Aim: Carry out the assay of sodium benzoate by redox Permanganometry titration.

Apparatus & Chemicals Required: Burette, pipette, Conical flask, Volumetric flask, 0.1N perchloric acid, Glacial acitic acid.

Principle: Non aqueous titration using standard Perchloric acid

Sodium benzoate is a weakly basic substance.. It is dissolved in acetic acid which exerts the leaving effect on it and increase the basic strength of sodium benzoate. Now this can easily titrated with strong acid like Perchloric acid 

Procedure:

Preparation & Standardization of0.1 N perchloric acid

  • Perchloric acid is available as 12N Strength.
  • Therefore 0.1N can be prepared by diluting 120times
  • Pipette out 8.5 ml of Perchloric acid solution,add 900ml of glacial acetic acid with continuous stirring
  • Add 30ml of acetic anhydride solution and make up the volume 1000ml with glacial acetic acid, allow the solution to keep for 24 hours.

Note: Perchloric acid must be diluted with glacial acetic acid before adding acetic anhydride otherwise explosive acetyl Perchlorate will be formed.

  • Dissolve 2.042 of potassium Hydrogen Phthalate in about 80ml of glacial acetic acid.
  • Heat on water bath until completely dissolves. Protect from humidity.
  • Cool at 20° C and make up the volume up to 10ml with glacial acetic acid.
  • This will produce 0.1N Potassium Hydrogen Phthalate.
  • Take 10ml of KHP solution in a clean conical flask add acetous crystal violet as indicator
  • Titrate the solution with prepare Perchloric acid until the color change from blue to green. Repeat the procedure take the average reading and calculate the exact normality of Perchloric acid by using the formula 

N1V1(perchloric acid)=N2V2(Potassium hydrogen phthalate)

Assay of Sodium Benzoate:

  • Dissolve the given sample in 10ml glacial acetic acid by warming 50°C if necessary
  • Cool the solution at room temperature and titrate with 0.1 N perchloric acid using alpha napthol benzein as an indicator until the color change from blue to green.
  • Repeat the procedure take the average reading. Run a blank titration and calculate the amount of sample present by using formula given below.

Observation and Calculation:

Standardization of 0.1N Perchloric acid

Burette               :0.1N Perchloric acid

Conical Flask    : 10ml 0.1N Potassium Hydrogen Phthalate+2drops Indicator

Indicator            : Crystals violet

End point           : Blue to Green

 

Sl No. Volume
of
Potassium
hydrogen
phthalate
(ml)
 Volume
Of
Perchloric
Acid
(0.1N)
 Volume
of
Perchloric
Acid
consumed
(ml)
 Color
Change
Initial
(Ml)		 Final
(Ml)
   

Calculation : 



Blank

Burette              :Perchloric acid

Conical Flask    : 10ml glacial acetic acid

Indicator            : Alpha napthol benzein

End point           : Blue to Green

Blank Vs 0.1N Perchloric Acid

 
Sl No. Volume
of
Glacial
acetic
acid(ml)
 Volume
Of
Perchloric
Acid
(0.1N)
 Volume
of
Perchloric
Acid
consumed
(ml)
 Color
Change
Initial
(Ml)		 Final
(Ml)
 
 Assay  of Sodium Benzoate

Burette               :Perchloric acid

Conical Flask    : 10ml Sample dissolved in glacial acetic acid

Indicator            : Alpha napthol benzein

End point           : Blue to Green

 

Sample Vs 0.1N Perchloric acid

Sl No. Volume
of
Potassium
hydrogen
phthalate
(ml)
 Volume
Of
Perchloric
Acid (0.1N)
 Volume
of Perchloric
Acid
consumed
(ml)
 Color
Change
Initial
(Ml)		 Final
(Ml)

IP Factor 
Each ml of 0.1N  Perchloric acid is equivalent to 0.01441g of sodium benzoate.
Amount of sample present in 100ml of the solution=
 
Report:
The percentage purity of the given sample of sodium benzoate was found to be _____%
 

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