1070-4272/05/7801-0085+2005 Pleiades Publishing, Inc.
Russian Journal of Applied Chemistry, Vol. 78, No. 1, 2005, pp. 85!88. Translated from Zhurnal Prikladnoi Khimii, Vol. 78, No. 1,
2005, pp. 86!89.
Original Russian Text Copyright + 2005 by Shvab, Gur’yanova, Omel’chuk.
AND CORROSION PROTECTION OF METALS
Kinetics and Mechanism of Reactions Occurring
in Electrodeposition of Silver from Thiosulfate Solutions
N. A. Shvab, I. A. Gur’yanova, and A. A. Omel’chuk
Vernadsky Institute of General and Inorganic Chemistry, Ukrainian National Academy of Sciences,
Received June 23, 2004
Abstract-The kinetics and mechanism of processes occurring in reduction of silver thiosulfate complexes on
stationary and rotating disc electrodes and on fluidized glass-bead electrode were studied voltammetrically.
Owing to depletion of natural resources of nonfer-
rous and noble metals, their recovery from various
wastes acquires growing importance. Utilization of
production waste is necessary not only from the stand-
point of saving raw materials but also from the en-
vironmental standpoint. Among wastes contaminating
the environment are spent thiosulfate fixing solutions
from treatment of X-ray films and photographic ma-
terials used in printing industry. These solutions con-
tain a considerable amount of silver (to 9310 g l
and sulfur compounds (up to 250 g l
). Only in the
United States, up to 1/3 of the total industrial con-
sumption of silver is spent for preparation of photo-
graphic materials , with approximately 1000 t of
silver passing annually to fixing solutions. The spent
fixing solutions, even those free of silver, are environ-
mentally hazardous, since in water reservoirs they de-
compose with the release of toxic sulfur compounds.
Silver is present in spent fixing solutions mainly
in the form of a complex compound Na
whose instability constant is K = 3.5010
standard potential of the silver electroreduction from
this complex is 0.017 V .
Various chemical and electrochemical methods of
silver recovery from spent fixing solutions are known
[1, 438]. Nevertheless, silver recovery from fixing
solution has not received wide acceptance. The tradi-
tional electrolysis methods are suitable for recovery of
pure silver only at very low current densities, hardly
acceptable for commercial processes. Silver sulfide,
which is reduced to silver with high specific con-
sumption of chemicals and electric power, is deposited
on the cathode at higher current densities. In this case,
the electrochemical processing of silver-containing
thiosulfate solutions is, in essence, a combination of
the electrochemical and electroless methods. Also, the
electroless methods do not allow recovery of pure
silver in one stage and are labor-consuming.
The lack of data on the features of the cathodic
reduction of silver thiosulfate complexes does not
allow purposeful control of this process, optimization
of the electrolysis conditions, and development of
the corresponding processes. Therefore, study of the
reactions proceeding on the cathode during elec-
trolysis of silver-containing thiosulfate solutions is
an urgent scientific and practical problem.
In this work we studied the reduction of a silver
thiosulfate complex on stationary and rotating disc
electrodes and in a fluidized bed of glass beads.
The processes occurring on the cathode in reduc-
tion of silver thiosulfate complexes under the static
conditions without stirring the electrolyte and with
active transfer of electroactive species toward the
cathode and removal of intermediate reduction prod-
ucts from the cathode surface were studied in a uni-
versal cell with a fluidized bed of glass beads .
The cathode and anode compartments of a poly(meth-
yl methacrylate) cell had rectangular shape and were
separated with a semitransparent diaphragm made of
thermally densified Khlorin fabric. A 4 cm
plate coated with silver and pressed into a flat poly-
(methyl methacrylate) holder, which was arranged
normally in the cathode space using guiding slots, was
the working electrode. Silver chloride reference elec-
trode was fed closely to the surface of the working
electrode using a Haber capillary. An electrolyte-flow
distributor made of stacked polyethylene grids with