Overview
ISO 23777:2023 - "Pulps - Kraft liquor - Determination of hydrosulphide ion concentration using potentiometric titration" - specifies a laboratory procedure to determine sulphide (hydrosulphide, HS−) concentration in kraft process liquors. The method applies to white, oxidized white, green and black liquors (black liquor up to 40% dry matter) and includes the sulphide fraction originating from any polysulphides present. The determination is based on potentiometric titration with silver nitrate, with pretreatment steps to dissolve polysulphides and clear interferences.
Key topics and requirements
- Scope and applicability: Suitable HS− ranges for white/green/black liquors (0.02–2.0 mol·L−1) and oxidized white liquors (0.5–50 mmol·L−1); sample volume adjusted accordingly.
- Principle: Potentiometric titration using AgNO3 to form Ag2S/AgSH precipitates; polysulphides are converted to HS− using alkaline sodium sulphite so each polysulphide contributes one sulphide equivalent.
- Reagents: Analytical-grade chemicals; oxygen-free water; 0.1 M AgNO3 (standardized against KCl); 1 M NaOH; alkaline sodium sulphite (~0.5 M); ammonia (~25%).
- Apparatus: Automatic potentiometric titrator (or manual titrator + pH meter), polished silver indicator electrode, reference electrode, calibrated pipettes.
- Sampling and pretreatment: Prevent oxidation (fill bottles fully); heat and react sample with alkaline sodium sulphite for polysulphide dissolution (different conditions for oxidized white liquor).
- Titration and calculation: Titrate to first inflection (~−650 mV) and calculate HS− concentration from AgNO3 consumption; results reported to two decimal places. Duplicate determinations and precision limits are provided (Annex A).
- Quality control: Electrode checks, appropriate dilution for samples >40% dry matter, and rejection/re-treatment rules if two inflection points indicate untreated polysulphide.
Applications and users
- Pulp and paper mills: Process control of kraft pulping and recausticizing operations; monitoring white and green liquor quality.
- Analytical laboratories: Routine laboratory method for QA/QC and compliance testing.
- Environmental and research labs: Measuring sulphide speciation in process effluents and research on sulfur balance in kraft recovery cycles.
- Plant chemists and laboratory analysts: Use this standard to ensure consistent, traceable hydrosulphide measurements and to interpret polysulphide contributions.
Related standards
- This ISO document contains no normative references. It was prepared by ISO/TC 6 (Paper, board and pulps); users may consult related ISO/TC 6 publications or national implementation guidance for complementary sampling and laboratory practices.
Keywords: ISO 23777:2023, kraft liquor, hydrosulphide ion, potentiometric titration, silver nitrate titration, polysulphide, white liquor, black liquor, sodium sulphite, pulp mill quality control.