Overview
ISO 21128:2006 specifies an iodometric titration method to determine oxidizing residues released by ready-to-use cork stoppers. The standard targets residues of bleaching or peroxide-based treatments and provides a repeatable laboratory procedure to quantify peroxide (H2O2) equivalents released from small groups of stoppers.
Key topics and technical requirements
- Scope and intent
- Method is intended for cork stoppers that may have been treated with oxidizing products; untreated cork typically gives a negative result.
- Sampling and test units
- Take a statistically representative gross sample and perform the test twice: two groups of four cork stoppers each.
- Conditioning
- Tests carried out at (21 ± 4) °C and (60 ± 20) % relative humidity.
- Principle
- Peroxide residues oxidize iodide to iodine; iodine is titrated with standard sodium thiosulfate (iodometric titration).
- Reagents
- Examples specified: dilute sulfuric acid (25 %), potassium iodide (50 g/L), ammonium heptamolybdate (3 %), sodium thiosulfate standard solution (0.02 N), starch indicator (0.1 %), acetic acid (50 %), and nitrogen (or ultrasound) to remove O2.
- Apparatus
- Common lab glassware plus orbital stirrer (≈150 rev/min), volumetric pipettes and class A burette.
- Procedure summary
- Add specified reagents to a screw-cap flask, introduce four stoppers, add 200 mL water, remove dissolved O2 (N2 flow or ultrasound), stir 2 h protected from light. A blue coloration indicates oxidizing residues; titrate with 0.02 N thiosulfate to a persistent endpoint (use starch near endpoint). Include a blank (no stoppers).
- Calculation and reporting
- Oxidizing residue per stopper (mg H2O2) is calculated as: mg H2O2 per stopper = (V × 0.02 × 17) / 4, where V (mL) is thiosulfate used after blank subtraction. Report the greatest value from the two groups, rounded to the nearest tenth, and include standard reference, product identification and any deviations.
Applications and who uses it
- Cork producers and finishers - verify residual oxidizing agents after bleaching or chemical treatments.
- Quality control labs - routine acceptance testing of cork batches destined for wine or other packaged goods.
- Wineries and bottling operations - assess closure compatibility and risk of off‑flavors or oxidative effects from residues.
- Regulatory and inspection bodies - confirm compliance with product-safety or labeling requirements.
Keywords: ISO 21128:2006, cork stoppers testing, oxidizing residues, iodometric titration, peroxide residues, sodium thiosulfate titration, cork quality control.
Related standards
- ISO 633 - Cork - Vocabulary (referenced in ISO 21128 for terms and definitions).
