Exercise

Practical 12 - Estimation of sodium Benzoate in Food Sample

Aim: To estimate the sodium benzoate contents in fruit and vegetable products.

Theory and Principle: Sodium benzoate is used as a class II preservative in preservation of pickles, sauces, squashes etc. Its estimation is important as the quantity of class II preservative shall not exceed maximum limit prescribed under FPO. In a sample containing NaCl, the benzoic acid is converted into water soluble sodium benzoate by addition of NaOH. When the Sodium benzoate solution is acidified with excess of HCl, water insoluble benzoic acid is formed which is extracted with chloroform. The chloroform is removed by evaporation and the residue containing benzoic acid is dissolved in alcohol and then titrated with standard sodium hydroxide.

Reagents, glassware and apparatus required
  1. Sodium chloride (standard salt solution)
  2. Sodium hydroxide 0.05 N: Dissolve 40.005g NaOH pellets in water and dilute to one litre in a volumetric flask. Then dissolve 5 ml of the above solution to 100 ml with water to make 0.05N NaOH.
  3. H2SO4 and HCl,
  4. Chloroform and Phenolphthalein indicator
  5. Distillation flask and condensers, conical flask, volumetric flask, pipette, burette and separating funnel.

General procedure for sample preparation:
  1. Mix the sample (ketchup, sauce and pickle) thoroughly and grind. Use 50-100g sample for estimation.
  2. Transfer to 500ml volumetric flask.
  3. Add powdered NaCl (15g) to saturate.
  4. Make the solution alkaline to litmus paper by adding 10 % NaOH and dilute to 500ml with saturated salt solution.
  5. Shake thoroughly and allow standing for 2 hrs with frequent shaking. Filter through Whatman No.4 paper or centrifuge.
  6. For sample containing fat, extract with ether before determination.

Procedure for sodium benzoate estimation
  1. Take 100 ml of filtrate in 500 ml volumetric flask.
  2. Neutralize to litmus paper with dilute HCl (1+3) and add 5 ml of HCl in excess.
  3. Extract carefully by mixing the sample with chloroform in separating funnel using 70 ml, 50 ml, 40 ml and 30 ml chloroform successively.
  4. To avoid the formation of emulsion, shake the funnel continuously.
  5. Draw off carefully clear chloroform solution after each extraction and wash the chloroform layer with water.
  6. Transfer the combined chloroform extract from the separating funnel to a 250 ml conical flask. Rinse the funnel with chloroform.
  7. Distill at low temperature to approximately ¼th of the original volume and evaporate to dryness at room temperature on water bath till only a few drops remain.
  8. Dry the residue overnight in desiccator containing H2SO4 (until no acetic acid odour is detected in ketchup sample) and dissolve in 50 ml alcohol.
  9. Add 12-15 ml of water and 1-2 drops of phenolphthalein indicator and titrate against 0.05 N NaOH.
  10. Calculate using following expression and express as ppm sodium benzoate or benzoic acid.

Observations

Sample No.

Weight of sample

Volume made up

Volume of sample taken for estimation

Titre value

ppm of anhydrous sodium benzoate

Benzoic acid (ppm)

1

2

3

Mean



Calculations:

1ml of 0.05N NaOH = 0.0072g of anhydrous Sodium Benzoate
Titre × Normality of NaOH × 144 × volume made up × 1,000,000
Anhydrous sodium benzoate (ppm) = --------------------------------------------------------------------------
Volume of sample taken for estimation × weight of sample × 1000
Titre × Normality of NaOH × 122 × volume made up × 1,000,000
Benzoic acid (ppm) = -----------------------------------------------------------------------------------
Volume of sample taken for estimation × weight of sample x 1000


Problem:
50g tomato ketchup was made alkaline using 10% NaOH and volume was made to 500 ml and filtered. 200ml of filtrate was used for extraction of benzoic acid using chloroform. The extracted residue after addition of 15ml water was titrated against 0.05NaOH to give a titre value of 1.5ml .Calculate sodium benzoate in the given sample.
Solution:
Given wt of sample= 50g
Volume made up=500ml
Volume of sample taken for estimation=200ml
Titre=1.5ml
Titre×Normality of NaOH×144×Volume made up×1,000,000
Sodium benzoate (ppm)=----------------------------------------------------------------------------
Wt of sample×Volume of aliquot taken for estimation ×1000
1.5×0.05×144×500×1,000,000
=-----------------------------------------
50×200×1000

=540 ppm

Inference: The given sample of tomato ketchup had 540 ppm sodium benzoate as preservative.
Last modified: Wednesday, 7 March 2012, 10:45 AM