Estimation of Sodium Benzoate: Sodium benzoate (C₆H₅COONa) is the sodium salt of benzoic acid and is widely used as a preservative in pharmaceutical preparations, foods, and beverages.
It can be estimated by acid-base titration. Since sodium benzoate is the salt of a weak acid and strong base, it reacts with a strong acid such as hydrochloric acid to liberate benzoic acid.
Reaction
C6H5COONa + HCl → C6H5COOH + NaCl
The reaction occurs in a 1:1 molar ratio.
Principle
A known quantity of sodium benzoate is dissolved in water and titrated with a standardized solution of 0.1 M hydrochloric acid, using a suitable indicator.
At the end point, all sodium benzoate is converted into benzoic acid.
Sodium benzoate + HCl → Benzoic acid + NaCl
Because the reaction is 1:1, the amount of HCl consumed is equivalent to the amount of sodium benzoate present.
Requirements
Apparatus
- Burette
- Pipette
- Conical flask
- Volumetric flask
- Measuring cylinder
- Analytical balance
- Funnel
Reagents
- Sodium benzoate sample
- Standardized 0.1 M HCl
- Distilled water
- Suitable indicator, commonly methyl orange
Procedure
- Accurately weigh a suitable quantity of sodium benzoate.
- Transfer it to a volumetric flask.
- Dissolve the sample in distilled water.
- Make up the volume to the required mark with distilled water and mix thoroughly.
- Pipette a suitable aliquot of the solution into a clean conical flask.
- Add a few drops of methyl orange indicator.
- Titrate against standardized 0.1 M HCl from the burette.
- The end point is indicated by the appropriate color change of the indicator.
- Repeat the titration until concordant readings are obtained.
- Calculate the amount and percentage purity of sodium benzoate.
Observation Table
| Trial | Initial burette reading (mL) | Final burette reading (mL) | Volume of HCl consumed (mL) |
| Rough | 0.0 | 10.2 | 10.2 |
| I | 0.0 | 10.0 | 10.0 |
| II | 0.0 | 10.0 | 10.0 |
| III | 0.0 | 10.0 | 10.0 |
Mean titre value = 10.0 mL
The values above are illustrative; actual readings should be obtained experimentally.
Calculation
Molecular weight of sodium benzoate:
C6H5COONa = 144.11 g/mol
The reaction is 1:1:
C6H5COONa : HCl = 1:1
Therefore:
Moles of sodium benzoate = Moles of HCl
If:
- Molarity of HCl = 0.1 M
- Volume of HCl = VV mL
then:
Therefore:
For 10.0 mL of 0.1 M HCl:
Thus, 10.0 mL of 0.1 M HCl is equivalent to approximately 0.1441 g of anhydrous sodium benzoate, assuming a 1:1 reaction.
Formula for Percentage Purity
If a correction factor is applicable:
Corrected titre = Observed titre × F
where FF is the standardization factor of the HCl solution.
Result
The amount and percentage purity of sodium benzoate in the given sample are determined by titration with standardized hydrochloric acid.
Important Note
For a pharmacopoeial assay, the exact solvent, indicator, concentration of titrant, sample quantity, and calculation should be taken from the applicable IP/BP/USP monograph, because the official method may differ from the general acid-base titration described above.
Editorial Note
This article has been carefully researched and written by Deepak Rajput with a focus on accuracy, clarity, and evidence-based healthcare information. Pharmaacademias.com
