Chemistry: Masking and Demasking Agents

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Questions and Answers

What is the purpose of adding a demasking agent in complexometric titration?

To react with the interfering metal ions

To remove the masking agent

To reverse the masking reaction and titrate the metal ion (correct)

To form a more stable complex with the metal ion

What is the result of direct titration of a mixture of Cu, Cd, and Ca with EDTA?

Only Cu is titrated

Cu and Cd are titrated, but not Ca

The sum of the three metals is titrated (correct)

Only Ca 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 (correct)

To demask the metal ions

To remove the EDTA from the solution

To react with EDTA and form a complex

What is the function of BAL (2,3-dimercapto-1-propan-1-ol) in complexometric titration?

To mask Sn, Sb, Cd, and Hg Signup and view all the answers

What is the type of titration that involves the addition of a titrant to the solution until the reaction is complete?

Direct titration Signup and view all the answers

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 Signup and view all the answers

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 Signup and view all the answers

What is the role of EDTA in complexometric titration?

To react with the metal ions and form a complex Signup and view all the answers

What is the primary reason for using the replacement titration method?

When direct or back titrations do not give sharp end points Signup and view all the answers

What is the role of mercury in the indirect titration of barbiturate?

It forms a complex with the barbiturate anion Signup and view all the answers

What is the purpose of using oxalate in metal ion complexation?

To precipitate out Ca ions Signup and view all the answers

What is the equation for the titration step in the replacement titration method?

Mg2+ + EDTA4- ↔ Mg-EDTA2- + 2H+ Signup and view all the answers

What is the purpose of using fluoride in metal ion complexation?

To react with Al ions Signup and view all the answers

What is the indirect titration method used for?

Analyzing the concentration of barbiturate anions Signup and view all the answers

Why are chelons considered unselective titrants?

Because they usually give the total metal content Signup and view all the answers

What is the role of hydroxide in metal ion complexation?

To react with some transitional metals, Fe and Mn Signup and view all the answers

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 Signup and view all the answers

Which of the following types of titration is based on the principle of acid-base reaction?

Complexometric Titration Signup and view all the answers

What is the significance of pH control during complexometric titration?

To control the role of H+ ion in chelation Signup and view all the answers

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 Signup and view all the answers

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 Signup and view all the answers

Which of the following types of titration involves the reaction between a metal ion and a complexing agent?

Complexometric Titration Signup and view all the answers

What is the primary role of a buffer solution in complexometric titration?

To maintain the pH of the solution Signup and view all the answers

How does the number of steps involved in complex formation affect the titration curve?

It increases the break in the titration curve Signup and view all the answers

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

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Description

This quiz covers the concepts of masking and demasking agents in chemistry, including their roles in titration and equilibrium reactions.