Complexometric titration, or chelatometry, is a type of titration (volumetric technique) that uses a colored compound to identify the endpoint of a chemical reaction. This is the most efficient method for determining the amount of a specific type of metal ion in a solution.
Complexometric titrations are used in analytical chemistry for determining the presence of different metal ions in sample solutions. Generally, the endpoint of the complexometric titration is determined using an indicator that changes color, such as eriochrome black T, murexide, fast sulphon black, calcein, etc.
Explain the theory of the metal ion indicator:
In complexometric titrations, organic dyes are the most commonly used indicators. These dyes work by forming a colored complex with the metal ion that is being titrated. During the reaction, the indicator is replaced by EDTA, which forms a more stable complex with the metal, and when the reaction is finished, the color changes.
The following reaction can be used to illustrate the use of a metal ion indicator in an EDTA titration:
M-In + EDTA M-EDTA + In
This reaction will take place, only if the m-EDTA complex is more stable than the m-In complex.
M + In M-In
The equation describing the stability of the metal indicator complex KIn be given here.
[M-In]
KIn = ________
[M]
[In]
Since the change in color of the indicator is also affected by the concentration of hydrogen ions, it is convenient to define a conditional indicator constant K’In, which fluctuates with pH.
K' In be more than 104, and the ratio of the conditional formation constant of the metal-EDTA K’ MY to K’In must be of the order 104 to offer a good endpoint. This will ensure that the error is minimal.
The strength of the metal-indicator complex in comparison to the metal-EDTA complex determines how accurate the end point of the indicator is. After the equivalence point, the color will change if the metal-indicator complex is too strong. But, if the metal-indicator complex is not strong enough, you will not reach the equivalence point until the endpoint is reached.
The majority of metallochromic indicators are also weak acids. This results, for example, in a pH dependence of the conditional formation constant for the metal-indicator complex.
As a result, the conditional formation constant for the metal–indicator complex relies on the pH of the titrant. This allows some control over the titration error of an indicator, as the strength of a metal–indicator complex can be altered by adjusting the pH at which the titration is performed. However, since the indicator is a weak acid, pH also affects the color of the uncomplexed indicator.
References:
- Complexometric titration with EDTA, Part III,lscollege, Available Here:
- ‘Complexation Titration’. Chemistry LibreTexts, 23 Nov. 2014, Available Here:
- ‘Complexometric Indicator’. Wikipedia, 12 Aug. 2022. Wikipedia, Available Here:
- ‘Colours of Transition Metal Ions’. Unacademy, Available Here:
Keywords:
Principle, EDTA titration curve, titrand, metallochromic indicators, metal-ligand complexes ph of buffer solution, , acid-base, types of metal ion indicator, blue, red, hardness of water, electrode, selectivity, Calmagite, PAN, masking and damasking agents, example, properties, name, role, redox.
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