Page:Text-book of Electrochemistry.djvu/197

 ��THE DISSOCIATION THEORY.

��CHAP.

��Catalytic Action of Hydrogen and Hydroxyl Ions. — As mentioned in Chapter VII., cane sugar in aqueous solution is converted in presence of acids into invert sugar.

Now, since the characteristic of all acids is the presence of hydrogen ions, it might be supposed that the hydrogen ion was the cause of this change. Further, since the degree of dLssociation of an acid is proportional to its molecular con- ductivity and inversely proportional to its conductivity at infinite dilution, it might be expected, since this latter value ia nearly the same for all acids, that the velocity of inversion of cane sugar would be proportional to the conductivity of the acid added if equivalent quantities of different acids were employed. In 1884 I (l:d) showed on theoretical grounds that this velocity must be proportional to the conductivity of the catalysing acid, and shortly afterwards Ostwald (17), who was then investigating reaction velocities, experimentally confirmed this conclusion. Ostwald obtained the numbers given in the next table for the conductivity (I) and the velocity of inversion (p) of different acids at the same concentration ; the conductivity of hydrochloric acid (in normal solution) is set = 100, and the velocity of inversion caused by this same acid (in 0'5-normal solution) is also set = 100.

��Acid.

��Hydrochloric acid. . Nitric acid .... Chloric acid .... Sulphuric acid. . . Benzenesulphonic acid Trichloracetic acid. Dichloracetic acid. . Monochloracetic acid. Acetic acid .... Formic acid ....

��i.

��a.

�UK.)

�...

�O-Ch

�2-:$

��The values given under cr are for the velocity of saponi- fication of esters in presence of 0G7-noimal solutions of the acids mentioned.

�� �