138 CONDUCTIVITY OF ELECTROLYTES. chap.
In this way the degree of dissociation can be determined for all salts of monovalent acids or bases, and for the strongest acids (hydrochloric, hydrobromic, hydriodic, nitric, chloric acids, etc.) and bases (potassium, sodium, calcium, strontium, barium hydroxides, ammonium bases, etc.). The value of Xoo (for a given temperature) is a measure of the mobility of the ion pair (K + 01).
Transport Number. — It is of interest to learn to what extent the conductivity is due to each of the two ions of a " binary " electrolyte. Let us assume that a quantity of electricity corresponding exactly with 1 gram-equivalent (in this case 1 gram-molecule), i.e. 96,500 coulombs, passes between the electrodes A and B (Fig. 34) through a solution
of potassium chloride. This quan- ^+ p -B tity of electricity is transported
partly by the K ions, which conduct positive electricity in the direction
��Fig. 84. ^B, and partly by the 01 ions,
which carry the negative electricity in the opposite direction (BA). If the 01 ions remained stationary, i.e. did not aid the transportation of the elec- tricity, a gram-equivalent (= 39* 15 grams) of potassium would pass in the direction AB through any cross section P of the column of liquid. On the other hand, if the K ions remained stationary and the 01 ions alone trans- ported the electricity, a gram-equivalent (= 35*45 grams) of chlorine would migrate through the section P in the direction BA, As a rule, however, both ions take part in the conduction. Let us assume that the K ions transport a fraction, u, of the electricity, then the 01 ions transport the remainder, (1 - u). The fractions u and (1 — t^) are tenned the " transport numbers " or " migration numbers " of the potassium and chlorine ions respectively in the potassium chloride solution. There must then migrate across P in the direction AB - u gram-equivalents (3915 x u grams) of potassium and in the direction BA (1 — u) gram-equivalents (= 35*45 - 35-45 x u grams) of chlorine. In order to
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