Photometric complexometric titration
May 25, 2020
Article
Complexometric titration was discovered in 1945 when Gerold Schwarzenbach observed that aminocarboxylic acids form stable complexes with metal ions, which can change their color by addition of an indicator. From the 1950’s on, this technique gained popularity for the determination of water hardness. Soon it was clear that aside from magnesium and calcium, other metal ions could also be titrated in this way. The use of masking agents and new indicators gave further possibilities to determine not only the whole amount of metal ions present in solution, but also to separate and analyze them. A new titration type was born: complexometric titration.
Have you ever performed a complexometric titration? Many of you will respond “yes” as it is one of the most frequently used types of titration. However, you have probably struggled over the detection of the endpoint and over the titration itself. In contrast to other types of titrations, the boundary conditions such as pH and reaction time play an even bigger role in complexometry since the complex binding constant is very pH dependent and the reaction might be slow. This article presents the most common challenges and how to overcome them when carrying out complexometric titrations.
For a complexometric titration analysis, it is very important to know the qualitative composition of your sample. This determines the indicator, the complexing agent, and the masking agent you need to use.
The following topics will be covered (click to jump directly to a section):.
Complexometric reactions always consist of a metal ion which reacts with a ligand to form a metal complex. Figure 1 shows an example of such a chemical reaction of a metal ion Mn+ with ethylenediaminetetraacetic acid (EDTA). EDTA is the most commonly used titrant for complexometric titrations and reacts in a stoichiometric ratio of 1:1. As shown on the right side of Figure 1, EDTA can form six coordinational bonds, in other words: EDTA has a denticity of six. The more coordinational bonds a ligand can form, the more stable the formed complex is.