Thionalide C10H7NHCOCH2SH may be used for the quantitative precipitation of mercury(II) as Hg(C12H10ONS)2. Sulphate does not interfere. Attention is drawn to the following experimental points.
The chloride ion concentration of
the solution should not exceed 0.1M; the
results are high if the chloride ion concentration is excessive.
If nitric acid solutions of
mercury (II) nitrate are used, the latter must be
converted into mercury (II) chloride by the addition of at least an equivalent
amount of chloride ion.
A three-fold excess of reagent
should be employed.
The sample solution may contain 5 to 75 mg of mercury(II). Heat the solution to 80-85 °C and add, with constant stirring, a three-fold excess of a 1 per cent solution of thionalide in acetic acid. The precipitate coagulates upon stirring. Filter the hot solution through a sintered-glass filtering crucible (porosity No. 3) which has been preheated by pouring hot water through it. (The use of a warm filtering crucible is essential; the separation of thionalide in the pores of the sintered plate of the crucible, which would render filtration difficult, is thus avoided.) Wash with hot water until free from acid, and dry to constant weight at 105 °C. Weigh as Hg(C12H10ONS)2.