SOP: Rapid Screening for Benzoic Acid and Sorbic Acid by Ultraviolet Absorption

GIFSQ — Global Institute for Food Safety & Quality

Principle

Benzoic acid and sorbic acid are measured by their ultraviolet absorption maxima (benzoic acid: 225 nm, with a characteristic triple peak at 282, 272 and 265 nm; sorbic acid: 250 nm; fumaric acid: 225 nm without the triple peak or a shoulder at 233 nm). At 225 nm the benzoic acid peak is ten times higher than at 272 nm, so it may be measured quantitatively there.

Apparatus required

• 100-ml stoppered conical flasks
• 100-ml volumetric flasks
• Spectrophotometer

Reagents

a. Benzoic acid stock solution: dissolve 0.59 g of sodium benzoate in water and dilute to 100 ml. Standard solutions: dilute 4, 8, 12, 16 and 20 ml of the stock solution to 100 ml with water.

b. Sorbic acid stock solution: dissolve 0.67 g of potassium sorbate in water and dilute to 200 ml. Standard solutions: dilute 2, 4, 8, 12 and 16 ml of the stock solution to 100 ml.

c. Dilute HCl (1+4).

d. Ethyl ether.

e. Petroleum ether.

f. Anhydrous sodium sulphate.

Procedure

Standard curve

To 1 ml of each standard solution in a 100-ml stoppered conical flask, add 0.5 ml of dilute HCl (1+4), 40 ml of ethyl ether, 40 ml of petroleum ether, and about 5 g of anhydrous sodium sulphate; shake after each addition

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Filter into a 100-ml volumetric flask, wash the conical flask thrice and transfer the washings to the volumetric flask through filter paper; make up to volume with ether

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Prepare a blank using 1 ml of water in place of the standard solution

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Measure the absorbance of the sorbic acid solutions at 250 nm and of the benzoic acid solutions at 225 nm in a spectrophotometer; plot absorbance against concentration

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The curve is linear for sorbic acid up to 0.4 mg/100 ml of ether solution, and up to 1 mg/100 ml of ether solution for benzoic acid

Sample

Treat 1 ml of the sample in the same way as the standard; note the absorbance

↓

From the standard curve, note the concentration of the preservative, and multiply by 100 to get the concentration in mg/100 g, or by 1000 to report in ppm

Precautions

• In the presence of combined preservatives, only a rough estimate of sorbic acid may be made; to estimate benzoic acid it must first be separated from sorbic acid by chromatographic methods.
• Fumaric acid, if present, can be identified by its absorption characteristics (225 nm, no triple peak at 282/272/265 nm, no shoulder at 233 nm).

References

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

Gantenbein, W.M. & A. B. Karasz, J. Assoc. Offic. Anal. Chem., 53, 738 (1969) — as cited in the book.