Thionalide reacts with a variety of metal ions to give chelate complexes which
are sparingly soluble in water. The difference in stability of complexes formed by
the reagent with various metal ions in some cases assures a certain analytical
selectivity.
1. Copper(II), silver(I), mercury(II), bismuth(III), arsenic(III), tin(IV), gold(I),
platinum(IV) and palladium(II) give precipitates with the reagent in acidic media
(mineral acids).
2. The copper(II), mercury(II), cadmium(II), thallium(I) and gold(I) complexes
precipitate from alkaline solutions containing tartrate.
3. Thallium(I), antimony(III) and bismuth(III) complexes are precipitated by
the reagent from solutions containing cyanide and tartrate.
4. The thallium(I) complex also precipitates from alkaline (sodium hydroxide)
solution containing cyanide and tartrate.
The above classification shows that the reaction can be made specific for
thallium by appropriate choice of the pH of the solution and application of
suitable masking agents. Similarly, the selectivity of the precipitation reaction can be increased for other ions too, by application of other masking agents. For
instance, tin(IV) can be masked selectively in the presence of arsenic(III) and
antimony(III) with phosphoric acid, while these latter two ions can be precipitated
with thionalide.
The analytical application of thionalide is stimulated by the fact that its metal
complexes are of stoichiometric composition, and can be dried at 105-110°C
and weighed directly.
The reagent may also be utilized in volumetric analytical procedures. The thionalide
content, which is equivalent to the metal to be determined, can be titrated
directly iodometrically, according to the equation:
2C10H7-NH-CO-CH2-SH+I2=C10H7-NH-CO-CH2-S-S-CH2-CO-NH-C10H7+2HI.
As a reagent for copper, mercury, silver, thallium, and bismuth.
