfactory atomic fluorescence signals for some elements, including cadmium, mercury, zinc, and thallium. 

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