18
MITROFANOVA
2+
The concentrations of succinic acid monosubsti-
Hydrolysis of Ca ions was not taken into account,
as being negligible at pH < 6 [7].
tuted derivatives HL in solution were calculated from
the precise weights of their portions and refined by
pH-potentiometric titration. Chemically pure grade
calcium nitrate and potassium chloride were used.
The content of Ca(II) in solutions were determined by
complexometric titration. The concentration of KOH
solution prepared by dilution of its saturated solution
[6] was refined by potentiometric titration. All the
solutions were prepared in freshly boiled distilled
water.
The equilibrium concentrations of species in solu-
tion were calculated from the following equations of
material balance and mass action law:
cL = [HL] + [L ] + [CaL+] + [CaL2],
cH = [HL] + [H+],
cM = [Ca2+] + [CaL+] + [CaL2],
Solutions of a succinic acid monosubstituted
[HL]
[CaL+]
derivative, Ca(NO ) , and KCl were placed in a tem-
3 2
B =
,
+ =
,
CaL
[L ][H+]
[Ca2+][L ]
perature-controlled cell. After adding each portion
(0.1 ml) of the titrant (0.3 M KOH), the pH values
were measured. The equilibrium was considered as
attained if pH remained constant for 2 3 min. The
glass electrode was calibrated by buffer solutions
before each measurement. The calibration straight line
was determined by the least-squares procedure and
then used to refine the pH values obtained during
titration. The pH meter readings were checked by
a buffer solution with pH 4.01 after each titration. If
the measured pH differed from this value by more
than 0.02, the results were discarded. Points of the
titration curve in the range pH 3.0 6.0 were used in
the calculations.
[CaL2]
=
,
CaL
[Ca2+][L ]2
2
where c , c , and c are total concentrations of the
L
H
M
ligand, hydrogen ions, and metal ions, respectively;
B, protonation constant of a monosubstituted succinic
+
acid derivative; and
and
, stability con-
CaL
CaL
stants of the correspondi2ng calcium compounds.
The formation function n for the system under
study has the following form:
+[L ] + 2
2[L ]2
CaL
CaL
The protonation constants of succinic acid mono-
amide and methyl succinate anions,
n =
.
1 +
This equation can be transformed to
1)[L ]2
+[L ] + 2
2[L ]2
CaL
CaL
[HL]
B =
,
[L ][H+]
n + (n
1)[L ]
+ + (n
= 0.
CaL
CaL
2
were determined by the potentiometric titration at
25 C and ionic strength I = 0.3 (KCl). Three to five
replicate determinations were performed for each acid.
In each determination, the protonation constant was
calculated as the average of the calculated values for
each point of the titration curve. The error in logB
was taken as the probable deviation of the arithmetic
mean at a confidence level of 0.95. The calculated
logB values for succinic acid monoamide and methyl
hydrogen succinate were 4.51 0.01 and 4.42 0.01,
respectively (I = 0.3, T = 25 C).
The experimental data were treated by the least-
squares procedure. The number of independent deter-
minations of complex formation constants for each
+
compound was 8 10. The obtained
and
CaL
CaL
values for Ca(II) complexes with succinic acid mono2-
amide and methyl hydrogen succinate at I = 0.3 (KCl)
and T = 25 C were recalculated to zero ionic strength
by the Davies equation. The experimental data were
approximated by y = a + bx functions with at con-
fidence level of 0.95 and with the confidence interval
no greater than 2. The logarithms of the complex for-
mation constants are presented below together with
the previous data [4] on the stability of calcium suc-
cinates, given for comparison.
The following equilibria were taken into account
when calculating the stability constants of calcium
complexes with succinic acid derivatives:
H+ + L
Ca2+ + L
Ca2+ + 2L
HL,
(1)
(2)
(3)
0
0
Compound
Succinic acid
Methyl hydrogen succinate
Succinic acid monoamide
log
+
log
CaL
CaL
2
CaL+,
CaL2.
2.08 0.02
1.91 0.02
1.86 0.05
3.41 0.06
3.56 0.03
3.58 0.06
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 76 No. 1 2003