Overview
ISO 6638-1:1985 specifies a gravimetric laboratory method for the determination of formic acid in fruit and vegetable products. The procedure uses steam entrainment (distillation) to transfer formic acid from a test portion, chemically reduces mercury(II) chloride with the formic acid to produce mercury(I) chloride (Hg2Cl2), and quantifies formic acid from the mass of precipitated mercury(I) chloride. Part 2 of ISO 6638 describes an alternative titrimetric method.
Key topics and technical requirements
- Principle: quantitative steam entrainment of formic acid, reduction of HgCl2 to Hg2Cl2, and gravimetric measurement of Hg2Cl2 yield.
- Apparatus: analytical balance, steam distillation assembly (steam generator, flasks A & B, condenser), conical flasks (500 ml and 2 l), Schott filter G4 (10–20 µm) or sintered glass, desiccator, oven (100 ± 2 °C), mixer/grinder, Whatman No. 1 filter paper, hotplate, evaporating dish, reflux condenser and boiling water bath.
- Reagents (analytical grade): barium carbonate or calcium carbonate; mercury(II) chloride + sodium chloride solution (100 g HgCl2 + 30 g NaCl per 1 000 ml); sodium acetate (500 g/l); 10% HCl (v/v); tartaric acid; ethanol; diethyl ether; distilled water.
- Sample preparation: liquids, pastes or solids are homogenized; test portions are typically 25–50 g (or ml) and must contain ≤ 0.15 g formic acid.
- Procedure highlights: distillation with added tartaric acid and carbonate in paired flasks, collection and processing of distillate/filtrate, evaporation, addition of reagents, 2-hour reflux to form Hg2Cl2, filtration on pre-dried Schott filter, washing (water, ethanol, ether), drying at 100 °C for 1 hour, and precise weighing (±0.0002 g).
- Calculations: formic acid content is calculated from the mass of Hg2Cl2 precipitate using the conversion factor given in the standard (conversion factor 0.0975 is used in the method).
- Quality control: perform two determinations; repeatability - the difference between paired results must not exceed 2% of the mean. Test reports must state method, results (to three decimals), and any non‑standard conditions.
Applications and users
- Food testing and quality control laboratories analyzing processed fruit and vegetable products for formic acid content.
- Analytical chemists and method developers validating gravimetric assays.
- Regulatory agencies and manufacturers requiring standardized, comparable results for product safety, labeling, or compliance.
- Research groups studying organic acid profiles in agricultural products.
Related standards
- ISO 6638/2 - Determination of formic acid content: titrimetric method (alternative technique).
- General ISO guidance on sampling and reporting for agricultural and food product chemical analysis.
Keywords: ISO 6638-1:1985, formic acid determination, gravimetric method, fruit and vegetable products, steam distillation, mercury(II) chloride reduction, mercury(I) chloride, food quality control, analytical method.
Note: The method uses mercury compounds; follow institutional safety and hazardous-waste disposal regulations.
