Overview
ISO 6638-2:1984 specifies a titrimetric (routine) method for determining formic acid content in fruit and vegetable products. The method is based on quantitative entrainment of formic acid by steam from a prepared test portion, followed by an indirect titration: the acid in the distillate reacts to consume bromine, the amount of iodine equivalent to the bromine consumed is liberated, and the liberated iodine is titrated with a standard sodium thiosulfate solution.
Key topics and technical requirements
- Principle: steam distillation (quantitative entrainment) of formic acid, followed by iodometric titration with Na2S2O3 to quantify the acid.
- Sample size and preparation:
- Typical test portion: 50 ml (or 50 g) of homogenized sample; solids/pastes weighed and diluted to ensure fluidity (about 50 ml water often sufficient).
- Frozen samples must be thawed and thaw liquid returned to the product.
- Distillation apparatus and conditions:
- Steam generator, condenser (refrigerant ~50 cm), 500 ml and 1 000 ml flasks, conical flask with ground stopper, burette, homogenizer/grinder.
- Collect ~1 000 ml distillate, adjust to mark and mix; take 200 ml aliquot for titration.
- Reagents:
- Tartaric acid, 10% sodium acetate solution, 10% potassium iodide solution, bromoacetic acid, standard sodium thiosulfate solution (≈0.1 mol/L), 1% starch paste.
- Titration procedure:
- Treat 200 ml distillate with acetate and bromoacetic acid, allow reaction, add iodide, titrate liberated iodine with sodium thiosulfate using starch as endpoint indicator.
- Run a blank in parallel.
- Calculation and reporting:
- Formic acid content is calculated from titrant volume and concentration, expressed as g per 100 ml or 100 g.
- Two determinations are performed and the arithmetic mean used if repeatability is met: difference between duplicates must not exceed 2% of the mean.
- Test record: method used, operational details, deviations and incidents must be recorded.
Applications and users
- Practical for routine analytical laboratories, food quality control, beverage and preserve manufacturers, and regulatory/inspection bodies monitoring formic acid in fruit- and vegetable-derived products.
- Useful for assessing product safety, compliance with specifications, and process control where formic acid concentration is relevant.
Related standards
- ISO 6638-1: Gravimetric method for formic acid (alternative method referenced in ISO 6638 series).
Keywords: ISO 6638-2, formic acid content, fruit and vegetable products, titrimetric method, steam distillation, sodium thiosulfate titration, bromoacetic acid, iodine.
