3.2. Optimum conditions for quantitati6e
precipitation of Sn(IV)
Our preliminary experiments showed that
TDTC precipitates at pH56. Generally the decomposition
rate of dithiocarbamates increases
with increasing acidity of the medium, a pH
range over 5.0–5.5 was chosen for the precipitation.
This is easily controlled by the use of an
acetate buffer of pH 5.2 with the aid of a
screened indicator. Under such conditions the
protonation effects of TDTC and the masking
agent used would be minimized too. It is an
added advantage.
An auxiliary complexing agent should be first
chosen to meet the following requirements: It
can prevent the hydrolysis of Sn(IV) and keep
all the concomitant species in solution, yet exerts
little effect on the main reaction. It was
found that tartaric acid serves the purpose and
in the presence of 0.5–1.0 g of it the precipitation
of Sn(IV) runs normally. As it complexes
cationic species in moles, its amount of addition
vary widely for different kinds of alloys, as specified
in note 4 of Section 2.4.
As revealed by our preliminary experiments,
the mass of the precipitate¬6the mass of Sn
added. Therefore, it can be inferred that the
precipitate would be Sn(C5H8NS2)4, because its
relative molecular mass, calculated on the basis
of values of relative atomic mass of elements
recently recommended by IUPAC [13] would be
703.74, which divided by 118.71, the relative
atomic mass of Sn, gives the quotient 5.928.