Physical Chemistry , 1st ed.

Viscosity is typically measured in units of poise, where

1 poise  1 

cm

g

s



Using the expression for Stokes’ law, the velocity of the ions becomes

vi

e

6 

zi





r



i

E



Substituting into the expression for current,Iibecomes

Iie^2 zi^2 

N

V

iA

6 

E

ri

 (8.61)

This equation shows that the ionic current is related to the square of the

charge on the ion. For virtually all ionic solutions, the ionic currents of the

positive and negative ions I and I will be different. In order to maintain

overall electrical neutrality, the oppositely charged ions have to move at dif-

ferent velocities.

Finally, the basic relationship between the voltage Vacross a conductor and

the current Iflowing through the conductor is known as Ohm’s law:

VI (8.62)

The proportionality constant is defined as the resistance, R, of the system:

VIR

Measurements of the resistances of ionic solutions show that the resistance is

directly proportional to the distance,, between two electrodes and inversely

proportional to the area Aof the electrodes (which usually are the same size):

R

A



 (8.63)

The proportionality constant is called the specific resistanceor the resistivity

of the solution, and has units of ohmmeter or ohmcm. We also define

the conductivity(also called the specific conductance) as the reciprocal of the

resistivity:





1

 (8.64)

Conductivities have units of ohm^1 m^1 .* Resistivities or conductivities are

extremely easy to measure experimentally using modern electrical equipment.

However, as one might expect, they are quite variable because would depend

not only on the charge on the ions but on the concentration of the solution.

It is better to define a quantity that takes these factors into account. The equiv-

alent conductanceof an ionic solute,, is defined as



N



 (8.65)

where Nis the normality of the solution (is the capital Greek letter lambda).

Recall that normalityis defined in terms of number of equivalents per liter of

solution. The use of equivalents rather than moles takes ionic charge into

account.

236 CHAPTER 8 Electrochemistry and Ionic Solutions

*The unit siemen(abbreviated S) is defined as ohm^1 , so conductivity values are some-

times given in units of S/m.