492
DUBEY ET AL.
bismuthate to obtain bismuth(V) in the solution. Such
solutions in HCIO4ϪHF mixture are not stable at
ambient temperature and also contain bismuth(III)
as an impurity. Moreover, the speciation offluoro-
bismuth(V) species in such solutions is yet to be re-
solved. Further, the method ofstandardization ofthis
solution requires critical conditions. However, such
problems to a large extent are not encountered for bis-
muth(V) solution in phosphoric acid medium.
The kinetic studies ofthe oxidation by bismuth(V)
reported so far are, therefore, scanty [11–16]. The for-
mation ofbismuth(IV) and OH were considered to be
the rate-determining step in one such study in per-
chloric acid medium where the rate was independent
ofthe substrate concentration [11]. Recently, kinetics
ofoxidation ofvarious compounds by bismuth(V) in
HCIO4ϪHF mixtures have been reported [17], but the
problem ofspeciation ofbismuth(V) species still re-
quires better resolution.
Our efforts in the past to bring bismuth(V) into the
solution phase had not been successful; a limited suc-
cess had, however, been achieved in dissolving so-
dium bismuthate in aqueous phosphoric acid. Never-
theless, such a solution ofbismuth(V) has distinct
advantages over the solution ofthe oxidant in
HCIO4ϪHF mixture mainly from the viewpoint of
avoiding the need to use HF.
glass bottles blackened from the outside at refrigerated
temperature (ϳ5ЊC). No traces ofH O2 could be
2
detected in such solutions even on long standing.
Nevertheless, these solutions, similar to solutions in
HCIO4ϪHF mixture, contained bismuth(III), which is
either present as an impurity in the sample ofbis-
muthate and/or formed during the digestion period of
the solid through the reduction ofbismuth(V) via ox-
idation ofwater to oxygen. It is, however, reported
[8,17] that neither pentaoxide nor bismuthates can be
prepared in a pure state but shall always contain a
considerable amount oftervalent bismuth. Since bis-
muth(III) did not affect the kinetics of the title reac-
tion, no efforts were made to remove bismuth(III) and
the solutions were used as prepared.
Spectrophotometric Analysis of the
Solution
Bismuth(V) solution in HCIO4ϪHF mixture is trans-
parent in the visible as well as in the ultraviolet region.
Solution ofbismuth(V) is aqueous phosphoric acid
absorbs strongly in the visibile region at 530 nm
1
1
(ꢀ 640 dm3 molϪ cmϪ ). This was done by taking
solution ofbismuth(V) in a 1-cm quartz cell against
the reference of phosphoric acid present in the solution
employing a spectrophotometer at ꢁ 530 nm.
The concentration ofHF employed in bismuth(V)
solution in HClO4ϪHF mixtures rendered glasswares
unserviceable and unusable due to the reaction ofthe
silicate ofthe glass with HF.
Kinetic Procedure
EXPERIMENTAL
The requisite quantities ofvarious components ofthe
reaction mixture, except formic acid, were taken in
glass-stoppered Erlenmeyer flasks, which were then
suspended in a water bath thermostated at the desired
temperature Ϯ0.1ЊC unless stated otherwise. The re-
action was initiated by adding the requisite solution of
the temperature pre-equilibrated formic acid into the
reaction mixture, and the time ofinitiation was re-
corded when halfofthe contents from the pipette were
released. Aliquot samples (5 to 10 cm3) were with-
drawn at different intervals of time and then quenched
in an ice-cold KI (10%) solution; the liberated iodine
was titrated against thiosulphate solution using starch
as an indicator. Measurements in triplicate without any
interference from the ingredients of the reaction mix-
ture were in agreement to within Ϯ5%.
Sodium bismuthate (Riedel AnalaR) was the source of
bismuth(V), and the sample was employed as supplied
without any further treatment. However, the sample of
sodium bismuthate contained bismuth(III) oxide as an
impurity, which can not be removed by any method
or technique. Bismuth nitrate or bismuth oxide (BDH
AnalaR) was employed for bismuth(III). Phosphoric
acid was ofE. Merck grade. Corning glassware was
employed both for storing the solutions and the kinet-
ics ofthe reactions unless specified otherwise.
Doubly distilled water was employed throughout
the study; the second distillation was from alkaline
permanganate solution in an all-glass apparatus.
The requisite quantity ofsodium bismuthate was
digested in aqueous H3PO4 (3.0 mol dmϪ ) solution
Initial rates were computed employing plane mirror
method [18]. Pseudo-first-order plots were also made
wherever reaction conditions permitted. Since the ki-
netics is not affected by the light, no further precau-
tions were taken to exclude light from the reaction
mixtures.
3
for 20–30 minutes on a mechanical shaker; a pink
solution with some undissolved material was obtained.
Undissolved material was centrifuged and the solution
was standardized iodometrically [12–15]. The solu-
tion is fairly stable if kept in brown glass bottles or