Instant Notes: Analytical Chemistry

Section C – Analytical reactions in solution

C10 CONDUCTIMETRY

Movement of ions When an electric field is applied across a solution, a force is exerted on the ions
that will cause them to move. Positive ions will move toward the more negative
electrode, negative ions toward the more positive, but each will carry current.
The speed with which the ions move usually depends on:

(i) the electric field strength (V m-^1 );

(ii) the charge zon the ion;

(iii) the size of the ion in solution; and

(iv) the viscosity of the solvent.

In water, the hydrogen ion, H 3 O+, and the hydroxyl ion, OH-, are small, and

move most rapidly by a very fast exchange with the molecules of water. In

dilute solutions, the ions move independently of each other.

For an electric field of 1 V m-^1 , the ionic speed is called the mobility, ui. Table

1 gives the ionic size and mobility of selected ions in aqueous solutions at 25∞C.

It is worth noting that despite their larger size in crystals, hydrated potassium

ions are smaller than hydrated sodium ions when in solution and move faster.

Key Notes

Ions in solution or in molten ionic solids will move when an electric field

is applied. The speed of movement will depend on the size and charge of

the ion.

In ionic solutions, the ions carry the current, but Ohm’s law still applies.

Increasing the number of ions increases the conductance.

Since analysts are concerned with concentrations, it is preferable to

compare molar conductivities, which depend on the chacteristics of the

ions.

If the number or nature of ions present in a cell change, then the

conductance will change. This is useful for many types of titration,

including some in nonaqueous media.

Related topics Other topics in Section C.

Movement of ions

Conductance

Conductivity

Conductimetric

titrations

Table 1. Selected vales of mobility and ionic radius

Ion 108 ui/m^2 s-^1 V-^1 ri/nm

Na+ 5.2 0.18

K+ 7.6 0.13

F- 5.5 0.17

Cl- 7.9 0.12

Br- 8.1 0.12