Chemistry, Third edition

The equivalence point is reached when the reactants have reacted together in an

exact ratio of quantities, as given in the balanced equation for the reaction. For

example, in an acid–base titration, the equivalence point involves addition of exactly

enough base to neutralize any acid present.

The technique whereby one reactant is slowly added to a second reagent until the

equivalence point is reached is called a titration.Chemical indicatorsare substances

that change colour at a particular ratio of reactant concentrations called the end-

point. In a titration, a chemical indicator is chosen so that the endpoint and equiva-

lence point are the same.

By measuring the volumes of the solutions that have reacted together, and using

the balanced equation for the reaction, the unknown solution concentration can be

determined. If carried out carefully, volumetric analysis is quite an accurate tech-

nique and a skilled worker should determine an unknown concentration within an

error of no more than 0.2%.

An acid–base titration

The (previously unknown) concentration of an acid can be determined by titrating

it with a standard alkaline solution. The acid is neutralized by the alkali. The pro-

cedure is as follows:

1.A measured volume of the acid is delivered into a conical flask, using a pipette as

shown in Fig. 9.3.

2.A few drops of a suitable indicator are added to the acid solution.

3.The standard alkaline solution is added from a burette. A burette is a long glass

tube with graduation marks on it, with a stopcock at the bottom end. It allows the

user to measure the volume of solution delivered through the stopcock. The alka-

line solution is added until the endpoint is reached and the indicator changes

colour; the total volume of alkaline solution added is then noted. A diagram of

this procedure is shown in Fig. 9.4.

Some typical results from such a titration are shown in the following example.

VOLUMETRIC ANALYSIS 141

Fig. 9.3Delivering a

measured volume of acid.

Fig. 9.4Addition of standard

alkaline solution.

Example 9.4

Hydrochloric acid, of unknown concentration, was titrated with

a standard solution of potassium hydroxide of concentration

0.500 mol dm^3. It was found that 25.0 cm^3 of the acid reacted

with 37.5 cm^3 of the potassium hydroxide solution. Calculate the

concentration of the hydrochloric acid.

Answer

Write the balanced equation for the reaction:

HCl(aq) + KOH(aq) KCl(aq) + H 2 O(I)

Work out the molar relationship between the two reactants:

1 mol HCl 1 mol KOH