ISSN 1070-4272. Russian Journal of Applied Chemistry, 2006, Vol. 79, No. 3, pp. 492! 493. + Pleiades Publishing, Inc., 2006.
Original Russian Text + Yu.P. Perelygin, I.V. Rashevskaya, 2006, published in Zhurnal Prikladnoi Khimii, 2006, Vol. 79, No. 3, pp. 501!503.
On the Term [pH of the Precipitation Onset of Heavy
Yu. P. Perelygin and I. V. Rashevskaya
Penza State University, Penza, Russia
Received September 28, 2005; in final form, January 2006
Abstract-It is suggested to take as pH of the precipitation onset of metal hydroxides the pH at which 10%
of the initial amount of metal ions has precipitated. An equation for calculating this quantity is presented.
Precipitation of metals in the form of hydroxides is
used in hydrometallurgy , analytical chemistry ,
and wastewater treatment [3, 4].
The pH values of the onset and completion of the
precipitation of hydroxides are determined using the
data of the reference book . Whereas by pH of
complete precipitation is meant a pH value at which
the concentration of metal ions in solution over the
precipitate does not exceed 10
M, the term [pH of
the precipitation onset] has no clear definition .
In , it was suggested to understand by pH of
the precipitation onset of metal hydroxides a pH value
at which the concentration of metal ions in solution
over a precipitated hydroxide is 1 M. However, the
initial concentration of the metal ion in solution,
which should exceed 1 M, remains unknown and,
consequently, it is unclear what part of the initial
amount of metal ions has precipitated. In , no
dependence of pH of the precipitation onset of metal
hydroxides on the initial concentration of a salt in
solution was demonstrated, whereas in , a depen-
dence of this kind was observed.
Thus, the reported pH values of the precipitation
onset of hydroxides [5, 6] can hardly be used for prac-
tical purposes for the reasons mentioned above. This
circumstance initiated the present study.
The concentration of the metal ion in solution,
], over a precipitated metal hydroxide M(OH)
can be found, if the solubility product (SP) of the
metal hydroxide is known, by the equation
] = SP/[OH
According to the material balance, the sum of the
amounts of the metal ion in the precipitate and solu-
tion is equal to the initial amount of the metal ion
V = m/M +[M
where z is the valence of the metal; V, solution vol-
ume; m and M, mass of the precipitate and the molecu-
lar weight of the metal hydroxide, respectively; and
, initial concentration of the metal ion in solution.
Substituting Eq. (2) into (1) and taking into account
is the ionic product of
water), we obtain the equation
V = m/M + (SP V[H
Dividing the left- and right-hand parts of Eq. (3) by
the initial amount of the metal ion in solution (c
we obtain an equation that can be used to determine
the relative fractions of the metal hydroxide [a =
V) 0 100%] and metal ion (100 3a) in solution
as functions of the solution pH:
a =[13 (SP[H
)] 0 100%. (4)
Relative fractions of cadmium ions (1) in the precipitate
(=) and (2) in solution (100 !=) vs. pH. Initial concentra-
tion of cadmium ions 1 M.