Page:Text-book of Electrochemistry.djvu/159

 ��CONDUCTIVITY OF ELECTROLYTES. chap.

��complex molecules than in the dilute solution, a conclusion which indeed would be expected.

Hittorf {11) and Lenz (iiF) have proved that in aqueous solutions of cadmium iodide which are more concentrated than normal, the transport number of the iodine exceeds 1 ; for 3-normal solution it is 1*3, whilst for 003-normal solution it is 0-61.

For analogous reasons it is found that the transport numbers of the majority of electrolytes sufier a greater or less change with the concentration; this is shown by the results obtained by Goldhaber {IS) for cadmium bromide at 18° contained in the following table, in which v indicates the volume of the solution in which a gram-molecule of the salt is dissolved, and u^ is the transport number for JCd : —

��tl«!

��«e

��On further dilution Uc remains constant.

In a 0'1-normal solution of copper sulphate, the transport number for the anion SO4 is 064, and in a 2-normal solution it is 0'73. In agreement with the explanation given, it is found from the depression of the freezing point that formation of molecular complexes does take place to a very consider- able extent. A comparative investigation of the relationships obtained by these methods would be of great interest.

Mobilities of Organic Ions. — The values of the mobilities l^ at 25° have been determined by Ostwald and Bredig for a large number of organic ions, both positive and negative. Ostwald {14) found that the mobility of the negative ions decreases as the number of atoms in the ion increases. It is easy to see why this should be so, for as the number of atoms increases, so also does the surface of the ion, and consequently its friction against the liquid. However, this friction does not increase with the mass of the atoms.

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