Concise Physical Chemistry

c13 JWBS043-Rogers September 13, 2010 11:27 Printer Name: Yet to Come

208 COULOMETRY AND CONDUCTIVITY

the contributions to◦as

◦KCl=λ◦K++λ◦Cl−

◦NaCl=λ◦Na++λ◦Cl−

..

.

and so on.

In this way, the difference found for

◦KCl−◦NaCl=◦KNO 3 −◦NaNO 3

would be merely the difference

◦KCl−◦NaCl=λ◦Na++λ◦Cl−−λ◦K+−λ◦Cl−

=λ◦Na+−λ◦K+

and would be the same for all pairs of sodium and potassium electrolytes because the

anions, Cl−,NO− 3 ,OH−..., would always cancel out. The same would be true for

pairs of electrolytes containing a common anion, because the cation would always

cancel out. In these and similar calculations, the integral charge on an ion must always

be taken into account so that the expression for Kohlrausch’s law of independent ion

migration is

◦=ν+λ++ν−λ−

whereνis the number of charges on each ion,ν=1 for the examples shown.

13.4 PARTIAL IONIZATION: WEAK ELECTROLYTES

So far we have considered mainly strong electrolytes, those for which ionization

is complete in aqueous solution. This is not always the case. Ionization of weak

electrolytes is partial. The usual example chosen is that of acetic acid for which

ionization at ordinary concentrations is very small. Examples of the vast difference in

behavior can be seen by comparing the Kohlrausch conductivity curves in Fig. 13.3

for HCl, which is completely ionized, with that of acetic acid (HOAc), which is not.

Ionization of HOAc does, however, rise rapidly in dilute solutions, and Kohlrausch’s

law of independent migration holds at infinite dilution. At infinite dilution, the weak

electrolyte HOAciscompletely dissociated (all electrolytes are), so we have a very

simple way of determining the approximatedegree of ionizationαat some finite

concentrationcby comparingto◦. We simply take the ratio

α=



◦