ISO 29681:2009

Overview

ISO 29681:2009 specifies a standardized method for the determination of pH of salted water extracts of paper, board and pulps. The method uses a 0.100 M KCl (salted water) extract to reduce Donnan ion‑exchange effects and provide pH values that better reflect real mill/process conditions-especially for bleached pulps from virgin fibres and other samples with low ionic strength. Results can differ notably from those obtained with ISO 6588-1 / ISO 6588-2.

Key topics and technical requirements

• Salted water: potassium chloride solution c(KCl) = 0.100 M (7.45 g KCl per 1 000 ml).
• Test portion: air-dry, cut to small pieces; weigh 2.0–2.2 g into a flask.
• Extraction/soaking: add 100 ml salted water and soak 1 hour; soak at 20–25 °C (cold) or (90 ± 5) °C (hot), shaking at least once during the period.
• Filtration: filter the suspension (fritted glass or low-ash filter paper) before measurement.
• pH measurement:
  • Calibrate pH-meter with at least two buffer solutions (e.g. pH ≈ 4, 7, 9) at 20–25 °C.
  • Measurement at 20–25 °C; electrode system must allow measurement error < 0.05 pH units.
  • Rinse electrodes with deionized water and a small aliquot of the extract before measuring.
• Reporting:
  • Run duplicate determinations; report the mean to nearest 0.1 pH unit.
  • Individual duplicates shall not differ by more than 0.2 pH units (otherwise repeat).
• Precision data: the standard includes repeatability and reproducibility statistics for typical paper and pulp types (cold and hot extracts).

Practical applications and users

Who uses ISO 29681:2009:

• Quality control and process laboratories in pulp and paper mills.
• Laboratory technicians performing pH testing of pulps, paper and board.
• R&D teams assessing chemical conditions of pulp suspensions and process waters.
• Third‑party testing bodies and compliance laboratories.

Why use it:

• Provides more realistic pH values for low‑ionic‑strength samples by neutralizing ion-exchange artifacts (Donnan effect).
• Improves stability and accuracy of pH readings compared with deionized-water extracts.
• Useful for process control, troubleshooting pulp bleaching, and comparing material behavior under mill‑like ionic conditions.

Related standards

• ISO 6588-1 / ISO 6588-2 - alternative pH methods using deionized water (comparison included in ISO 29681).
• ISO 186, ISO 7213 - sampling standards referenced for representative test portions.

Keywords: ISO 29681:2009, pH determination, salted water extracts, 0.100 M KCl, paper testing, pulp pH, low ionic strength, Donnan effect, ISO 6588.