Ethylenediamine, acid dissociation constant

The c/s-[diaquabis(ethylenediamine)chromium(IlI)] bromide salt is sensitive to light. If stored in the dark at -20°, it is stable for years. In acidified solution it is converted slowly to an equilibrium mixture containing the trans-isomer. The equilibrium constant is = [cis]/[trans] = 12.8 (25°, 1 MNaNOs). The acid dissociation constants are pK = 4.8 and pA 2 = 7.17 (25°, 1 MNaNOa). ... 

Rossotti et al [Fa 70] found that the ratio of stability constants obtained by equilibrium measurements in solvent mixtures similar to the above was independent of the composition of the solvent. McBryde et al showed that in most of the systems they examined, the ratio of the first two stepwise complex stability constants did not depend on the solvent. In the case of the stepwise acid dissociation constants this correlation was approximately true only for ethylenediamine for glycine the ratio of the protonation constants increase with increase in the concentration of the organic component. However, if the formation of the glycine complexes is defined as a reaction between the protonated ligand and the metal ion, accompanied by the liberation of a hydrogen ion, the ratio of the equilibrium constants is approximately independent of the composition of the solvent mixture. 

Bis(ethylenediamine)oxohydroxorhenium(V) perchlorate consists of purple needles which dissolve in water or acids to give a purple solution. It is acidic, with a dissociation constant of 5 X 10 In water it partially reverts to the dioxo form but is stable in dilute acid. It is insoluble in absolute methanol and acetone. An absorption maximum at 487 m/x, molar absorbancy of 19.1, is observed together with a weak absorption at 650 m/t. 

S. J. Westerback and A. . Martell. Nature 178, 321-2 (1956). Dissociation constants, substituted ethylenediamine acids. 

For example, in acetic acid (c = 6), perchloric acid is completely ionized, but the dissociation constant of the ion pair H3CC00H2C10J is only about 10" mol/L and HCIO4 can be classified as a weak acid in this medium. The ionization of acids is enhanced in basic solvents and diminished in acidic solvents, but the dissociation of the ion pair depends only on the dielectric constant of the solvent. Nevertheless, the acidity of weak acids is enhanced in basic solvents, and the basicity of weak bases is enhanced in acidic solvents. Examples are solutions of phenol in ethylenediamine ... 

Hence, in water, which has a very high dielectric constant, the so-called strong electrolytes include all salts and strong acids and bases. The weak electrolytes are almost all organic acids and bases. However, this distinction depends on the characteristics of the solvent. Some solvents are more acidic than water (e.g., formic and acetic acid), while the others are more basic (e.g., ethylenediamine). Besides, the permittivity of organic solvents is generally smaller than the permittivity of water. So, the dissolution of a potential electrolyte in a nonaqueous medium proceeds in two steps ionization and dissociation. 

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