Ti(III) was chosen for this purpose, since it
serves to reduce also the higher valence states of
As and Sb quantitatively to their trivalent ones,
the presence of its excess is indicated by its own
color and both Ti(III) and Ti(IV) are non-interferents.
If sodium sulfite were used instead, the
separation of Sb(III) would be incomplete owing
to reoxidation [17]. Moreover, sulfite would react
with hydrogen sulfide, one of the acid decomposition
products of DDTC [18] to form colloidal
elemental sulfur, and hence is the only interferent
possibly used among the reductants.
The reaction between Mo(VI) and DDTC was
reported to take place over the pH range of 2–9
[14]. This statement is questionable, as our experiments
showed that it, like Sn(IV), reacts with
either DDTC or TDTC at pH56 only. Hence,
Mo(VI) would accompany Sn(IV) in the filtrate
after the preliminary separation at pH 9. This
interference from Mo can not be eliminated by
masking with EDTA or hydroxylamine (which
was used to mask the precipitation of MoS3 in
acid solution) [19] alone, nor as Mo(VI)–
NH2OH–EDTA [20].
V, like Mo, would accompany Sn(IV) in the
filtrate too. As shown in its predominance-zone
diagram, it is at pH 5.0–5.5 in the state of
vanadyl vanadate, i.e. partly as V(IV) and partly
as V(V) [21]. Hence, effects of both V(V) and
V(IV) should be studied. It was found that in the
presence of EDTA, TDTC does not precipitate
either of them at the stated pH to form the same
yellow product. The only difference lies in that
this precipitation takes place at once in the case of
V(V), but slows down somewhat in the case of
V(IV).
It should be noted that at pH 9 V(V) is the
predominate species in the filtrate; whereas at
pH50 V(IV) is [21]. Therefore, subsequent
acidification of the alkaline filtrate makes the
unstable V(V) an oxidant. Under such conditions
V(V) would oxidize DDTC or its decomposition
products and thus render the analyte solution
colloidal. In order to avoid this additional interference
ethanol is purposely added prior to