Peculiarities of electrolytic dissociation of ethyl derivatives of succinic acid Текст научной статьи по специальности «Химические науки»

KÉMYA PROBLEML9RÎ № 1 2018 ISSN 2221-8688

63

UDC 547.541

PECULIARITIES OF ELECTROLYTIC DISSOCIATION OF ETHYL DERIVATIVES

OF 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]

The original method suggested by authors for the analysis of processes of electrolytic dissociation of weak multibasic organic acids was used for the calculation of dissociation parameters of ethyl derivatives of succinic acids: DL-2,3-diethylsuccinic and meso-2,3-diethylsuccinic acids. The values of dissociation degrees of separate steps (including "partial" dissociation degrees of the second step), concentrations of various ionized and non-ionized forms, activity coefficients of hydrogen ions and mono and dianions in the dilute (0.0001-0.01M) solutions of these acids have been calculated. The concentration intervals of predominance of various charged and uncharged substances in the dilute solutions of the above mentioned acids determined. Also suggested are simple empirical equations for fast approximate calculation of the dissociation parameters.

Keywords: weak organic acids, dissociation constant, dissociation step ,hydrogen ions concentration, equations

1. INTRODUCTION

Succinic acid and its various derivatives are widely used in the organic synthesis and polymer industry and involved in the biologically important Krebs cycle. Many useful properties of these acids are directly connected with interesting peculiarities of behavior of these acids in the solutions and, in particular, with regularities of their electrolytic dissociation. The paper provides an analysis of

regularities in the electrolytic dissociation of DL-2,3-diethylsuccinic and meso-2,3-diethylsuccinic acids in the diluted (0.0001-0.01M) solutions which was carried out with the help of a new method of determination of dissociation parameters of weak multibasic organic acids with the "overlapping "equilibriums previously described by the authors[1-5].

2. EQUATIONS

In case of weak dibasic organic acid H2A the mass action equations for both dissociation steps may be written as follows [1-4]:

K c(a -a22) F ca2[1 - (a2)2]

K1 =—;-r-F1 =-;-r-F1(1)

1 -a1

1 -a1

caÇaL+a) ^ caq2(1 + a^)

K 2 --f2 --:-'-f2

C^! Ci 2

1 -a.

(2)

where K1 and K2 are thermodynamic dissociation constants of first and second steps; and a1 and a2 are usual degrees of dissociation of corresponding steps; a 2 is "partial" degree of dissociation for second step; c is total concentration of the acid; F1 and F2 are quotients of activity coefficients:

fh + fHA~

~ (3)

F =

f,

h 2 A

fu + fA 2-

f h j a2 F _ , (4)

f

ha-

Note that degrees of dissociation ay, a2 and a2 may be evaluated successively by iterative solution of the following quadratic equations:

a, _ — 1 2

K,

+ .

cFi \

r \2

'Kl

v cfi j

+ 4

a22 +

Kl cFi

(5)

a2 _ — 2 2

El

CF

V 2

+ a1

+ .

El

CF

V2

2

+ a1

+

4 K 2a1

cFn

(6)

a2 _-

1 K2

1 + 2

aicF2 J ^

+ .

K2 i + 2

aicF2 J

+

4K 2

a,cF2

(7)

1

1

2

The values of "partial" degrees of dissociation may be calculated with the help of the following equations:

a2 = a2 / a1 (8) or with the aid of the more complex equation presented in [3].

The values of activity coefficients can be approximated by the Debye-Huckel equation:

lg f = z2 A41

1 + afijï

(9)

where ai is cation-anion distance of the closest approach; A and B are constants depending on the properties of water at given temperature; zi is ion charge with the ionic strength

I = c(a1 + 2a2) = cal(l + 2a2). Note that the values of a;, A and B at 25°C have been taken from [6], while the activity coefficient of undissociated acid is assumed to make up integrity. The values of dissociation constants of ethyl derivatives of succinic acid at 25°C needed for calculations were taken from [7-8]: DL-2,3-diethylsuccinic acid: Ki=2.34x10-4;

_n

K2=7.47*10 , meso-2,3-diethylsuccinic acid: K1=2.88x10-4; K2=2.57x10-7.

Using the above mentioned dissociation parameters we can determine the equilibrium concentrations of hydrogen ions, mono and dianions and undissociated acid molecules:

[HA-] _ c(a1 -a2) _ ca1(1 -a2) [A2" ] _ ca2 _ caa2 [H+] _ c(a1 +a2) _ ca1(1 + a2)(12)[H2 A] _ c(1 -a1)

(10) (11)

(13)

With the help of the Eqs. (10)-(13) we possible due to the following equations that

can determine intervals of the acid disclose equality conditions of the various

concentration where various charged or products of dissociation: uncharged substances dominate. This is

1 -a 1

[ H+] = [ H2 A]: a = -02 = —- (14)

2 a2 + 2

[ HA-] = [ H2 A]: a = i+02 = (15)

2 2 - a2

2 1 [ A2" ] = [ H2 A]: a = 1 - a = ——7 (16)

a2 +1

[ A2" ] = [ HA~ ]:a1 = 2a2 (17)

a2 = 0.5 (18)

Pursuant to the conditions of mono and dianions, as well as undissociated concentrations equality, areas of dominance of acid molecules may be formulated.

3. RESULTS AND DISCUSSION

Tables 1 and 2 provide the a1, a2, a2 and 0.01M) of the reviewed ethyl derivatives of pH values for the diluted solutions (0.0001- succinic acid.

Table 1. Values of dissociation parameters for the diluted solutions of DL-2,3-diethylsuccinic acid at 25°C

Acid concentration, M «1 «2 «2 pH

0.0001 0.7588 0.003579 0.004717 4.112

0.0002 0.6491 0.001818 0.002801 3.891

0.0004 0.5320 0.000924 0.001736 3.678

0.0006 0.4655 0.000622 0.001337 3.561

0.0008 0.4203 0.000470 0.001118 3.482

0.001 0.3871 0.000378 0.000977 3.421

0.002 0.2950 0.000193 0.000654 3.240

0.004 0.2206 0.000099 0.000447 3.068

0.006 0.1848 0.000067 0.000361 2.970

0.008 0.1627 0.000051 0.000311 2.902

0.01 0.1472 0.000041 0.000277 2.849

Table 2. Values of dissociation parameters for the diluted solutions of meso-2,3-diethylsuccinic acid at 25°C

Acid concentration, M «1 «2 «2 pH

0.0001 0.7885 0.002660 0.003373 4.106

0.0002 0.6838 0.001350 0.001974 3.869

0.0004 0.5676 0.000686 0.001209 3.651

0.0006 0.4997 0.000462 0.000925 3.531

0.0008 0.4533 0.000349 0.000771 3.449

0.001 0.4186 0.000281 0.000672 3.388

0.002 0.3214 0.000144 0.000447 3.204

0.004 0.2417 0.000074 0.000304 3.029

0.006 0.2031 0.000050 0.000245 2.930

0.008 0.1790 0.000038 0.000210 2.861

0.01 0.1621 0.000031 0.000189 2.808

The comparison of the a2 and a2 values shows that the values of "partial" degree of dissociation (which more correctly characterizes the completeness of dissociation at the second step) exceed appreciably the values of usual degree of dissociation (especially in case of lower Ki values and higher acid concentrations).

With the help of the equations (14)-(18) areas of a predominance of various charged and uncharged forms in the diluted solutions of the reviewed methyl derivatives of succinic acid were determined. Calculations go to show that the monoanion concentration exceeds the [H2A] value where c < 0.0005M (for both acids). In all cases the in

1

2 -an

are

1 + a2

equalities: a1 >—-— and a >

fulfilled. Areas of prevailing [H+] values in comparison with the [H2A] values are as follows: c < 0.0005M (DL-2,3-diethylsuccinic acid) and c < 0.0006M(meso-2,3-diethylsuccinic acid). In all cases the

1 -a2 , ^ 1

inequalities: a1 >—-— anda1 >

2

2 + a0

are fulfilled.

To conclude, when adjusted for the comparative complexity of calculations through the use of the equations (5)-(7), we suggest simple empirical equations for the prompt approximate determination of the values of usual and "partial" degrees of dissociation and pH in the diluted solutions of all reviewed derivatives of succinic acid.

DL-2,3-diethylsuccinic acid

(19)

a1 - lg(0.18664c "°372)

(up to c = 0.002M)

a2 - 4.46684 x10-7 c"0976 (20)

(up to c = 0.01M)

a2 - 9.0365 x10-6c-0675

(up toc = 0.001M)

pH -1.324 - 0.697 lg c

(up to c = 0.01M)

(21) (22)

Meso-2,3-diethylsuccinic acid

a = lg(0.19953c "0 372)

(up to c = 0.002M)

a2 = 3.31894 x10-7 с "0 976

(up to c = 0.01M)

(23)

(24)

a2 = 5.19996 x10-6 с "07

(up to c = 0.003M)

pH = 1.221 - 0.719lg с

(up to c = 0.001M)

(25)

(26)

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. 787792.

2. Kvaratskhelia E. and Kvaratskhelia R. The Electrolytic Dissociation of Mellitic Acid. J. Solution Chem. 2008, vol. 37, no. 9. pp. 1063-1070.

3. 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.

4. Kvaratskhelia R. and Kvaratskhelia E. On Dissociation of Weak Dibasic and Tribasic Organic Acids Participating in Krebs Cycle. Russ. J. Electrochem., 2009, vol. 45, no. 2. pp. 221-224.

5. Kvaratskhelia E. and Kurtanidze R. The Electrolytic Dissociation of Methyl Derivatives of Succinic Acids. Kimya Problemleri - Chemical Problems. 2016, vol. 4, pp. 349-355. (In Azerbaijan).

6. Koryta J., Dvorak J., Kavan L. Principles of Electrochemistry. 2nd Edition. L. Wiley and Sons, Chichester, New York, Brisbane, Toronto, Singapore, 1993.

7. Dean J.A. Lange's Handbook of Chemistry; 15th Edition, McGraw-Hill: New York, St. Louis, San Francisco etc, 1999.

8. Dippy J.F.J., Hughes S.R.C., Rozanski A. The dissociation constants of some symmetrically disubstituted succinic acids. J. Chem. Soc. 1959, pp. 24922498.

ОСОБЕННОСТИ ЭЛЕКТРОЛИТИЧЕСКОЙ ДИССОЦИАЦИИ ЭТИЛПРОИЗВОДНЫХ

ЯНТАРНОЙ КИСЛОТЫ

Е. Кварацхелия и 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: elickoamail. ru, ekvarats@yahoo. com

Оригинальный метод, предложенный авторами для анализа процессов электролитической диссоциации слабых многоосновных органических кислот, был использован для расчета параметров диссоциации этилпроизводных янтарной кислоты: ПЬ-2,3-диэтилянтарной и мезо-2,3-диэтилянтарной кислот. Были рассчитаны значения степеней диссоциации отдельных ступеней (включая «парциальные» степени диссоциации второй ступени), концентраций различных ионизированных и

неионизированных форм, коэффициентов активности ионов водорода и моно- и дианионов в разбавленных (0.0001-0.01М) растворах данных кислот. Также рассчитаны концентрационные интервалы преобладания различных заряженных и незаряженных частиц в разбавленных растворах указанных кислот. Предложены простые эмпирические уравнения для быстрого приближённого расчёта значений параметров диссоциации.

Ключевые слова: слабые органические кислоты, константы диссоциации, степени диссоциации, концентрация ионов водорода, уравнения.

Received 12.02.2018.