38 Questions
What is the fundamental basis of precipitimetry as a method of analysis?
Ability of some ions to form a precipitate upon treatment with certain reagents
What is the condition for the precipitate in precipitimetry?
The precipitate must be practically insoluble
What is the purpose of the solubility product rule?
To determine the solubility of sparingly soluble salts
What happens when a sparingly soluble salt is shaken in water or other solvent?
A very small amount of the solid phase is converted to soluble molecules
What is the general formula for the solubility product (Ksp)?
Ksp = [A-]n [B+]m
What is the significance of the solubility product (Ksp)?
It is a constant at constant temperature and pressure
What is the condition for the formation of a precipitate according to the solubility product rule?
The product of molar concentrations of ions each raised to a power equal to the number of ions produced is greater than Ksp
What is the importance of the end point in precipitimetry?
It must be easily detected
What is the effect of adding BaCl2 to a solution of BaSO4?
It decreases the solubility of BaSO4
What is the effect of adding KCl to a solution of BaSO4?
It increases the solubility of BaSO4
What is the effect of adding CN- to a solution of AgNO3?
It dissolves the precipitate of AgCN
What is the effect of adding NH3 to a solution of AgCl?
It dissolves the precipitate of AgCl
What is the common ion effect?
The effect of a similar ion on the solubility of a sparingly soluble salt
What is the diverse ion effect?
The effect of a diverse ion on the solubility of a sparingly soluble salt
What happens when a sparingly soluble salt is dissolved in a solution containing a common ion?
The solubility of the sparingly soluble salt decreases
What happens when a sparingly soluble salt is dissolved in a solution containing a diverse ion?
The solubility of the sparingly soluble salt increases
What is the purpose of adding excess AgNO3 in the first drop?
To react with Ag(CN)2- to form a white precipitate
What is the role of KI in Denige's method?
To detect the end point by forming a yellow turbidity of AgI
What is the color of the turbidity that indicates the end point in Denige's method?
Yellow
What is the advantage of using Denige's method over Liebeg's method?
It renders the end point more clear
What is the formula of the complex formed by Ag+ ions with KCN?
Ag(CN)2-
What is the purpose of using KCN in the reaction?
To form a complex with Ag+ ions
What is the minimum concentration of Ag+ required for AgCl to precipitate?
10^-9
Why does AgI precipitate before AgCl?
AgI has a lower solubility product than AgCl
What is the term for the type of successive precipitation using the same precipitating agent?
Fractional Precipitation
What is the purpose of a titration curve in precipitimetry?
To determine the sharpness of the end point and choose the most suitable indicator
What determines the shape of the titration curve in precipitimetry?
All of the above
What is the significance of the inflection point on the titration curve?
It determines the sharpness of the end point
Why does a higher concentration of the precipitating agent and analyte result in a sharper end point?
Because the reaction is more complete at higher concentrations
What is the effect of the solubility product of the precipitate on the sharpness of the end point?
A lower solubility product results in a sharper end point
What is the product formed when silver is added to a solution containing cyanide ions?
Silver cyanide
What is the purpose of adding more cyanide to a solution containing silver cyanide?
To form a soluble argentocyanide complex
What is the end point of the precipitation reaction in argentometry?
Disappearance of the yellow color of the chromate indicator
What is the medium required for the titration reaction in argentometry?
Neutral to slightly alkaline
What is the indicator used in Mohr's method?
Chromate
What is the reason for using a dilute solution of chromate in Mohr's method?
To mask the trace color of silver chromate
What is the condition required for the formation of a precipitate in argentometry?
pH range of 7-9
What is the limitation of Mohr's method?
It is not suitable for highly acidic or basic solutions
Study Notes
Precipitimetry
- Precipitimetry is a quantitative volumetric method of analysis based on the ability of some ions or compounds to form a precipitate upon treatment with certain reagents.
Conditions for Good Application of Precipitimetry
- The precipitate must be practically insoluble.
- Precipitation should be rapid.
- The end point must be easily detected.
Solubility Product Rule
- The solubility product rule controls the formation of precipitates and is applied only for sparingly soluble salts.
- For a sparingly soluble salt having the general formula AnBm, the solubility product will be: Ksp = [A-]n [B+]m, where Ksp is constant at constant temperature and pressure.
Factors Affecting Solubility of Sparing Soluble Salts
- Common ion effect: the presence of a common ion decreases the solubility of the sparingly soluble salt.
- Diverse ion effect: the presence of a diverse ion increases the solubility of the sparingly soluble salt.
- Complex formation: the formation of a complex can increase the solubility of the precipitate.
Titration Curves in Precipitation Reactions
- A titration curve is a plot of –log molar concentration of the precipitated ion versus ml of titrant.
- The sharpness of the inflection point on the curve determines the end point.
Fractional Precipitation
- Fractional precipitation is a type of successive precipitation using the same precipitating agent.
- The substance with the lower Ksp precipitates first.
Denige's Method
- Denige's method is a modification of Liebeg's method to render the end point more clear.
- It uses KI in an ammonical medium to detect the end point.
Applications of Precipitimetry
- Precipitimetry can be used for the determination of Cl- and Br-.
- The choice of method depends on the type of indicator used.
Mohr's Method
- Mohr's method is used for the determination of Cl- and Br-.
- The titrant is Ag+, and the indicator is chromate (concentration: 0.015 M).
- The end point is a brick-red color.
Learn about precipitimetry, a quantitative method of analysis based on the formation of precipitates upon treatment with certain reagents. Presented by Dr. Nada Samy.
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