Instant Notes: Analytical Chemistry

Lm=k/c =3.85/100 =0.0385W-^1 m^2 mol-^1

For strong electrolytes, it must be noted that, because of ionic interaction and

the effects of the ionic atmosphere, the molar conductivity decreases as the

concentration increases, according to the Onsager equation:

Lm =Lom-Kc

where Lomis the molar conductivity at infinite dilution, and Kis a constant,

dependent on temperature and the solute.

For weak electrolytes, the ionic concentration depends on the dissociation

constant, and since the relationship of the dissociation constant, Kto the degree

of ionization ais, for a weak acid, approximately:

K =ca^2 /(1 -a)

The molar conductivity of a weak electrolyte may be written:

Lm =aLom

These equations show that, while Lmfor a strong electrolyte decreases almost

linearly with the (c)1/2, for weak electrolyes such as ethanoic acid, it decreases

approximately as (1/c)1/2. More details will be found in textbooks of physical

chemistry.

Direct measurement of concentration is possible. The presence of sodium

chloride, or other ionic species will increase conductance, and by knowing

the molar conductivity, the concentration may be found by measuring the

conductivity of the solution.

When very pure water is required for the preparation of pure organics, or the

manufacture of semiconductor components, conductance measurements are

most useful. Provided dissolved CO 2 and other intrinsic factors are eliminated,

extremely pure water should have a conductivity of around 150 mS m-^1 (or

1.5 mS cm-^1 ) at 25∞C. In order to comply with regulations for some work, it

must be less than 200 mS m-^1.

Whenever titrations of ionic solutions are carried out, the number and nature of

the ions change throughout the entire titration. If a strong base, say NaOH, is

titrated with a strong acid, HCl, the reaction

Na++OH-+H++Cl-=Na++Cl-+H 2 O

will first of all remove the OH-as nonconducting water, so the conductance will

decrease until the end point. Then excess HCl will increase the conductance.

This gives a ‘V’ shaped graph of conductance versus titer. If a mixture of acids is

used, they will be titrated in the order of their strength. Figure 1(a)shows the

conductimetric titration of a mixture of HCl, boric acid and ammonium chloride

using NaOH.

For precipitation reactions:

Ag++NO 3 - +Na++Cl-=AgCl(s) +Na++NO 3 -

The sodium and silver ions have similar conductivities, so adding NaCl solution

does not change the conductance much until the end point is reached. Then the

conductance rises with excess NaCl. This is shown in Figure 1(b). The titrant is

often about ten-times more concentrated than the sample solution, to avoid

dilution errors.

Conductimetric
titrations

106 Section C – Analytical reactions in solution