CC BY
11
408
CHEMICAL PROBLEMS 2019 no. 3 (17) ISSN 2221-8688
UDC 541.132.132.4
THE ELECTROLYTIC DISSOCIATION OF METHYLENE SUCCINIC ACID
E. Kvaratskhelia and R. Kurtanidze
R. Agladze Institute of Inorganic Chemistry and Electrochemistry, Iv. Javakhishvili Tbilisi State University 11 Mindely st., 0186 Tbilisi, Georgia E-mail: [email protected], [email protected]
Received 09.07.2019
Abstract: The work deals with the analysis of regularities of the electrolytic dissociation of alkene derivative of the succinic acid (methylene succinic acid) in their dilute (0.0001-0.01M) solutions carried out by means of a new method of determination of dissociation parameters of weak multibasic organic acids with "overlapping equilibriums " effect previously described by the authors. Values of usual and "partial" degrees of dissociation, concentrations of all anions, hydrogen ions and undissociated acid molecules, as well as activity coefficients of all charged dissociation products were calculated. The concentration intervals of predominance of various charged and uncharged substances in the dilute solutions of the above mentioned acid were determined. Together with accurate equations, there were also suggested simple empirical equations for quick approximate determination of various dissociation parameters.
Keywords: dissociation constants, degree of dissociation, weak organic acids, equations, dissociation step.
DOI: 10.32737/2221-8688-2019-3-408-412
Introduction
Previously we described an original method for determination of various dissociation parameters of weak multibasic organic acids with close values of stepwise dissociation constants (effect of "overlapping equilibriums") [1-4]. This method was widely used for an analysis of various weak organic acids with different basicity in their dilute solutions. A term the "partial" degree of dissociation was also suggested [2] and used for more detailed analysis of complex equilibriums typical for dissolution processes of such acids. In this work our method is
applied for the research into peculiarities of electrolytic dissociation of alkene derivative of the succinic acid (methylene succinic acid) in their dilute (0.0001-0.01M) solutions. Succinic acid and its various derivatives are widely used in the organic synthesis and polymer industry, take part in biologically important Krebs cycle. Methylene succinic acid is widely used in the production of polyacrylonitrile and polyacrilic fiber. The useful properties of these acids are directly connected with peculiarities of their electrolytic dissociation.
The Equations
In case of weak dibasic organic acid H2A the steps may be written as follows [1-4]: mass action equations for both dissociation
K c(a -a22) F ca2[1 - (a2)2] F
K1 =—;-F1 =—;-F1 (1)
1 - a1 1 - a1
ca2(a1 + a2) ca1a^(1 + a2) K 2 =-F2 =-:-;-F2
a2
1 -a0
(2)
where Kj and K2 are the thermodynamic dissociation constants of first and second steps, aj and a2 are usual degrees of dissociation of the corresponding steps, a2 is the "partial" degree of dissociation for second step, c is the total concentration of acid, Fj and F2 are the quotients of the activity coefficients:
F =
fH + f
H +J HA-
f
(3)
H 2 A
F2 =
fH + fA2
f
(4)
HA~
Degrees of dissociation aj, a2 and a 2 may be evaluated successively by iterative solution of the following quadratic equations:
a1 = —
cFi D
/ \2 cFi
+ 4
a22 +
Kl cFi
(5)
a2 =-
K2
cF
+ .
K2
cF
2
+
4 K 2ai
cF
(6)
a2 =
1 K2
1 + 2
aicF2 j
+ .
K2
1 + 2
a1cF2 j
+ ■
4 K 2
alcF2
(7)
2
2
Values of "partial" degrees of dissociation may be calculated with the help of the following equations:
a2 = a2 / a1 (8)
or with the help of the more complex equation K2=3.55x10-6. as set forth in [2].
Note that values of activity coefficients can be approximated by the Debye-Huckel equation:
from [5]. The activity coefficient of undissociated acid is assumed to be unity. Values of dissociation constants of methylene succinic acid at 25°C necessary for calculations
were taken from [6-7]: K1=1.41x10-4;
6
lg f =
z,2
1 + a,,b41
Using the above mentioned dissociation parameters, we can determine equilibrium concentrations of hydrogen ions, mono and dianions and undissociated acid molecules:
(9)
[HA ] = c(a1 -a2) = ca1(1 -a2) (10)
where a; is the cation-anion distance of closest [A2 ] = ca2 = approach, A and B are constants depending on
properties of water at a given temperature, z is [H+] = c(a1 + a2) = cax(1 + a2) (12) the charge of ion. The ionic strength
is I = c(a1 + 2a2) = ca1(1 + 2a2). Note [H2 A] = c(1 -a1)
that values of ai, A and B at 25°C were taken
(11)
(13)
With the aid of the Eqs. (10)-(13), we can determine intervals of the acid concentration where various charged or uncharged substances dominate. This is possible due to equations below that express equality conditions of various products of dissociation:
1 -a 1
[H+] = [H2 A]: a = —a = - (14)
[HA-] = [H2 A] : a
2
1 + a2 2
a2 + 2
_1_
2 - a
"(15
[A2"] = [H2A]: a! = 1 -a2
a2 +1
[ A2" ] = [HA~ ] : a1 = 2a2
a2 = 0.5
(16)
(17)
(18)
In line with concentration conditions, the equality of dominance areas of mono and dianions and undissociated acid molecules may be formulated.
The results and discussion
In Table 1, the a1, a2, a2 and pH values for dilute solutions (0.0001-0.01M) of analyzed acid are presented.
Table 1. Values of dissociation parameters of dilute solutions of methylene succinic acid at 25°C
1
Acid concentration, M a1 a2 a 2 pH
0.0001 0.6796 0.03222 0.04741 4.152
0.0002 0.5625 0.01753 0.03116 3.941
0.0004 0.4481 0.009069 0.02024 3.745
0.0006 0.3864 0.006145 0.01590 3.635
0.0008 0.3460 0.004659 0.01347 3.560
0.001 0.3167 0.003758 0.01187 3.503
0.002 0.2376 0.001925 0.008102 3.330
0.004 0.1755 0.000986 0.005620 3.164
0.006 0.1463 0.000667 0.004561 3.068
0.008 0.1283 0.000506 0.003942 3.002
0.01 0.1158 0.000408 0.003524 2.950
The comparison of the a2 and a2 values goes to show that values of the "partial" degree of dissociation (which more correctly characterize the completeness of dissociation at second step) exceed appreciably those of the usual degree of dissociation (especially in higher acid concentrations).
With the aid of the equations (14)-(18), areas of predominance of various charged and uncharged forms in the dilute solutions of methylene succinic acid were identified. Calculations show that the monoanion concentration exceeds the [H2A] value when c < 0.0003M. In all these cases the inequalities:
1 + a2 1
a >-- and a >-- were fulfilled.
2 2 -a2
Regions of prevailing [H+] values in comparison with the [H2A] values are also: c < 0.0003M. In
1 — a2
all these cases the inequalities: a1 > —-— and
a1 >
1
2 + a9
were fulfilled. As a whole,
analyzed concentration interval the [HA-] and [H+] values exceed the [A2-] values.
To sum up, allowing for the comparative complexity of calculations by
means of equations (5)-(7), we suggest simple degrees of dissociation and pH in the dilute empirical equations for quick approximate solutions of methylene succinic acid. determination of values of usual and "partial"
Methylene succinic acid
a = lg(0.17742c_0355)
a2 = 5.7544-10-6 c "a939
a2'= 1.85353 -10-4 c "0 602 pH = 1.575-0.641lgc
(up to 0.0001M)
(up to 0.006M)
(up to 0.004M) (up to 0.01M)
(19)
(20)
(21) (22)
References
1 Kvaratskhelia E. and Kvaratskhelia R. The Degrees of Dissociation of Weak Multibasic Organic Acids. J. Solution Chem, 2007, vol. 36, no. 6, pp. 787-792.
2 Kvaratskhelia E. and Kvaratskhelia R. The "Partial" Degrees of Dissociation of Weak Multibasic Organic Acids. J. Solution Chem, 2009, vol. 38, no. 3, pp. 345-349.
3 Kvaratskhelia E. and Kvaratskhelia R. The Forms of the Law of Dilution for Weak Multibasic Organic Acids in Case of Overlapping Equilibria. ECS Transactions. 2009, vol. 16, no. 38, pp. 49-54.
4. Kvaratskhelia E. and Kvaratskhelia R. The Determination of Acidity of the Dilute
Solutions of Weak Multibasic Organic Acids. J. Biomedical Science and Engineering. 2010, vol. 3, no. 5, pp. 484487.
5. Koryta J., Dvorak J., Kavan L. Principles of Electrochemistry. 2nd Edition. L. Wiley and Sons, Chichester, New York, Brisbane, Toronto, Singapore, 1993.
6. Dean J. A., Lange's Handbook of Chemistry; 15th Edition, McGraw-Hill: New York, St. Louis, San Francisco etc, 1999.
7. Dippy J. F. J., Hughes S. R. C., Rozanski A. The dissociation constants of some symmetrically disubstituted succinic acids. J. Chem. Soc. 1959, pp. 2492-2498.
METiLENKBHRBBA TURÇUSUNUN ELEKTROLiTiK DiSSOSiASiYASI
E. Kvaratsxelia andR. Kurtanidze
iv. Cavaxiçvili ad. Tbilisi Dovlat Universitetinin R.Aqladze ad. Qeyri-Uzvi va Elektrokimya institutu 0186, Tbilisi, Mindeli kuç., 11 E-mail: [email protected], [email protected]
Çoxasasli zaif uzvi turçularin dissosiasiya parametrlarinin yeni tayin metodu ila metilenkahraba turçusunun duru mahlullarda (0.0001-0.01 M) elektrolitik dissosiasiyasinin qanunauygunlugu tahlil edilmiçdir. Dissosiasiya parametrterinin tayini uçun daqiq tanlikterte yanaçi sada empirik tanlikter taklif olunur.
Açar sôzhr : metilenkahraba turçusu, elektrolitik dissosiasiya, zaif uzvi turçular, tanlik
ЭЛЕКТРОЛИТИЧЕСКАЯ ДИССОЦИАЦИЯ МЕТИЛЕНЯНТАРНОЙ КИСЛОТЫ
Е.Р. Кварацхелия и Р.Р. Куртанидзе
Институт неорганической химии и электрохимии им. Р.И. Агладзе Тбилисского Государственного Университета им. Ив. Джавахишвили ул. Миндели 11, 0186 Тбилиси, Республика Грузия E-mail: [email protected], [email protected]
В данной работе проведен анализ закономерностей электролитической диссоциации алкенпроизводной янтарной кислоты (метиленянтарной кислоты) в ее разбавленных (0.0001-0.01М) растворах с помощью нового метода определения параметров диссоциации слабых многоосновных органических кислот с эффектом «перекрывающихся равновесий», ранее описанным авторами. Были рассчитаны значения обычных и «парциальных» степеней диссоциации, концентрации всех анионов, ионов водорода и недиссоциированных молекул кислот, коэффициенты активности всех заряженных продуктов диссоциации. Также рассчитаны концентрационные интервалы преобладания различных заряженных и незаряженных частиц в разбавленных растворах указанной кислоты. Вместе с точными уравнениями были также предложены простые эмпирические уравнения для быстрого приближённого расчёта значений параметров диссоциации.
Ключевые слова: константы диссоциации, степень диссоциации, слабые органические кислоты, уравнения, ступени диссоциации.
