Overview
ISO 6560:1983 specifies a molecular absorption spectrometric method for the determination of benzoic acid in fruit and vegetable products when concentrations are greater than 200 mg per litre or per kilogram. The method is designed for food matrices such as fruit juices, tomato purée (salted or unsalted) and products preserved with vinegar or lactic acid. It combines solvent extraction, chemical derivatization (nitration and reduction), a modified Mohler reaction and spectrophotometric measurement of the resulting red complex.
Key Topics and Requirements
- Scope: Applicable to fruit and vegetable products with benzoic acid > 200 mg/L or kg. For lower concentrations see ISO 5518.
- Analytical principle: Extraction of benzoic acid into diethyl ether, nitration, reduction, and color development via Mohler’s reaction modified with hydroxylamine hydrochloride; measure absorbance of the red complex at 533 nm by spectrometer.
- Reagents & purity: Requires analytical-grade reagents (e.g., diethyl ether, concentrated sulfuric acid, potassium nitrate, hydroxylamine HCl, ammonia, NaOH, Carrez I/II clarifying solutions). Standard benzoic acid solution (1 g/L) is used for calibration.
- Apparatus: Separating funnels, pipettes, volumetric flasks, controlled water/oven baths, centrifuge or filtration equipment, and a spectrometer with 10 mm pathlength cells capable of measuring at 533 nm.
- Procedure highlights:
- Specific sample preparation for liquid, semi-thick and thick products (pressing/clarification where required).
- Ether extraction followed by conversion to alkaline extracts and concentration.
- Nitration, reduction and modified Mohler colour development under controlled temperature and time.
- Spectrometric measurement within a defined time window after colour development.
- Calibration & calculation: Use a multi-point calibration graph from standard solutions; formulae provided to calculate mg/L or mg/kg.
- Quality criteria: Two determinations per sample; repeatability requirement - difference between duplicate results should not exceed 10 mg benzoic acid per litre or per kilogram.
- Interferences: Formic acid and sorbic acid do not interfere with the determination.
Applications and Users
- Routine food quality control and regulatory compliance testing for manufacturers of fruit and vegetable products.
- Food safety and public health laboratories validating preservative levels.
- Analytical laboratories performing benzoic acid quantification in juices, tomato products and pickled/fermented vegetable products.
- Useful where benzoic acid concentrations are relatively high (industrial/process monitoring).
Related Standards
- ISO 5518 - Spectrophotometric method for determination of benzoic acid at lower concentrations (referenced for samples below 200 mg/L or kg).
Keywords: ISO 6560:1983, benzoic acid determination, fruit and vegetable products, molecular absorption spectrometric method, spectrophotometry, Mohler reaction, 533 nm, diethyl ether extraction.