Overview - ISO 23774:2023 (Kraft liquor, potentiometric titration)
ISO 23774:2023 specifies a potentiometric titration method for determining total alkali, active alkali and effective alkali in white and green kraft (sulphate) liquors. The procedure uses hydrochloric acid titration with continuous pH (or derivative) monitoring to identify three inflection points in the titration curve. Results can be reported in mol·L‑1 or converted to conventional units (g NaOH/L or g Na2O/L). The standard excludes liquors with significant polysulphides (e.g., oxidized white liquors) and is not intended for quantifying individual ions such as sulphides or carbonates.
Key technical topics and requirements
- Principle: Titrate a homogenized liquor sample with standardized HCl while recording pH vs titrant volume; identify inflection points corresponding to effective (1st), active (2nd), and total (3rd) alkali.
- Titration end-points: Typical inflection pH values are near 11 (1st), 8–9 (2nd) and 4 (3rd).
- Reagents: Analytical-grade chemicals; hydrochloric acid ~1 M (known to ±0.005 M); buffer solutions at pH 4.01 and pH 9.18 for pH meter calibration.
- Apparatus: Preferably automatic titration equipment with pH/combined glass electrode and curve‑analysis (derivative) capability; manual setup permitted if adequate.
- Calibration & QA: pH meter calibration with two buffers; electrode performance check (reading within 0.2 pH units). Precision and reproducibility data are provided in Annex A from inter-laboratory studies.
- Safety: Titration can generate hydrogen sulphide (H2S) - perform under a fume hood or equivalent controls.
- Calculations & reporting: Use volumes at the three inflection points (a, b, c), sample volume (v) and titrant molarity (m) to compute alkali concentrations. Report to nearest 0.01 mol·L‑1 or 1 g/L and include reference to ISO 23774:2023 and any departures.
Practical applications and users
- Industrial and laboratory measurement of alkali strength in kraft pulping process control (white liquor causticity, green liquor recovery).
- Quality control in pulp mills for causticizing efficiency, liquor balancing, and chemical charge calculations.
- Users: pulp mill chemists, process control engineers, analytical laboratories, and equipment vendors supplying automatic titrators or pH measurement systems.
Related standards and notes
- Precision assessment follows ISO 5725‑2 practices (inter-laboratory precision estimation).
- The document was prepared by ISO/TC 6 (Paper, board and pulps) and aligns with ISO/IEC Directives editorial rules.
- Not applicable for liquors with high polysulphide content; method does not replace targeted ionic analyses.
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