Types of complexometric titration

Direct titration
In this case, the metal ion is titrated with a standard solution of EDTA. The solution containing the metal ion is buffered to an appropriate pH at which the stability constant of the metal-EDTA complex is large. The free indicator has a different colour from that of the metal-indicator complex.

Back titration
In certain circumstances a particular metal ion cannot be titrated directly. This includes situations where:

• The metal ion precipitates in the absence of EDTA.
• The metal ion reacts too slowly with EDTA.
• The metal ion forms an inert complex.
• No suitable indicator is available.

In these cases a back titration is required. This involves addition of a known excess of EDTA to the metal ion (buffered to an appropriate pH). Then, the excess EDT A is titrated with a standard solution of a different metal ion. The choice of a second metal ion is important as it must not displace the analyte metal ion from its EDTA complex.

Table 23.2 Properties of selected indicators