24 Questions
What is the purpose of adding a demasking agent in complexometric titration?
To reverse the masking reaction and titrate the metal ion
What is the result of direct titration of a mixture of Cu, Cd, and Ca with EDTA?
The sum of the three metals is titrated
What is the purpose of adding cyanide to the solution in the analysis of a mixture of Cu, Cd, and Ca?
To mask Cu and Cd, leaving only Ca to be titrated
What is the function of BAL (2,3-dimercapto-1-propan-1-ol) in complexometric titration?
To mask Sn, Sb, Cd, and Hg
What is the type of titration that involves the addition of a titrant to the solution until the reaction is complete?
Direct titration
What is the purpose of using masking agents in complexometric titration?
To selectively react with interfering metal ions and prevent them from reacting with the EDTA
What is the result of adding formaldehyde to the cyanide-containing mixture in the analysis of a mixture of Cu, Cd, and Ca?
Only Cd is demasked and titrated
What is the role of EDTA in complexometric titration?
To react with the metal ions and form a complex
What is the primary reason for using the replacement titration method?
When direct or back titrations do not give sharp end points
What is the role of mercury in the indirect titration of barbiturate?
It forms a complex with the barbiturate anion
What is the purpose of using oxalate in metal ion complexation?
To precipitate out Ca ions
What is the equation for the titration step in the replacement titration method?
Mg2+ + EDTA4- ↔ Mg-EDTA2- + 2H+
What is the purpose of using fluoride in metal ion complexation?
To react with Al ions
What is the indirect titration method used for?
Analyzing the concentration of barbiturate anions
Why are chelons considered unselective titrants?
Because they usually give the total metal content
What is the role of hydroxide in metal ion complexation?
To react with some transitional metals, Fe and Mn
What is the primary reason why stability constants are only valid for a specific pH?
Due to the effect of pH on the formation of complex ions
Which of the following types of titration is based on the principle of acid-base reaction?
Complexometric Titration
What is the significance of pH control during complexometric titration?
To control the role of H+ ion in chelation
What is the primary factor that determines the magnitude of break in the titration curve at the end point?
The stability of the complex formed
What is the significance of the sudden increase in pM at the end point in complexometric titration?
It indicates the removal of traces of metal ions from solution
Which of the following types of titration involves the reaction between a metal ion and a complexing agent?
Complexometric Titration
What is the primary role of a buffer solution in complexometric titration?
To maintain the pH of the solution
How does the number of steps involved in complex formation affect the titration curve?
It increases the break in the titration curve
Study Notes
Complexometric Titration
- Complexometric titration involves the reaction between a metal ion and a complexing agent (chelon) to form a stable complex.
- Interfering metal ions can be masked by adding auxiliary chelating compounds that selectively react with them, forming more stable complexes than the chelon.
Masking Agents
- Cyanide: masks Ni, Co, Zn, Cd, Cu, Fe2+
- Iodide: masks Hg
- Ascorbic acid: masks Fe3+, Cu
- Triethanolamine: masks Fe3+, Al, Mn
- BAL (2,3-dimercapto-1-propan-1-ol): masks Sn, Sb, Cd, Hg, Zn
- Tiron (disodium catechol-3,5-disulfonate): masks Al, Fe3+
Demasking Agents
- Formaldehyde: demasks Cd in the presence of cyanide
- Chloral hydrate: demasks metals in the presence of masking agents
Titration Steps
- Step 1: Direct titration with EDTA to determine the sum of metal ions (e.g., Cu, Cd, Ca)
- Step 2: Masking with cyanide to remove Cu and Cd, leaving only Ca to be titrated with EDTA
- Step 3: Demasking with formaldehyde to titrate Ca and Cd with EDTA
Advantages of Complexometric Titration
- High accuracy and precision
- Applicable to a wide range of metal ions
- Can be used to determine the stability constants of complexes
Principle of Complexometric Titration
- Acid-base reaction between metal ions and complexing agents
- Similar to acid-base titrations, with the end point marked by a sudden change in pM (negative log of metal ion concentration)
Factors Affecting the Titration Curve
- Stability of the complex formed
- Number of steps involved in complex formation
- Effect of pH (constant pH is crucial, controlled by buffer solutions)
Indirect Titration
- Used for determination of anions that do not react with EDTA
- Involves displacing Mg from Mg-EDTA chelate with the metal ion to be analyzed, then titrating the freed Mg with standard EDTA solution
Titration Selectivity and Masking Reagents
- Chelons are unselective titrants, requiring methods to titrate individual metal ions in mixtures
- Selective precipitants can be used to separate out interfering metal ions or groups of metal ions
This quiz covers the concepts of masking and demasking agents in chemistry, including their roles in titration and equilibrium reactions.
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