The Electrolytic Dissociation of 1,4-Cyclohexanedicarboxylic Acid

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 trans-1,4-cyclohexanedicarboxylic acid 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:

where K₁ and K₂ are the thermodynamic dissociation constants of first and second steps, α₁ and α₂ 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, F₁ and F₂are the quotients of the activity coefficients for the corresponding steps:

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

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_iis the charge of ion. Ionic strength .

According to equations [1] and [2] the degrees of dissociation α₁, α₂ and α^′₂can be evaluated successively by iterative solution of following two quadratic equations:

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:

The dissociation constants of trans-1,4-cyclohexanedicarboxylic acid have the following values: K₁=6.607×10ˉ⁵; K₂=3.802×10ˉ⁶.

In Tables I the values of α₁, α₂, α^′₂and pH for the dilute solutions of acid are presented.

Table I. The values of the dissociation parameters

                of trans-1,4-cyclohexanedicarboxylic acid    

                at 16ºC

 

Concentration, M	α1	α2	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	0.5507 0.4365 0.3358 0.2849 0.2525 0.2295 0.1691 0.1233 0.1021 0.08926 0.08035	0.03472 0.01826 0.009473 0.006427 0.004875 0.003932 0.002014 0.001030 0.0006954 0.0005267 0.0004246	0.06305 0.04183 0.02821 0.02256 0.01931 0.01713 0.01191 0.008354 0.006811 0.005901 0.005284	4.236 4.046 3.866 3.764 3.693 3.639 3.474 3.314 3.221 3.156 3.106

We suggest also the simple empirical equations for fast approximate calculation of the dissociation parameters of trans-1,4-cyclohexanedicarboxylic acid: