Overview
ISO 6636-1:1986 specifies a polarographic method for the determination of zinc content in fruits, vegetables and derived products. The procedure covers sample drying and ashing, acid digestion of the ash, pH adjustment with ammonia, and quantification by polarography using an ammonia/ammonium chloride electrolyte and a mercury-dropping electrode. The method is suitable for zinc concentrations > 0.05 mg/kg. Copper, tin, lead and cadmium are reported not to interfere.
Key topics and technical requirements
- Scope: Determination of zinc in agricultural, fruit and vegetable products by polarography.
- Sample prep:
- Typical test portion: 10–25 g (weighed to 0.01 g).
- Drying at 110–120 °C, then ashing in a muffle furnace with controlled ramping to 525 ± 25 °C; total ashing ≈ 6 h.
- If carbon remains, wet with H2O and HNO3, evaporate and re-ash until carbon-free.
- Chemical treatment:
- Dissolve ash in diluted HCl, transfer to 50 mL volumetric flask, adjust pH to about 10 with 25% ammonia, dilute and filter.
- Use ammonium chloride/ammonia electrolyte (specified concentrations in the standard).
- Add sodium sulfite (1 mol/L) as stated before polarographic measurement.
- Instrumentation:
- Polarograph with a mercury-dropping electrode (cathode) and mercury anode in the electrolyser.
- Scan polarogram between approximately –1.0 and –1.6 V; zinc half-wave near –1.2 V.
- Standards & calibration:
- Preparation of zinc standard solutions from metallic zinc (≥99% purity) and use of standard additions to determine recovery by extrapolation.
- Quality and repeatability:
- Method measures zinc >0.05 mg/kg.
- Repeatability: difference between two determinations (same operator, apparatus, lab) should not exceed 5% of the mean.
- Reporting:
- Results expressed in mg/kg, reported to one decimal place; include method and any non‑standard conditions.
Applications and users
- Food testing and quality-control laboratories performing zinc content analysis for:
- Nutritional labeling and composition analysis.
- Regulatory compliance and monitoring of produce and processed fruit/vegetable products.
- Research studies on micronutrient content in agricultural products.
- Useful for manufacturers, public health agencies, and analytical service providers requiring a validated polarographic procedure.
Related standards
- ISO 6636-2: atomic absorption spectrometric method for zinc (complementary technique).
- ISO 6636-3: dithizone spectrometric method.
- ISO 874: sampling of fresh fruits and vegetables (sampling guidance referenced by this method).
Keywords: ISO 6636-1:1986, zinc determination, polarographic method, fruits and vegetables, muffle furnace, ammonia/ammonium chloride electrolyte, food testing, analytical method.