Questions and Answers
What is the mass of disodium EDTA required to prepare 500 mL of a 0.25 M solution?
46.50 g
Which indicator is commonly used in direct EDTA titrations for determining the endpoint?
Eriochrome Black T
What is the molarity of the EDTA solution if a 200 mg sample of pure calcium carbonate was dissolved in 500 mL and required 25 mL of the EDTA solution for titration?
0.05 M
In a zinc sulfate titration using EDTA, what is the role of the indicator?
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What is the purpose of the residual EDTA titration in the analysis of a metal ion?
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What is the indicator used in the direct EDTA titration of calcium?
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Why is residual EDTA titration applied to the analysis of Al and Bi complexes?
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What is the indicator used in the standardization of disodium EDTA solution?
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Which indicator is specifically used in the titration of Bismuth?
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What happens at the endpoint in Zinc sulfate titration?
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How is the displacement of Ca+2 from the EDTA complex achieved in direct EDTA titration?
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What is the primary role of EDTA in analytical chemistry?
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Which of the following metal ions form relatively weak or unstable complexes with EDTA?
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Why is disodium EDTA preferred over the free acid form for preparing standard EDTA solutions?
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What is the purpose of masking agents in EDTA titrations?
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Which of the following is a common method for masking aluminum ions in the presence of magnesium ions during EDTA titrations?
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In a residual EDTA titration, what is the purpose of the back-titration step?
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Study Notes
Disodium EDTA
- Disodium EDTA (C10H14N2Na2O8·2H2O) is an analytical agent that reacts with metal ions to form a water-soluble, stable complex, or chelate compound.
- It is preferred over the free acid in preparing the standard solution of EDTA since it is more water-soluble, non-hygroscopic, and very stable.
EDTA Formula and Calculations
- The formula to calculate the weight of disodium EDTA required is: wt = M x MW x L
- Example calculation: wt = 0.1M x 372g/mol x 0.5L = 18.6g disodium EDTA
Indicator Used in Direct EDTA Titration
- Indicators used in direct EDTA may be: EBT, Hydroxynaphthol blue, and pyridyl azonaphthol (red with Zn+2 which forms a violet complex)
Titration Methods
- Mercuric-EDTA complex reaction is used as the masking agent in the titration of calcium
- Residual EDTA titration is applied to the analysis of Al and Bi complex
- In the case of Zinc sulfate titration, a sensitive indicator is used to the titrant, and the metal being determined remains complexed with EDTA and does not interfere with Zn-EDTA complexation
- Bismuth titration forms a highly stable complex that can be titrated at a pH as low as 1 or 2, with dithizone as the indicator
Masking
- Masking is the term used to indicate the determination of a metal in the presence of another metal
- The masking agent is capable of entering into a complication reaction which is accomplished by:
- Adjusting the pH of the titration medium so that it will be favorable for the complexation of the metal that is being determined and not the other metal
- Using an auxiliary complexing agent such as Triethanolamine (TEA)
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