16 Questions
What type of adsorption indicator is used when the titrant is Ag+?
Anionic dye
What is the primary requirement for the precipitate in Fajan's method?
It should be in the colloidal state
Why should the sample solution not be too dilute in Fajan's method?
Because the amount of precipitate formed will be small
What is the purpose of using anionic adsorption indicators in Silver Nitrate titrations?
To detect the end point of the titration
What is the characteristic of weak acid adsorption indicators like Eosin and Fluorescein?
They can donate a proton (H+)
Why can't eosin (tetrabromo fluorescein) be used in the titration of Cl- with Ag+?
It is strongly adsorbed by the precipitate before the equivalence point
What is the main advantage of using adsorption indicators in Fajan's method?
They provide a sharp end point
What is the purpose of using Rhodamine-6-G as an adsorption indicator?
To detect the end point of the titration when the titrant is Cl-
Why is fluorescein not suitable for titration in slightly acid solutions?
It is a very weak acid
What is the minimum pH range for using dichlorofluorescein as an indicator?
pH 4.4
What is the advantage of using eosin as an indicator?
It can be used in highly acidic solutions
What is the purpose of adjusting the pH of the solution during a titration?
To ensure the indicator remains in its ionic form
Why should titrations with adsorption indicators be carried out with a minimum exposure to sunlight?
To ensure silver halides are not sensitized to the action of light
What type of complex is formed during the titration of silver in the presence of free nitric acid with standard potassium thiocyanate or ammonium thiocyanate solution?
Soluble colored complex
Which of the following indicators is suitable for the titration of iodides?
Rose Bengal
What is the color of the precipitate at the end point during the titration of cyanide ion with standard silver diphenylcarbazide nitrate solution?
Pale violet
Study Notes
Principle of Adsorption Indicators
- Used to determine chloride, bromide, iodide, thiocyanate, etc.
- Method employs adsorption indicators to detect endpoint
- Adsorption indicators are organic acidic or basic dyes with 2 different colors:
- One color when not adsorbed (in solution)
- Different color when adsorbed on the ppt at the endpoint
Conditions for Choosing a Suitable Adsorption Indicator
- Precipitate should be in the colloidal state to maximize indicator adsorption
- Precipitate must strongly adsorb its own ions
- Sample solution should not be too dilute to ensure a sharp endpoint
- Indicator ion must be of opposite charge to the ion of the precipitating agent
- Indicator ion should not be adsorbed before the particular compound is completely precipitated
- Concentration of indicator must not precipitate during titration or after the endpoint
- pH of the solution must be adjusted such that the ionic form of the indicator predominates
Examples of Adsorption Indicators
- Eosin (tetrabromo fluorescein): can be used for I-, Br-, or SCN- but not Cl- due to strong adsorption before the equivalence point
- Fluorescein: can be used for Cl- and has an optimum pH range of 7-10
- Dichlorofluorescein: stronger acid than fluorescein, applicable in slightly acid solutions of pH > 4.4, and can be used in more dilute solutions
- Eosin (tetrabromofluorescein): stronger acid, can be used down to pH 1-2, and has a sharpest color change in an acetic acid solution (pH < 3)
- Rose Bengal (dichloro tetraiodo fluorescein) and dimethyl diiodo-fluorescein: recommended for the titration of iodides
- Other dye-stuffs have been recommended as adsorption indicators for the titration of halides and other ions
Disadvantage of Adsorption Indicators
- Silver halides are sensitized to the action of light by the layer of adsorbed indicator, so titrations should be carried out with a minimum exposure to sunlight
This quiz covers the principles and conditions of titration of chlorides, including the use of indicators and the adjustment of pH. It also discusses the importance of concentration and ionic form of the indicator.
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