Instant Notes: Analytical Chemistry

allows the determination of volumes to 0.01 cm^3. This corresponds to about

0.05% for a typical titration volume of 20 cm^3.

Weighing solutes prior to dissolution gives comparable accuracy. Pre-deter-

mined volumes, for example 25 cm^3 are measured with manual or mechanical

pipets, while cumulative volumes of titrant during addition are measured by a

manual or automated buret.

Any reaction used in titrimetry will cause the concentrations of the species in

solution to change. For acid-base reactions, the concentration of hydrogen ions,

and hence the pH, will alter and similar changes in the ionic concentrations

occur with other reactions.

Figure 1shows the pH values in the titration of a strong base by a strong acid

as a function of the volume of acid added. pH is discussed in Topic C4. The

figure shows that there is only a small change in pH before about 24 cm^3 where

there is still an excess of base, and also after 26 cm^3 where there is an excess of

acid. However, in the region around 25 cm^3 , where the acid exactly neutralizes

the base, the change in pH is very large. This signifies the end point of the

titration.

Provided that the end point coincides with the equivalence pointor the stoi-

chiometry of the reaction studied, the amounts of titrant and titrand measured

should correspond to the actual amounts present.

For a general reaction:

aA +bB =cC +dD

a moles of A react with b moles of B to produce c moles of C and d moles of D.

A molar solution contains 1 mole of solute per 1000 cm^3 (or 1 l) of solution.

Therefore, 1 cm^3 of a 1 M solution contains 1 mmol of solute.

C5 – Titrimetry I: acid–base titrations 81

14

12

10

8

6

4

2

0

pH

0 1020304050

Volume of acid/cm^3

Fig. 1. Titration of 25 cm^3 of 0.1 M NaOH with 0.1 M HCl (pH against volume added).