1064
The Regularities of the Electrolytic Dissociation of 1,3-Cyclohexanedicarboxylic Acids
E. Kvaratskhelia, R. Kvaratskhelia and R. Kurtanidze
R. Agladze Institute of Inorganic Chemistry
and Electrochemistry,
Iv. Javakhishvili Tbilisi State University
11 Mindely str., Tbilisi, 0186, Georgia
Cyclohexanedicarboxylic acids are widely used as the members of the positive resist radiation-sensitive composition, the coating resinous compositions, for production the high-solid polyurethane coatings for such demanding markets as automotive, transportation, maintenance, and aerospace. In this communication the regularities of electrolytic dissociation of cis and trans-1,3-cyclohexanedicarboxylic acids are studied by the suggested by authors original method for calculation the parameters of dissociation of weak multibasic organic acids with the “overlapping” equilibria effect.
For weak dibasic organic acids the law of dilution for both dissociation steps may be expressed as follows:
[1]
[2]
where K_{1} and K_{2} are the thermodynamic dissociation constants of first and second steps, α_{1} and α_{2} are the usual degrees of dissociation of these steps, α^{′}_{2} is the “partial” degree of dissociation of second step, c is the total (analytical) concentration of acid, F_{1} and F_{2}are the quotients of the activity coefficients for the corresponding steps:
[3]
[4]
The values of the activity coefficients were approximated by the Debye-Huckel equation:
[5]
where a_{i} is the cation-anion distance of closest approach, A and B are constants depending on the properties of water at given temperature, z_{i} is the charge of ion. Ionic strength I=c(α_{1}+2α_{2})=cα_{1}(1+2α'_{2}).
According to equations [1] and [2] the degrees of dissociation α_{1}, α_{2} and α^{′}_{2}can be evaluated successively by iterative solution of following two quadratic equations:
[6]
[7]
[8]
The equilibrium concentrations of the dissociation products: hydrogen ions, mono- and dianions and undissociated acid molecules can be calculated with the aid of the following equations:
[9]
[10]
[11]
[12]
The dissociation constants of cis-acid have the following values: K_{1}=7.943×10ˉ^{5}; K_{2}=3.467×10ˉ^{6}. The corresponding values for trans-acid are: K_{1}=4.898×10ˉ^{5}; K_{2}=1.862×10ˉ^{6}.
In Tables I and II the values of α_{1}, α_{2}, α^{′}_{2}and pH for the dilute solutions of both acids are presented.
Table I. The values of the dissociation parameters
of cis-1,3-cyclohexanedicarboxylic acid
at 16ºC
Concentration, M |
α_{1} |
α_{2} |
α^{′}_{2} |
pH |
0.0001 0.0002 0.0004 0.0006 0.0008 0.001 0.002 0.004 0.006 0.008 0.01 |
0.5822 0.4660 0.3612 0.3075 0.2732 0.2487 0.1839 0.1344 0.1115 0.09753 0.08785 |
0.03218 0.01686 0.008723 0.005912^{} 0.004482^{} 0.003614^{} 0.001849^{} 0.000945^{} 0.000639^{} 0.000484^{} 0.000390 |
0.05527 0.03618 0.02415 0.01923 0.01641 0.01453 0.01005 0.007030^{} 0.005728^{} 0.004958^{} 0.004437 |
4.215 4.020 3.836 3.732 3.661 3.606 3.439 3.277 3.184 3.118 3.068 |
Table II. The values of the dissociation parameters
of trans-1,3-cyclohexanedicarboxylic acid
at 19ºC
Concentration, M |
α_{1} |
α_{2} |
α^{′}_{2} |
pH |
0.0001 0.0002 0.0004 0.0006 0.0008 0.001 0.002 0.004 0.006 0.008 0.01 |
0.4995 0.3904 0.2971 0.2508 0.2216 0.2010 0.1473 0.1070 0.08845 0.07722 0.06945 |
0.01791 0.009248 0.004739 0.003198^{} 0.002418^{} 0.001946^{} 9.91·10^{–4} 5.05·10^{–4} 3.40·10^{–4} 2.57·10^{–4} 2.07·10^{–4} |
0.03586 0.02369 0.01595 0.01275 0.01091 0.009682 0.006730 0.004720^{} 0.003849^{} 0.003333^{} 0.002986 |
4.290 4.102 3.924 3.823 3.753 3.699 3.536 3.376 3.284 3.219 3.169 |
We suggest also the simple empirical equations for fast approximate calculation of the dissociation parameters of both acids:
Cis-1,3-cyclohexanedicarboxylic acid
[13]
[14]
[15]
[16]
Trans-1,3-cyclohexanedicarboxylic acid
[17]
[18]
[19]