[K] Knox, in Abegg's laboratory, Trans. Faraday Soc., 4, 43 (1908).
The difference in the tendencies of acids to ionize, as expressed in the table, may be recognized in equivalent solutions by any of the properties dependent on the ionization, such as the conductivity, the chemical activity, the osmotic pressure and allied effects, and so forth. If the conductivities of equal volumes of equivalent (e.g. normal) solutions of hydrochloric, phosphoric and acetic acids are compared (exp.)[190], it is readily seen that hydrochloric acid is the best conductor, phosphoric acid a much poorer one, and acetic acid an exceedingly poor one (the conductivity of normal acetic acid is about 1 / 200 that of normal hydrochloric acid, and the conductivity of normal phosphoric acid is about 1 / 14 that of normal hydrochloric acid). Since the conductivity is approximately proportional to the concentration of the hydrogen-ion[191] in each of the solutions, it is evident that the hydrochloric acid is ionized to a considerably greater extent than either of the other acids—than acetic acid, in particular. Similarly, if a drop (0.05 c.c.) of molar hydrochloric acid and a drop of molar acetic acid are added to equal volumes (50 c.c.) of a very dilute solution of methyl orange (exp.), the color will be changed decidedly by the hydrochloric acid to a bright pink, but by the acetic acid only to an orange hue. Again, if a precipitate of barium chromate or calcium oxalate is treated with some strong acid, hydrochloric or nitric, for instance, it dissolves readily, while a considerable excess of acetic acid (exp.) only dissolves traces of either precipitate.[192] In this way, the chemical behavior of these acids differs in degree, as a result of the different tendencies to ionize, which are expressed in the constants of the table. Advantage is taken, in analysis, of such differences. Acetic acid, for instance, is used when only a slight degree of acidity is desired—as in recognizing barium-ion by its chromate, or oxalic acid by means of its calcium salt. Hydrochloric or nitric acid is used when decided acidity is required—as in the separation of groups by hydrogen sulphide (Chap. XI).
The Ionization of Bases.
The Ionization Constants[A] of Bases
| Base. | Ratio. | K. |
|---|---|---|
| Potassium hydroxide | [K+]×[HO−]/[KOH] | (1) |
| Sodium hydroxide | [Na+]×[HO−]/[NaOH] | (1) |
| Barium hydroxide | [Ba2+]×[HO−]2/[Ba(OH)2] | (0.03) |
| Strontium hydroxide | [Sr2+]×[HO−]2/[Sr(OH)2] | (0.03) |
| Calcium hydroxide[B] | [Ca2+]×[HO−]2/[Ca(OH)2] | (0.03) |
| Ammonium hydroxide[C],[D] | [NH4+]×[HO−]/([NH4OH]+[NH3]) | 1.8E−5 |
| Hydrazine[E] | [N2H5+]×[HO−]/([N2H5OH]+[N2H4]) | 0.3E−5 |
| Aniline[F] | [C6H5NH3+]×[HO−]/ ([C6H5NH3OH]+[C6H5NH2]) | 0.5E−9 |
| Water | [H+]×[HO−]/[H2O] at 25° | 0.2E−15 |
| at 100° | 0.9E−14 | |
| [H+]×[HO−] at 25° | 1.2E−14 | |
| at 100° | 0.5E−11 |
[A] As explained above, the bracketed values under K for strong bases are not constants, but express the values of the ratios for 0.1 molar solutions only. (For the alkaline earths 0.05 molar solutions are referred to.)
[B] Estimated value.