Precipitation Titration Part- 2: Volhard & Modified Volhard Method, MCQ GPAT, UGC NET JRF
Volhard Method :-
A precipitation titration in which Ag ion is precipitated by thiocyanate ion in presence of Fe(III) ions indicator in acidic medium is called volhard method.
The titration will be carried out in acidic medium for use of nitric acid in this Titration it should be free of anides of Nitrogen.
The volhard method is used to determine the concentration of Ag ion or concentration of halide ions indirect titration ( Back Titration).
Initially
Ag + SCN → AgSCN
At the end point
Fe³ + SCN → [ FeSCN]
Procedure For Titration :-
Pipette out 25 ml of std. AgNO3 in 250 ml conical flask
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Add 10 ml of purified nitric acid and 1 ml of feric alum indicator.
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Titration with thiocyanate solution until reddish of red blood colour begins to spread the solution.
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It state add thiocyanate solution drop by drop.
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The end point is marked by appearance of permanent red blood colour of iron thiocyanate complex
Precaution :-
- During the titration in resulting solution concentration of nitric acid should be 0.5 M – 1 M. Here strong nitric acid is not used because it prevent the formation of thiocyanate iron (III) complex. Here acidic medium prevent the hydrolysis of fe (III) as Fe(OH)3.
- The solution must be free from Nitrous acid otherwise it give the instant ref colouration forming the thiocynic acid.
- Temperature of the solution must be maintained below 250 c since at an elevated temperature the red colour of the ferric thiocyanate complex fades away rapidly.
Modified volhard method :-
1. When chloride are analysed the volhard method has to slightly modified.
2. During Titration the solution is contact with two precipitation stimuantaneously silver chloride and ammonium thiocyanate which has different solubility.
3. So, after the titration excess of thiocyanate ions are react with AgCl and the red colour of iron thiocyanate complex gradually disappear.
4. Addition of more SCN‾ cause restoration of red colour which induced huge error in analysis so to avoid this error before Titration with thiocyanate the solution of few ml of organic liquid which does not mix with water is added.
5. This compound will moisten the surface of the ppt formingon the surface insoluble in water and thus isolating it form the solution.
6. This effectively prevent the exchange of ion between ppt and solution.
MCQ
1. Volhard method required medium is
A. Acidic
B. Basic
C. A and B
D. None of the above
2. Which acid are used in volhard method ?
A. HNO3
B. HCL
C. A and B
D. None of the above
3. Volhard method is
A. BACK titration
B. Direct Titration
C. A and B
D. None of the above
4. Which metal are used as indicator in volhard method ?
A. Fe (II)
B. Na
C. Br
D. Fe(III)
5. How much temperature is required for volhard method ?
A. < 250°
B. > 250°
C. > 150°
D. > 100°
Answer key
1. A
2. A
3. A
4. D
5. A
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