17 Questions
What is the main purpose of the titration described in the text?
To determine the concentration of chloride ions
What is the titrant used in the precipitation titration described in the text?
Silver nitrate solution
What is the indicator used in the precipitation titration described in the text?
Potassium chromate
What is the endpoint indication in the precipitation titration described in the text?
Formation of a reddish-brown precipitate
What is a limitation of precipitation titrations mentioned in the text?
They are limited to analytes that form insoluble precipitates with the titrant.
Why is it difficult to prepare and store standardized solutions of iodine, as mentioned in the text?
Iodine is a volatile compound.
What is the primary reason for using a small quantity of potassium chromate (K2CrO4) in Mohr's method?
To ensure that silver chloride precipitates before silver chromate
Which of the following statements accurately describes the role of the reddish-brown precipitate in Mohr's method?
It indicates the end point of the titration
Which of the following is a limitation of precipitation titrations like Mohr's method?
Difficulty in detecting the end point due to the formation of colloidal precipitates
In Mohr's method, what is the purpose of using silver nitrate (AgNO3) as the titrant?
To precipitate the chloride ions as silver chloride
What is the primary advantage of using a precipitation titration like Mohr's method over other analytical techniques?
Simplicity and cost-effectiveness for determining specific anions
Which statement accurately describes a limitation of precipitation titrations?
Only a few halide ions can be titrated using this method.
In the determination of chloride ions using silver nitrate, what is the significance of the reddish-brown color remaining stable?
It suggests the presence of excess silver ions in the solution.
What is a disadvantage of the Mohr's method of precipitation titration?
It can only be carried out between pH 7 and 10.
Which statement accurately describes the reaction between chloride ions and silver ions in the determination of chloride concentration?
Chloride ions react with silver ions to form a white precipitate of silver chloride.
What is a common issue associated with precipitation titrations?
The endpoint is difficult to determine due to co-precipitation reactions.
In the determination of chloride ions using silver nitrate, what happens when the reddish-brown color remains stable?
It means that the endpoint has been reached, and no more silver ions are needed.
Study Notes
Precipitation Titration
- Precipitation titration is a type of titration that involves the formation of a precipitate (insoluble solid) at the end point of the titrimetric analysis.
- It is based on the production of precipitation at the end point of the titrimetric analysis.
Limitations of Precipitation Titration
- Limited to reactions involving the precipitation of silver ions with anions like halogens or thiocyanate (SCN–).
- Co-precipitation reactions can occur, making it difficult to determine the composition of the precipitate.
- Only very few halide ions can be titrated using this method.
- Endpoint is difficult to analyze.
Determination of Chloride using Silver Ion
- Concentration of chloride ion can be determined by titrating with a solution of a known concentration of silver nitrate.
- Silver chloride precipitates out as a white substance.
- Endpoint is reached when the reddish-brown color remains stable and does not disappear with stirring.
Mohr's Method of Precipitation Titration
- Uses AgNO3 as the titrant and a small quantity of potassium chromate (K2CrO4) as the indicator to determine the concentration of chloride in a given sample.
- Silver chromate has a much lower Ksp value, but AgCl preferentially precipitates due to the higher concentration of chloride ions.
- When all chloride ions have been converted to AgCl, additional AgNO3 reacts with yellow K2CrO4 to produce a reddish-brown precipitate of Ag2CrO4.
Limitations of Mohr's Method
- Can only be carried out between pH 7 and 10.
- In acidic environments, chromate (CrO4 2–) is converted to chromic acid, which does not react with excess silver ions.
Iodometry
- Involves the formation of iodine in an acidic or neutral media, which is then titrated using a reducing agent like thiosulphate.
- Iodine is oxidized first and subsequently reduced by the reducing agent.
- Endpoint is reached when the dark-blue color disappears.
Experiment 7: Determination of Chloride
- Titrant: AgNO3 solution.
- Analyte: Cl− (from NaCl).
- Indicator: K2CrO4.
- Endpoint: reddish-brown precipitate.
Learn about different methods of precipitation titration including Volhard’s Method, Fajan’s Method, and Mohr's Method. Understand how Mohr’s method uses K2Cr2O4 to preferentially precipitate AgCl over Ag2CrO4 by keeping chloride concentration higher than chromate concentration.
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