ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2011, Vol. 85, No. 6, pp. 1072–1076. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © Vu Tkhi Duyen, O.V. Dolgikh, N.V. Sotskaya, 2011, published in Zhurnal Fizicheskoi Khimii, 2011, Vol. 85, No. 6, pp. 1173–1177.
1072
INTRODUCTION
An important characteristic of electrolytes used for
the electrodeposition of metals is their stability, that is,
the time during which they can work without a
decrease in current efficiency and properties of coat-
ings formed. It is known that the main reason for a
decrease in the stability of nickel plating electrolytes is
solution alkalization as a result of the occurrence of
the side hydrogen release reaction. This reaction
results in the formation of nickel hydroxides and basic
salts and, as a consequence, a decrease in precipitate
quality and current efficiency.
On the one hand, solution alkalization can be pre-
vented by the introduction of substances with buffer
properties, the main role of which is to keep pH at a
constant value. Effective buffer admixtures to nickel
plating electrolytes are carboxylic acids [2] and amino
acids, glycine in particular [3]. It was shown in [4] that
an important role in the formation of solution buffer
properties was also played by various salts usually used
as a background. In addition, the anionic composition
of the electrolyte substantially influences the charac-
teristics of the deposition process and the properties of
coatings [5].
On the other hand, an attempt can be made to
increase pH of hydrate formation (рН
hydr
). It is gener-
ally accepted that рН
hydr
is the medium pH value at
which sparingly soluble metal hydroxides or basic salts
are formed. The solubility product of a sparingly solu-
ble Ni(OH)
n
L
2 –n
compound is
Therefore,
(1)
where pK
w
is the ionic product of water and , ,
and are the activities of nickel, hydroxyl, and
ligand ions, respectively.
It follows from (1) that the lower the activity of
nickel ions the higher the рН
hydr
value. The introduc-
tion of anions capable of forming coordination com-
pounds with metal ions into electrolytes increases
рН
hydr
because of a decrease in the concentration of
free nickel ions.
1
The influence of anions on рН
hydr
,
however, depends substantially on the nature of the
sparingly soluble compounds formed.
The following cases are possible. In the simplest
situation, nickel hydroxides Ni(OH)
2
precipitate from
1
Here and throughout, nickel complexes only containing water
molecules are understood to be free nickel ions.
SP a
Ni
2+a
OH
–
n
a
L
–
2 n –
. =
pH
hydr
1
n
-- a
Ni
2+ log – pK
w
+ =
+
1
n
-- SP ( ) log 1
2
n
-- –
a
L
–, log +
a 2+
Ni
a -
OH
a -
L
Ni H
2
O ( )
6
2 +
The Influence of Electrolyte Composition
on pH of Nickel Hydrate Formation
Vu Thi Duyen, O. V. Dolgikh, and N. V. Sotskaya
Voronezh State University, Universitetskaya pl. 1, Voronezh, 394893 Russia
e-mail: nvs@chem.vsu.ru
Received July 21, 2010
Abstract—The influence of the composition of nickel plating electrolytes containing various ligands (Ac
–
,
Gly
–
, and Asp
2–
) on pH of nickel hydrate formation (pH
hydr
) was studied. The data obtained were interpreted
from the point of view of ionic equilibria in the systems studied. The nature of sparingly soluble precipitates
formed in these electrolytes when pH
hydr
was reached was determined. Sparingly soluble precipitates were
found to be pure Ni(OH)
2
hydroxides or basic salts with the composition Ni(OH)
n
Cl
2– n
, in which the con-
tent of chloride ions increased as the concentration of NiCl
2
grew. Organic substance anions were absent in
the precipitates formed and only influenced pH
hydr
through changes in the concentration of free nickel ions
as a result of complex formation. A principle of selecting the composition of nickel plating electrolytes was
suggested on the basis of the results obtained.
Keywords: pH of hydrate formation, complex nickel plating electrolytes, glycine, asparagine, acetate.
DOI: 10.1134/S0036024411060343
COLLOIDAL CHEMISTRY
AND ELECTROCHEMISTRY