1070-4272/02/7508-1237 $27.00 C 2002 MAIK [Nauka/Interperiodica]
Russian Journal of Applied Chemistry, Vol. 75, No. 8, 2002, pp. 1237!1239. Translated from Zhurnal Prikladnoi Khimii, Vol. 75, No. 8, 2002,
Original Russian Text Copyright + 2002 by Turygin, Smetanin, Khudenko, Tomilov.
AND CORROSION PROTECTION OF METALS
Electrochemical Synthesis of Arsenic Acid
V. V. Turygin, A. V. Smetanin, A. V. Khudenko, and A. P. Tomilov
State Research Institute of Organic Chemistry and Technology, Moscow, Russia
Received November 26, 2001
Abstract-The possibility of oxidizing arsenic(III) oxide to arsenic acid in quantitative yield in the presence
of hydrochloric or hydrobromic acid as a catalyst was studied.
Arsenic acid and arsenic(V) oxide are used to syn-
thesize dyes and pharmaceuticals, to manufacture op-
tical glass, etc. Commonly, arsenic acid is prepared
by oxidation of As
with nitric acid . A short-
coming of the method is that toxic wastes are formed.
Of interest in this connection is the possibility of per-
forming this process electrochemically. As is known,
oxidizes on a rotating platinum electrode in
acid medium at a potential of about 1.3 V [2, 3]. The
oxidation rate is the highest in a sulfate solution at
pH < 1 .
A procedure for preparing arsenic acid by oxidation
suspension on a tin dioxide anode in a 1%
sulfuric acid solution containing 1% Na
scribed in . However, this technique fails to yield
pure, impurity-free arsenic acid.
Much more attention has been given to oxidation
in an alkaline solution to obtain reagent-
grade arsenates, e.g., those of sodium  and lithium
The aim of this study was to develop a method for
synthesis of pure arsenic acid.
We used in the study reagent-grade arsenic(III)
oxide containing 99% main substance. Other reagents
were of chemically pure grade.
The experiments were performed in a filter-press
electrolyzer with platinum or ruthenium oxide an-
ode and a tungsten or titanium plate as a cathode.
The anode and cathode spaces were separated by a
MF34SK or Nafion cation-exchange membrane. The
anolyte was circulated at a rate sufficient for maintain-
in the form of a suspension with a centrifu-
gal pump. The working area of the anode was 30 cm
In a typical run, 0.5M HCL solution (60 ml) was
fed into the cathode chamber of the filter-press elec-
trolyzer equipped with tungsten cathode and ruthe-
nium oxide anode, and 0.5 M HCL solution (70 ml)
and 15 g As
(0.15 mole As), into the anode cham-
ber. The electrolysis was performed with a current of
6.0 A (current density i = 0.2 A cm
) for 1.5 h (110%
of the theoretically necessary quantity of electricity,
Q). In the course of the experiment, the anolyte tem-
perature increased from 20 to 45oC. The catholyte
volume increased by 18 ml owing to electrophoresis.
was found in the catholyte in amount of
0.0012 mol, i.e., 0.8% of the initial As
The anolyte was evaporated until dry, together with
the wash water from the anode chamber, and calcined
at 250oC. The resulting dry residue (18.2 g) contained
. Arsenic acid was obtained by dissolv-
ing the residue in water. The yield of the arsenic acid
by substance was 97%, and the current efficiency by
the acid, 88%.
First, we checked the possibility of oxidizing an
aqueous suspension of As
on platinum or graphite
without addition of any supporting electrolytes. As is
known, arsenic(III) oxide is poorly soluble in water
and is present in aqueous solutions in the form of
a weak arsenous acid, responsible for low electrical
conductivity of the solution. The results of these ex-
periments are presented in Table 1.
It can be seen that the voltage across the electro-
lyzer is high at the beginning of the process and then
gradually decreases owing to accumulation of the
stronger arsenic acid, which raises the electrical con-
ductivity of the solution. The oxidation is slow, and
a nearly sixfold current excess is necessary for oxi-
dizing only half the feed at the Pt anode.