SOP: Determination of Benzoic Acid and Saccharin by Selective Extraction

GIFSQ — Global Institute for Food Safety & Quality

Principle

Benzoic acid is selectively extracted with diethyl ether at pH 4.0, and then saccharin is extracted by making the liquid strongly acidic. Each residue is titrated with standard sodium hydroxide: benzoic acid against phenol red in 60% acetone, saccharin against bromothymol blue in water.

Apparatus required

• Separating funnels
• Water bath
• Air blower

Reagents

a. pH 4 buffer solution: the book gives its composition as containing 10% sodium carbonate and 6.5% citric acid (carried exactly as printed; verify against the printed book before use).

b. Diethyl ether.

c. 60% acetone.

d. 0.05 N sodium hydroxide solution.

e. Phenol red indicator.

f. Bromothymol blue indicator.

g. Concentrated hydrochloric acid.

h. Distilled water.

Procedure

Benzoic acid

To 50 ml of the sample in a separating funnel add 25 ml of the pH 4 buffer solution

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Add 25 ml of diethyl ether, shake, allow to separate, and draw off the aqueous phase into another separating funnel

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Extract the aqueous phase twice more in the same way with 25-ml portions of ether

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Wash each ether extract with the same 5-ml portion of water, transferring the wash water from one funnel to another

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Combine the ether extracts, filter, and carefully evaporate the ether over a water bath; remove the last 5 ml of solvent at room temperature using an air blower

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Dissolve the residue in 5 ml of 60% acetone and titrate with 0.05 N sodium hydroxide using phenol red as indicator

Saccharin

To the aqueous phase remaining in the separating funnel (containing the sample), add the water used for washing and 10 ml of concentrated HCl

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Extract the saccharin three times with 25-ml portions of diethyl ether

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Wash the combined ether extracts with three 5-ml portions of distilled water — do not discard the wash water

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Combine the washings and extract them with 10 ml of ether; mix this ether extract with the main ether extract, filter, and wash the filter paper with more ether

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Evaporate the ether on a water bath; dissolve the residue in 5 ml of acetone and evaporate to dryness

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Add 4 ml of distilled water, warm to dissolve, cool to room temperature, and titrate with 0.05 N NaOH using bromothymol blue as indicator

Calculation

Benzoic acid: 1 ml of 0.05 N NaOH = 0.0061 g of benzoic acid

Saccharin: 1 ml of 0.05 N NaOH = 0.00916 g of saccharin

Precautions

• Keep the washings of the ether extracts and re-extract them as directed — saccharin is partly water-soluble and is lost if the wash water is discarded.
• Remove the last traces of ether at room temperature; heating drives off benzoic acid with the solvent vapour.
• The pH 4 buffer composition is carried exactly as printed in the available copy; verify it from the printed book before routine use.

References

Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.