Page:Text-book of Electrochemistry.djvu/202

 former, and an excess of 05 x 10~^ x 10"^^ gram-ions of chlorine over the number of sodium ions should be sufficient to prevent a further separation of the chlorine ions by diffusion. Such small quantities (5 x 10"^ gram-equivalent) cannot be detected by chemical means.

Since a millivolt can be detected by an electrometer, the 10"''th part of the charge mentioned can be determined. The smallest weighable quantity is usually 01 milligram, so that if the equivalent weight ia 100, 10"^ milligram-equiva- lent, or 10"* gram-equivalent can be measured. The electro- metric analysis is therefore in this case 10^^ times more delicate than the chemical method. «

In the diffusion the more mobile chlorine moves slightly quicker than the sodium, and the liquid becomes negatively charged at A where pure water is, and positively charged at B, so that an electric current can be obtained when unpolarisable electrodes are placed at the ends and joined by a wire. We return later to these so-called concentration currents. The separation of the ions can, at any rate, be effected by taking from them their electric charges, as is done in electro-analysis.

Since electric forces come into play when electrolytic dissociation takes place and do not in the case of ordinary dissociation, the number of phenomena which accompany the former is greater than that associated with the latter. On account of the delicacy of the electrical methods of measure- ment, no other dissociation has been so thoroughly studied, and from so many points of view, as that of electrolytes into ions.

�� �