Since acids in solution owe their acidic properties to the hydrogen ion, their relative strengths may be compared by measuring the relative ionisations in solutions containing equivalent weights of the acids in identical volumes. The ionisation is most conveniently determined by the conductivity of the solution. Since the hydrogen ion is much more mobile than any of the anions of acids (as is seen from the table above), it carries most of the current, and the relative conductivities of different acids are therefore approximately proportional to the ionisations.
Expt. 5. - Distilled water and N/50 solutions of acetic, sulphuric, and hydrochloric acids are poured into four glass tubes, fitted with electrodes.  | Fig: Comparison of conductivities of acids |
The electrodes are the same distance apart in the three tubes, and in series with each tube is a carbon-filament lamp. The tubes are connected in parallel with the mains. The lamps in circuit with the water and acetic acid remain dark, because the conductivities are so small that practically no current passes. The lamps connected with the hydrochloric and sulphuric acids light up, but the former is brighter than the latter. The order of conductivities of the three acids: HCl > H 2SO 4 > CH 3-CO 2H, is therefore the same as the order of strengths found by the relative rates of solution of zinc in the acids.
|
