1070-4272/05/7804-0552 C 2005 Pleiades Publishing, Inc.
Russian Journal of Applied Chemistry, Vol. 78, No. 4, 2005, pp. 552!555. Translated from Zhurnal Prikladnoi Khimii, Vol. 78, No. 4, 2005,
Original Russian Text Copyright + 2005 by Kiryushov, Skvortsova.
AND INDUSTRIAL INORGANIC CHEMISTRY
Optimal Conditions for Masking of Sulfite Anions
with Formaldehyde in Iodometric Titration
of Photographic Fixing Solutions
V. N. Kiryushov and L. I. Skvortsova
Institute of Solid State and Mechanochemistry, Siberian Division, Russian Academy of Sciences,
Received October 22, 2004
Abstract-The iodometric titration of photographic fixing solutions can be simplified by masking sulfite
anions with 3% aqueous formaldehyde in a weakly acidic medium, instead of their standard masking with
concentrated aqueous formaldehyde in weakly alkaline solutions.
The concentrations of sodium thiosulfate and sodi-
um sulfite in photographic fixing solutions are com-
monly determined by iodometric titration. In this
method, two identical aliquots of a fixer are titrated
with iodine. In the first aliquot, the sum of S
anions is determined. The second aliquot is
titrated in the presence of an agent masking SO
anions to determine the content of S
The sulfite anions can be masked with formaldehyde
 or acetone [4, 6], or by precipitating poorly
soluble strontium sulfite [7, 8]. The accuracy of sodi-
um thiosulfate and sodium sulfite determination de-
pends on the reliability of masking sulfite anions.
As a rule, acetone is not used for masking of sulfite
anions because of its volatility and inflammability.
Futuremore, in iodometric titration in water3acetone
mixture, the color of the iodine-sensitive indicator
is strongly weakened and the color transition at
the equivalent point is puurly defined.
The precipitation of sulfite anions is expedient only
if other interfering components of the fixer are co-
precipitated. For example, in determination of sodium
thiosulfate in bleaching fixers, the precipitation of
strontium sulfite is apparently accompanied by co-
precipitation of thiourea interfering with the thiosul-
As a rule, in iodometric titration of fixers, sulfite
anions are masked with formaldehyde. Formaldehyde
is bound with sulfite anions to form a complex unox-
idizable with iodine. The formation of this complex is
described by reaction (1) in an acidic medium  and
by reaction (2) in a neutral or weakly alkaline medium
O 6 H
Na + NaOH. (2)
In iodometric titration of photographic fixers, sul-
fite anions are commonly masked in an alkaline me-
dium. The initial aqueous solution of the fixer is
diluted with water and alkalized to pH 8310, with
phenolphthalein as indicator. Thereafter, 37% aqueous
formaldehyde is added, and the resulting mixture is
allowed to stand for several minutes. This mixture is
acidified with 10% aqueous acetic acid (10 ml), and
a standard solution of iodine is added. The amount
of formaldehyde, recommended for masking of sulfite
anions (ml of formaldehyde solution per ml of fixer),
varies over a wide range: 1.0 , 2.0 , 2.5 , 5
, and even 10 ml [4, 11]. It has also been reported
 that sulfite anions can be masked in a weakly
acidified fixer (pH 33 4) by adding a large excess of
37% aqueous formaldehyde.
Published data on masking of sulfite anions with
formaldehyde in iodometric titration of fixers show
that the recommended formaldehyde additions sig-
nificantly exceed its stoichiometric amount required
for complexation of sulfite anions by reactions (1)
and (2). These reactions show that 1 mol of form-
aldehyde is bound with 1 mol of sulfite anions.
The maximum content of sodium sulfite in the fixers
does not exceed 40 g l
(0.32 M). Therefore, to mask
sulfite anions present in 1 ml of this fixer, 0.027 ml
of 37% (11.6 M) aqueous formaldehyde is required.
In practice, the formalehyde addition exceeds its stoi-
chiometric amount by a factor of 403400.