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Russian Journal of Applied Chemistry, Vol. 75, No. 5, 2002, pp. 846 !848. Translated from Zhurnal Prikladnoi Khimii, Vol. 75, No. 5,
2002, pp. 864!865.
Original Russian Text Copyright + 2002 by Uritskaya, Kitaev, Belova.
Kinetics of Cadmium Sulfide Precipitation
from Aqueous Thiourea Solutions
A. A. Uritskaya, G. A. Kitaev, and N. S. Belova
Ural State Technical University, Yekaterinburg, Russia
Received December 26, 2000; in final form, August 2001
Abstract-Kinetics of cadmium sulfide precipitation with thiourea from solutions of cadmium(II) ammine
complexes under conditions of spontaneous nucleation of the solid phase in the bulk of solution was studied.
Cadmium sulfide is widely used in semiconductor
devices . One of the methods for obtaining cadmi-
um sulfide as precipitates and thin films on the sur-
face of dielectrics is its chemical precipitation with
thiourea from alkaline solutions of cadmium com-
plexes . The properties of semiconducting materials
based on CdS essentially depend on their preparation
In this work we studied the kinetics of chemical
precipitation of cadmium sulfide. When thiourea is
added to an alkaline solution of a complex cadmium
salt, the transparent solution becomes turbid owing
to formation of the CdS solid phase by the reaction
CS + 2OH
= CdS + nNH
NCN + 2H
The surface of the solid phase forming under con-
ditions of homogeneous nucleation continuously
changes owing to an increase in the size of CdS par-
ticles during the process.
The kinetics of this heterogeneous process was
monitored by sampling from a reactor. The cadmi-
um(II) concentration in samples was determined with
Dithizone . The experiments were carried out at
298, 308, and 318 K. Working solutions were pre-
pared in volumetric flasks by combining appropriate
volumes of the starting solutions in the following se-
quence: 1 M solution of cadmium sulfate, 25% aque-
ous ammonia, 0.5 M solution of thiourea, 1 M solu-
tion of alkali, and distilled water.
Typical kinetic curves are S-shaped, which is char-
acteristic of heterogeneous autocatalytic processes.
Since cadmium sulfide is formed by the reaction of
two compounds, a complex cadmium salt and thiourea,
we described the reaction rate by the equation
)/dt = K
are the concentrations of the com-
plex cadmium salt (M) at t = 0 and t > 0, respectively;
are the concentrations of thiourea (M)
at t = 0 and t >0;(c
amount of cadmium sulfide (M) formed in time t from
the beginning of the reaction; (c
a quantity proportional to the surface area of CdS
particles formed in a homogeneous solution.
The number of particles is assumed to remain con-
stant, and their weight and size increase in the course
of the process .
Equation (2) is integrated by reducing it to the
tabular integral . After introducing the dimension-
less parameters a = c
and b = c
equation takes the form
Fig. 1. Kinetic curves at 308 K: (a) c
and (t) time.
pOH and c
(M): (1) 1.4 and 1; (2) 1.8 and 1.25; the
same for Fig. 2.