Overview
ISO 23772:2023 - Pulps: Kraft liquor - Determination of residual alkali using potentiometric titration defines a laboratory procedure to measure residual effective alkali in Kraft black liquor. The method is intended for black liquors with up to 40% dry matter and initial pH ≥ 11.0, using a potentiometric titration with hydrochloric acid (HCl). The method’s lower reporting limit is 0.1 g/L equivalent NaOH.
Key topics and technical requirements
- Principle: Alkaline species in the sample are titrated potentiometrically to the first inflection point (pH ≈ 11.0–11.5). Sodium carbonate is added to sharpen the inflection; NaOH is optionally added for samples with pH 11.0–12.5.
- Reagents: Analytical-grade HCl (1 M, known to 0.001 M), Na2CO3 (1 M), NaOH (1 M, known to 0.005 M), ethanol for electrode cleaning, and pH 7.0 & 11.0 buffers.
- Apparatus: Automatic titrator or pH meter with a motorized burette and recorder; combined or separate glass/reference electrodes (high-sodium pH electrode recommended); fume hood.
- Sampling & pretreatment: Keep sample bottles full and sealed to prevent oxidation; swirl (do not shake) when homogenizing; dilute if dry matter > 40% (target ≈ 20%).
- Procedure highlights: Sample volumes typically 10–20 mL (measured to ±1%); titrate under stirring; record HCl volume at the first inflection point (or at pH 11.5 if no clear inflection).
- Calculations: Residual alkali is calculated from acid consumption, corrected by a factor (0.95) and a correction term (−0.012) to remove contributions from other alkaline species. Results are reported as g/L NaOH or g/L Na2O (conversion factors given).
- Safety: Hydrogen sulfide may form-perform titration under a hood.
Applications and users
ISO 23772:2023 is applicable to:
- Quality control and process laboratories in pulp & paper mills monitoring black liquor causticity.
- Analytical and environmental labs performing routine alkali balance or recovery system checks.
- R&D teams studying kraft pulping chemistry and process optimization.
Benefits include standardized, reproducible measurements for process control, material balance, and regulatory reporting.
Related information
- Precision and reproducibility data are provided in Annex A of the standard (inter-laboratory results).
- The document references background work such as SCAN N33-94 for correction-factor justification.
- Prepared by ISO/TC 6 (Paper, board and pulps); see ISO for related pulp and analytical standards.
Keywords: ISO 23772:2023, Kraft liquor, residual alkali, potentiometric titration, black liquor, NaOH, HCl, pH 11, laboratory method.