ISO 6588-1:2021

Overview

Standard Reference: ISO 6588-1:2021
Title: Paper, board and pulps - Determination of pH of aqueous extracts - Part 1: Cold extraction

ISO 6588-1:2021 specifies a laboratory method to determine the pH of electrolytes extractable by cold water from samples of paper, board or pulp. The procedure uses distilled/deionised water for extraction and is applicable to all types of paper, board and pulp. Results reflect the pH of water-extractable ionic material and can differ from mill-process pH for low‑ionic-strength samples.

Key topics and technical requirements

• Principle: Extract 2.0 g (±0.1 g) air‑dry sample in 100 ml distilled/deionised water for 1 hour at 20–25 °C, filter the extract, add 2 ml of 1 M KCl, and measure pH.
• Water quality: Use distilled or deionised water with conductivity ≤ 0.1 mS/m after boiling and cooling; pH of water should be ~6.8–7.3 (see ISO 3696).
• Instrumentation: pH‑meter with glass and calomel electrodes or combined electrode; readable to at least 0.05 pH unit. Calibrate with at least two buffer solutions (examples: pH ≈ 4, 7, 9).
• Calibration & measurement: Calibrate at 20–25 °C; acceptable buffer agreement ±0.1 pH unit. Record pH when stable (no measurable drift within 30 s).
• Repeatability & reporting: Run duplicates, report the mean to nearest 0.1 pH unit. Individual results should not differ by >0.2 pH unit; if they do, perform two additional extracts and report mean and range.
• Quality controls: Clean glassware (acid cleaned, rinsed), avoid sample contamination (use gloves), follow sampling standards (ISO 186, ISO 7213).

Practical applications and users

Who uses ISO 6588-1:2021:

• Quality control laboratories in pulp and paper mills
• R&D teams developing pulp treatments, bleaching and coatings
• Suppliers and buyers verifying paper acidity/alkalinity specifications
• Environmental and testing labs assessing extractable ionic content
• Certification bodies and contract testing providers

Why it’s used:

• Routine monitoring of paper/pulp acidity that impacts permanence, processing, and product performance
• Troubleshooting process water or chemical interactions with fibres
• Baseline pH measurement for product specifications and acceptance testing

Limitations and related guidance

• For highly purified or bleached pulps with low ionic strength, cold-water extraction may give poor precision or results not representative of mill conditions. In such cases consider ISO 29681 (salt-added extraction) for results that better reflect process waters.
• For hot extraction, see ISO 6588-2. For cellulosic papers for electrical uses, see IEC 60554-2.
• Referenced sampling and water standards: ISO 186, ISO 7213, ISO 3696.

ISO 6588-1:2021 provides a consistent, repeatable procedure for determining aqueous-extract pH in paper, board and pulps - essential for quality control, specification compliance and material performance assessment.