Precipitation Titration Part-3: Fajan’s method, Mohr method,Volhard method & Question Answers Discussion

Precipitation Titration Part-3: Fajan’s method, Mohr method,Volhard method & Question Answers Discussion

Fajan’s method :-

The precipitation titration in which silver ion is titrated with Halide or thiocyanate ions in presence of adsorption indicator is called as fajan’s method.

The indicator which is a dye existing in solution as the ionized from usually anion .

This method is generally used for the quantitative analysis of halide ions or thiocyanate ions.

Indicator :- flurosine

End point :- pink colour

Procedure:-

Take 10 ml sample of NaCl.

Add few drop of fluorescein indicator

Titrate the above mixture solution by using standard 0.1 N AgNO3 solution.

At the end point pink colour precipitation will occur on the surface of AgCl precipitation.

Precaution :-

  • The pH of the titration  medium must be controlled to ensure a sufficient concentration of the ion of the weak acid or weak base indicator.
  • Fluorescein for has a Ka 10-7 and in solution  more than acidic than pH 7 the concentration of fluroscence in ion is so small that no colour change is observed.
  • The AgCl should not be allowed to coagulation into large particle at the eq.point since this will greatly decrease the surface available for adsorption of the indicator.
  • A protective colloid such as dextrin should be added to keep the prrcipitate highly dispersing.

Mohr method

Advantage :- sample method

Disadvantage :- Alkaline solution only, Not suitable for iodide

Fajan’s method

Advantage :- capability for different PH range and selectively with different indicator.

Disadvantage :- Difficulty with dilute solution should not be a high background ionic level

Volhard method :-

Advantage :- capable of direct silver and indirect halide analyses, very clear colour change.

Disadvantage :- must be 1M nitric acid solution, some problem with specific anion.

MCQ

1. Which compound are used as indicator in fajan’s method?

A. Axin

B. Sodium bicarbonate

C. Sodium chloride

D.flurosine

2. Fajan’s method are used to detect?

A. Halide

B. Thiocyanate

C. A and B

D. None of the above

3. What is the advantage of volhard method ?

A.very clear colour change

B. Sample method

C. capability for different PH

D. must be 1M nitric acid solution.

4. What is the advantage of Mohr method ?

A.very clear colour change

B. Sample method

C. capability for different PH

D. must be 1M nitric acid solution.

5. What is the advantage of the fajan method ?

A.very clear colour change

B. Sample method

C. capability for different PH

D. must be 1M nitric acid solution.

6.What is the disadvantage of the volhard method ?

A.very clear colour change

B. Sample method

C. capability for different PH

D. must be 1M nitric acid solution.

7.What is the disadvantage of the mohr method ?

A.very clear colour change

B. Alkaline solution is not suitable for iodine.

C. capability for different PH

D. must be 1M nitric acid solution.

8.What is the advantage of the volhard method ?

A.some problem with specific anion.

B. Alkaline solution is not suitable for iodine.

C. capability for different PH

D. Sample method

Answer key :-

1.  D

2. C

3. A

4. B

5. C

6.  D

7. B

8. A

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