Estimation of Sodium Benzoate

Sodium Benzoate

Molecular Formula : C7H5NaO2 

Molecular weight: 144.1

Description : A white, crystalline or granular powder or flakes; odourless or with a faint odour; hygroscopic.

Molecular structure :

Estimation of Sodium Benzoate

Weigh accurately about 0.25 g of Sodium Benzoate, dissolve in 20 ml of anhydrous glacial acetic acid, warming to 50º if necessary, cool. Titrate with 0.1 M perchloric acid, using 0.05 ml of 1-naphtholbenzein solution as indicator. Carry out a blank titration.

1ml of 0.1M perchloric acid is equivalent to 0.01441g of sodium benzoate

Strength =       X x M x 0.01441

                   weight of sodium benzoate x 0.1

M= Molarity 0.1M Perchloric acid 

X =Volume run down of 0.1M HclO4 

Preparation and Standardization of 0.1N Perchloric acid 

Reagents : Perchloric acid, Glacial acetic acid, Acetic anhydrate, Potassium hydrogen phthalate, crystal violet  and water.

Glassware: Volumetric Flask, measuring cylinder, Burette, Conical Flask, beaker and Pipettes. 

Preparation:

Gradually mix 8.5 ml of perchloric acid to 900 ml of glacial acetic acid with vigorous and continuous stirring. Now add 30 ml acetic anhydride and make up the volume to 1 liter with glacial acetic acid and allow to stand for 24 hours before use. The acetic anhydride reacts with the water in perchloric acid and some traces in glacial acetic acid thereby making the resulting mixture anhydrous. 

Principle 

Perchloric Acid is a very strong acid and when it is made to dissolve in acetic acid, the latter can behave as a base and forms an ‘onium ion’ after combining with protons donated by the perchloric acid(HClO4). Thus, we have : 

 

Potassium in the Potassium Hydrogen Phthalate reacts with perchlorate ion(ClO4-) gives KClO4(Potassium perchlorate) and Potassium Hydrogen Phthalate ion. 

Potassium Hydrogen Phthalate ion reacts with CH3COOH2⁺ ion (onium ion) converted into Phthalic acid and  Acetic acid. in this reaction onium ion donates proton and phthalate ion accepts proton. 

Standardization:

Weight accurately about dissolve 0.35 g of potassium hydrogen phthalate in 50 ml of anhydrous acetic acid, warming gently if necessary. Allow to cool protected from the air and titrate with the perchloric acid solution using 0.05 ml of crystal violet solution as indicator. A light green colour is the end point. 

Each ml of 0.1M perchloric acid VS is equivalent to 20.42 mg of C8H5KO4. Perform a duplicate and calculate the molarity factor.(MF) Note down the average value .

Calculation

MF = W x actual molarity of perchloric acid / V x 0.02042 (factor) 

Where

V = Volume of perchloric acid consumed in ml.

W = Weight of potassium hydrogen phthalate in g.

Example:

V=16 ml

W=0.35014gm

 = 0.35014x0.1

   16 x0.02042

 =0.107168 M

 Note :

•Re-standardize before use.

•Store in amber bottle with well fitted suitable stoppers which prevents access to atmospheric carbon dioxide.

•Discard the solution after 30 days.

•For storage and labeling follow the general procedure.