CH3CO2C2H5 + K+ + HO− → CH3CO2− + K+ + C2H5OH
or CH3CO2C2H5 + HO− → CH3CO2− + C2H5OH.
For ammonium hydroxide we have similarly,
CH3CO2C2H5 + NH4+ + HO− → CH3CO2− + NH4+ + C2H5OH.
Now, Arrhenius[140] proved that the rate of saponification of ethyl acetate by ammonium hydroxide, which is very much slower than the rate of saponification by potassium hydroxide of equivalent concentration, does agree quantitatively, indeed, with the rate demanded by the theory of ionization, when the hydroxide-ion is considered the active component of the bases, to which the saponification is due.
Exp. A rough idea of the difference in the chemical actions of the two bases may be obtained by observing their effects on the ester, methyl acetate, which is decomposed into an acetate and methyl alcohol rather rapidly. To 50 c.c. of (CO2 free) water containing some phenolphthaleïn, 10 c.c. of 0.1 molar potassium hydroxide is added; a similar mixture with 10 c.c. of 0.1 molar ammonium hydroxide solution is prepared. To each of the solutions, 2 c.c. (an excess) of methyl acetate is added (to the ammonium hydroxide solution first), and the mixtures are shaken for a moment. At room temperature, the mixture containing potassium hydroxide will become pale pink in a few minutes, and colorless soon thereafter, while the mixture [p081] containing ammonium hydroxide will still be deep red at the end of 45 minutes.[141]
In the following tables are summarized some of the results which have been obtained in comparing the activity of bases, in saponifying methyl acetate, and the concentrations of the hydroxide-ion, in the solutions of the bases, as determined by conductivity measurements. The comparisons are made by representing the activity of the hydroxide-ion in a solution of lithium hydroxide by 100 and by expressing the ratio of the activity of a given base to that of the lithium hydroxide in percentages of the activity of the latter. All the bases were used in 0.025 molar concentration, and their degrees of ionization are given in the last column of the table.
Chemical Activity of Bases and Their Ionization[A]
| Base. | Activity. | Relative Concentration of HO−. |
|---|---|---|
| Lithium hydroxide | 100 | 97 |
| Potassium hydroxide | 98 | 97 |
| Sodium hydroxide | 98 | 97 |
| Ammonium hydroxide | 2 | 2.5 |
| Ethyl ammonium hydroxide | 12 | 16. |
[A] Whetham, Theory of Solutions, p. 338 (1902). (Cf. Walker, Introduction to Physical Chemistry, p. 277 (1899).)
An ester is decomposed also under the influence of acids, in aqueous solution, into an organic acid and an alcohol, and cane sugar is similarly decomposed into glucose and fructose (grape sugar and fruit sugar): C12H22O11 + H2O → C6H12O6 + C6H12O6. Both actions are found to be caused by the influence of the hydrogen ions of the acids used and to proceed, at a given temperature, with a velocity proportional to the concentration of the hydrogen ions. Now, in 0.1 molar solution, acetic acid is very little ionized (1.3%), as compared with hydrochloric acid (91%), the degrees of ionization being determined by conductivity measurements (p. [50]); the relation may easily be demonstrated with the aid of the conductivity apparatus used to show the difference in ionization between potassium hydroxide and ammonium hydroxide. In the presence of 0.1 molar hydrochloric acid, the decomposition of cane sugar actually proceeds at 79 times the rate that it does in the presence of 0.1 molar acetic acid. The ratio of the concentrations of the hydrogen-ion in the two [p082] solutions is, in fact, 70 : 1. There is, therefore, close agreement[142] between the relative chemical activity of the two acids and the relation demanded, if we assume, on the basis of the theory of ionization (p. [77]), that the chemically active components of the acids are their ions and particularly their hydrogen ions.