ISO 6588-2:2021

Overview

ISO 6588-2:2021 specifies a laboratory method for determining the pH of aqueous extracts obtained by hot water extraction from samples of paper, board and pulp. Known as the “hot extraction” pH test, the standard defines sample preparation, extraction, filtration, conditioning of the extract and pH measurement conditions so that results are comparable between laboratories and suitable for quality control, R&D and regulatory records.

Key topics and technical requirements

• Scope: Applicable to all kinds of paper, board and pulp. Note limitations for very low ionic-strength (highly purified) pulps where precision becomes poor.
• Extraction principle: Typical procedure uses 2.0 g ±0.1 g air-dry sample extracted with 100 ml boiling distilled/deionised water under reflux for 1 h ±5 min.
• Post-extraction handling: Cool to 20–25 °C, filter (fritted glass), add 2 ml of 1 M KCl to stabilise the ionic strength, then measure pH.
• Reagents and water quality: Distilled or deionised water with conductivity ≤ 0.1 mS/m after boiling; water pH about 6.8–7.3. Use standard buffer solutions (pH ~4, 7 and 9) for calibration.
• Equipment: Chemically resistant glassware (reflux condenser, fritted filter), and a pH meter (combined or separate glass and reference electrodes) readable to at least 0.05 pH unit.
• Measurement and reporting:
  • Calibrate pH meter at 20–25 °C with two buffer points bracketing the expected pH.
  • Run duplicate extractions; report the mean to the nearest 0.1 pH unit.
  • Individual duplicates should not differ by more than 0.2 pH unit; otherwise perform two additional determinations and report mean and range.
• Quality checks: Re-check electrode performance after measurement; buffers should agree within 0.1 pH unit.

Practical applications and users

ISO 6588-2:2021 is used for:

• Quality control in pulp and paper mills (monitoring acidity/alkalinity of products).
• Incoming materials inspection and supplier compliance testing.
• Laboratory R&D assessing finishing additives, bleaching effects or contamination.
• Regulatory documentation and specification verification for packaging and archival papers.

Typical users include mill laboratories, independent testing labs, paper manufacturers, suppliers of pulp and chemicals, and standards/compliance teams.

Related standards

• ISO 6588-1 - cold extraction variant (different extraction conditions).
• ISO 29681 - hot extraction with added salt (preferred for bleached or very low ionic-strength pulps to simulate mill conditions).
• ISO 186 and ISO 7213 - sampling standards referenced for representative sampling.
• IEC 60554-2 - alternative pH method for cellulosic papers used in electrical applications.

Keywords: ISO 6588-2:2021, pH determination, hot extraction, paper testing, pulp testing, aqueous extracts, quality control, laboratory method.