Mr Ghittock, The Migration Constants, etc. 55 



The Migration Constants of Dilute Solutions of Hydrochloric 

 Acid. By C. Chittock, M.A., Trinity College. 



[Read 23 November 1908.] 

 Introduction, 



The experiments described in the present paper were under- 

 taken with the object of throwing light on the cause of the 

 abnormally low electrical conductivity of dilute aqueous solutions 

 of strong acids and alkalies. It has long been known that whereas 

 the equivalent conductivity of a neutral salt becomes approxi- 

 mately constant at great dilution, that of a strong acid or alkali 

 attains a maximum value at a dilution of about one-thousandth 

 normal, and then decreases rapidly as the concentration is still 

 further diminished. This decrease might be due to one of two 

 causes ; first, to some interaction between the ions of the acid 

 or alkali and the solvent, which might lead to a decrease in 

 the amount of ionization or in the velocity of one or both of the 

 ions ; or secondly, to some interaction with the small quantities 

 of impurity which are always present in the most carefully pre- 

 pared water. 



It was thought that measurements of the migration constant 

 of an acid at varying degrees of dilution might be of assistance 

 in deciding between these alternatives. If a small current is 

 passed for a given time through a solution of some acid such as 

 hydrochloric acid, hydrogen will be evolved at the cathode and 

 oxygen at the anode. The total quantity of acid present will 

 remain unaltered, but near the cathode there will be a loss of 

 acid, and near the anode an equal gain. If it is assumed that 

 the whole of the conductivity of the solution is due to the ions 

 of the acid, measurements of these changes of concentration give 

 us the means of calculating the ratio of the ionic velocities of 

 hydrogen and chlorine. Such experiments would therefore enable 

 us to determine whether the abnormally small conductivity of 

 a dilute solution is associated with an abnormally small mobility 

 of one of its constituent ions. 



Let u, V represent the velocities of the chlorine and hydrogen 

 ions respectively under unit potential gradient. Then the migra- 

 tion constant for the anion is given by ^ = u/(u + v). If a quantity 

 of electricity Q coulombs is passed through the solution, the 

 number of gram-equivalents of acid gained at the anode and 



rtO 

 lost at the cathode will be equal to -^-^ , where q (= 96440 



coulombs) is the charge carried by one gram-equivalent of 

 either ion. 



