ISO 23774:2023

Pulps - Kraft liquor - Determination of total, active and effective alkali using potentiometric titration

This document specifies a potentiometric titration procedure for the determination of total, active and effective alkali in white and green liquors obtained and used in the kraft or sulphate pulping process. This document is not applicable for the analysis of liquors such as oxidized white liquors which contain significant amounts of polysulphides. The method given in this document is not intended for the determination of particular ionic species, such as sulphides or carbonates.

Pâtes — Liqueur Kraft — Détermination des alcalis totaux, actifs et effectif par titrage potentiométrique

General Information

• Status: Published
• Publication Date: 09-Mar-2023
• ICS: 85.040 - Pulps
• Technical Committee: ISO/TC 6 - Paper, board and pulps
• Drafting Committee: ISO/TC 6 - Paper, board and pulps
• Current Stage: 6060 - International Standard published
• Start Date: 10-Mar-2023
• Due Date: 18-Dec-2023
• Completion Date: 10-Mar-2023

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.

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 23774
First edition
2023-03
Pulps — Kraft liquor — Determination
of total, active and effective alkali
using potentiometric titration
Pâtes — Liqueur Kraft — Détermination des alcalis totaux, actifs et
effectif par titrage potentiométrique
Reference number
© ISO 2023
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Published in Switzerland
ii
Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 2
5 Reagents . 2
6 Apparatus . 2
7 Calibration and check of pH meter . 3
8 Procedure .3
9 Calculation . 3
10 Report . 4
Annex A (informative) Precision data . 5
Annex B (informative) Quantities and units for the characterization of white and green
liquors . 7
Bibliography .10
iii
Foreword
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described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
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www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 6, Paper, board and pulps.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
iv
INTERNATIONAL STANDARD ISO 23774:2023(E)
Pulps — Kraft liquor — Determination of total, active and
effective alkali using potentiometric titration
1 Scope
This document specifies a potentiometric titration procedure for the determination of total, active and
effective alkali in white and green liquors obtained and used in the kraft or sulphate pulping process.
This document is not applicable for the analysis of liquors such as oxidized white liquors which contain
significant amounts of polysulphides. The method given in this document is not intended for the
determination of particular ionic species, such as sulphides or carbonates.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
total alkali
total concentration of alkaline constituents determined by titration of a sample of the liquor with strong
acid to the third inflection point according to specific requirements
Note 1 to entry: The specific requirements are described in this document, ISO 23774:2023, Clauses 8 and 9
Note 2 to entry: See also Annex B.
3.2
active alkali
total concentration of alkaline constituents, except carbonates, as determined by titration of a sample
of the liquor with strong acid according to specific requirements
Note 1 to entry: The specific requirements are described in this document, ISO 23774:2023, Clauses 8 and 9
Note 2 to entry: In practice, active alkali is considered to be the sum of the concentrations of hydroxyl and
hydrosulphide ions, including hydroxyl ions formed by hydrolysis of sulphides. See also Annex B.
3.3
effective alkali
concentration of strongly alkaline constituents determined by titration of a sample of the liquor with
strong acid to the first inflection point according to specific requirements
Note 1 to entry: The specific requirements are described in this document, ISO 23774:2023, Clauses 8 and 9
Note 2 to entry: In practice, this is considered to be the concentration of hydroxyl ions, including those formed
from sulphides by hydrolysis. See also Annex B.
4 Principle
The determination of total, active and effective alkali in a liquor sample is based on an acidic dosage by
a strong acid of the alkaline species contained in the sample. The major alkaline species in green liquor
2−
− −
are hydroxyl ions (OH ), hydrosulphur ions (HS ), carbonate (CO ) and hydrogenocarbonate ions (
−
− −
HCO ), whereas in white liquor they are mainly hydroxyl ions (OH ) and hydrosulphur ions (HS ).
During the acidic dosage, these alkaline species lead to different inflexion points for specific pH ranges
as described in Clause 8.
A sample of the liquor is titrated with hydrochloric acid of known concentration. The pH value (or
a suitable function of the pH value) of the reaction mixture and the volume of hydrochloric acid are
recorded continuously and from the recorded data the consumption of acid at the inflection points is
determined.
NOTE The titration is preferably performed using an automatic titration equipment.
From the amounts of acid required to reach the three inflection points, the effective, active and total
alkali of the sample are calculated.
Precision data are available in Annex A.
5 Reagents
WARNING — The use of this document can involve hazardous materials, operations and
equipment. It does not purport to address all of the safety or environmental problems associated
with its use.
All chemicals shall be of analytical grade. The water used in the titration and in the preparation of
reagents shall be distilled or deionized.
5.1 Hydrochloric acid 1 M, the actual concentration of hydrochloric acid, HCl, shall be known to the
nearest 0,005 M.
5.2 Buffer solutions of known pH values near 4 and 9. Suitable buffer solutions are commercially
available. They can also be prepared in the laboratory as follows:
— Buffer solution, pH 4,01: In a 1 000 ml volumetric flask dissolve 10,12 g of potassium hydrogen
phthalate, KHC H O , in water and dilute to the mark. There is normally no need to dry the salt. The
8 4 4
solution is stable for 2 months.
— Buffer solution, pH 9,18: In a 1 000 ml volumetric flask dissolve 3,80 g of sodium tetraborate
decahydrate (borax), Na B O ⋅10H O, in water from which carbon dioxide has been expelled by
2 4 7 2
boiling. Dilute to the mark. The solution is stable for 6 weeks. The solution will absorb carbon
dioxide when in contact with ambient air. Therefore, keep the solution in a stoppered bottle and do
not leave the bottle open more than absolutely necessary.
6 Apparatus
The usual laboratory apparatus and, in particular, the following shall be used.
6.1 Automatic titration equipment, including glass and reference electrodes, that records the
titration curve (pH against titrant consumed) or any function of the titration curve, such as the first
derivative, so that the inflection points of the titration curve can be determined. A combined pH glass
electrode may be used. Check the precision of the equipment as instructed in Clause 7.
For the analysis of green liquors, titratio
...