5.8
EDTA: Indirect and Alkalimetric Titration
Unlike direct titration, back-titration, and displacement titration, indirect titration is an EDTA titration method for quantifying anions. In the ind…
Like direct, back, and displacement titration, indirect titration is an EDTA titration involving metal ions.
In an indirect titration, anions are quantified by precipitating their insoluble salts with excess metal ions and then either directly titrating the excess metal in the filtrate or extracting and back-titrating the metal in the precipitate with EDTA.
For example, the addition of a known excess of barium ions to a sulfate solution generates a barium sulfate precipitate, which is filtered and washed.
The filtrate containing the excess barium ions is titrated with standard EDTA until the end point is reached.
The last type is alkalimetric titration, which is similar to acid–base titration.
This method involves the addition of disodium EDTA to a metal ion solution, which forms the metal–EDTA complex with the liberation of two equivalents of hydrogen ion.
This hydrogen ion solution is titrated with standard sodium hydroxide using a suitable acid–base indicator to obtain the end-point concentration.
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Q1: What is indirect titration and how does it differ from direct titration?
Indirect titration quantifies anions by precipitating them as insoluble salts with excess metal ions, then titrating the remaining metal in the filtrate with standard EDTA. Unlike direct titration, which measures the analyte directly, indirect titration measures excess reagent to calculate the original analyte amount. This approach is useful when direct titration is impractical or when the analyte cannot be directly complexed.
Q2: How does the back-titration approach work in indirect EDTA titration?
In back-titration, anions are first precipitated with excess metal ions. The precipitate is then filtered and washed. The metal ion is extracted from the precipitate and back-titrated with standard EDTA to determine the endpoint. This method allows indirect calculation of the original anion concentration by measuring the amount of metal that was bound to the precipitate.
Q3: What happens during the filtration and washing steps in indirect titration?
After precipitating anions as insoluble salts with excess metal ions, the mixture is filtered to separate the solid precipitate from the liquid filtrate. Washing removes residual soluble salts and impurities from the precipitate surface. The filtrate containing excess metal ions is then directly titrated with standard EDTA to reach the endpoint and determine the anion concentration.
Q4: What is alkalimetric titration and how does it relate to acid-base titration?
Alkalimetric titration is similar to acid-base titration but uses disodium EDTA added to a metal ion solution. The metal-EDTA complex formation liberates two equivalents of hydrogen ions. These hydrogen ions are then titrated with standard sodium hydroxide using an appropriate acid-base indicator until the endpoint is reached, allowing quantification of the metal ion.
Q5: Why are hydrogen ions liberated when disodium EDTA reacts with metal ions?
Disodium EDTA contains two ionizable hydrogen atoms. When it forms a complex with metal ions, these hydrogen ions are released into solution as the EDTA ligand binds to the metal. The liberation of two equivalents of hydrogen ions creates an acidic solution that can be quantified through titration with standard sodium hydroxide, providing an indirect measure of metal ion concentration.
Q6: What role does the acid-base indicator play in alkalimetric titration?
The acid-base indicator detects the endpoint of the alkalimetric titration by changing color when the pH reaches a specific value. As standard sodium hydroxide is added to neutralize the hydrogen ions liberated from metal-EDTA complex formation, the indicator signals when all hydrogen ions have been neutralized. This color change marks the endpoint, allowing calculation of the original metal ion concentration.
Q7: How is the anion concentration calculated from indirect titration data?
The anion concentration is calculated indirectly by determining how much metal ion was consumed during precipitation. By measuring the excess metal ions remaining in the filtrate through EDTA titration, the amount of metal that precipitated with the anions can be determined. Since the stoichiometry of the metal-anion precipitation is known, the original anion concentration is calculated from the metal consumption.