Subject arsenic hydrides

Arsenic does not combine directly with molecular hydrogen,9 and the element may be purified by sublimation in that gas. Hydrides, however, may be obtained by indirect methods (see pp. 79-84). Arsenic may be displaced by the gas from solutions of its salts at high temperatures and pressures. Thus arsenic separates in large well-defined crystals when a solution of sodium arsenate is subjected to the action of hydrogen at 25 atm. pressure 10 the action commences at 300° C., 15 per cent, of the arsenic being precipitated at this temperature, but it increases rapidly with rising temperature and at 350° C. 77 per cent, of the arsenic is liberated. Arsine is not produced in the reaction. 

Some metals, for example, arsenic and selenium, are difficult to analyze by atomic absorption because their analytical wavelengths are subject to considerable interference. These metals, however, are readily converted to gaseous hydrides by treatment with strong reducing reagents such as sodium borohydride. Since the hydrides can be readily separated from the sample matrix, interferences are much reduced. A typical hydride generation AAS is... 

Mercury and those elements (antimony, arsenic, bismuth, germanium, lead, selenium, tellurium and tin) which form volatile covalent hydrides may be separated from the matrix by vapour generation. The use of tin(II) chloride to generate elemental mercury and its subsequent aeration into a long-path absorption cell with silica windows has been described elsewhere in this book, as has the use of sodium borohydride to produce hydrides which are swept to a flame or heated tube for atomisation. This approach is far more successful for mercury than for the other elements, as the hydride generation technique is subject to interference from a large number of transition metals and oxyanions. 

---