ISO 23497:2019

Determination of pH value - Technical buffer solutions for the calibration of technical measuring installations

This document specifies requirements for technical buffer solutions. These buffer solutions are preferably used for the calibration and adjustment of technical pH measuring equipment as well as pH measuring installations in laboratories. Measuring methods are given to determine pH values and buffer capacities of such buffer solutions based on pH reference buffer solutions in accordance with ISO 23496. NOTE Annex A gives examples of technical buffer solutions.

Détermination de la valeur pH — Solutions tampons techniques pour l'étalonnage des installations techniques de mesure du pH

General Information

• Status: Published
• Publication Date: 11-Jul-2019
• ICS: 87.040 - Paints and varnishes
• Technical Committee: ISO/TC 35/SC 9 - General test methods for paints and varnishes
• Drafting Committee: ISO/TC 35/SC 9/WG 30 - Properties prior to and during application and drying
• Current Stage: 9093 - International Standard confirmed
• Start Date: 17-Mar-2025
• Completion Date: 13-Dec-2025

Overview

ISO 23497:2019 specifies requirements and measurement methods for technical buffer solutions used to calibrate and adjust technical pH measuring equipment and pH measuring installations. The standard defines how to determine pH values and buffer capacity of technical buffers by traceable comparison to reference buffer solutions in accordance with ISO 23496. Annex A provides practical examples of commonly used technical buffer solutions (e.g., glycine, acetate, citric/phosphate, boric acid/NaOH, HCl, NaOH).

Key topics and requirements

• Purpose: Provide higher ionic strength (higher buffer capacity) solutions suitable for industrial and technical pH calibration where reference buffers (low ionic strength) are impractical.
• Traceability: pH values of technical buffers are measured based on ISO 23496 reference buffers and in accordance with DIN 19268 to ensure traceability to primary standards.
• Measurement uncertainty:
  • pH < 10 at 25 °C: expanded uncertainty U ≤ 0.03 (k = 2).
  • pH ≥ 10 at 25 °C: expanded uncertainty U ≤ 0.05 (k = 2).
• Buffer capacity requirement: Technical buffers shall have β ≥ 0.05 mol·L⁻¹.
• Measurement method: Use combined electrodes with transference in the configuration: Reference electrode || buffer | pH measuring electrode. pH measurement procedures follow DIN 19268.
• Buffer capacity determination: Calculated from titration: β = (Δn / V) · (1 / ΔpH), where Δn is moles of added acid/base, V is volume, and ΔpH is the observed change. Measurements are typically done at (25 ± 1) °C.
• Practical considerations:
  • Minimize alkali error of glass electrode; follow manufacturer guidance.
  • Use combined electrodes with liquid reference electrolyte; KCl refill solution should be ≥ 3.0 mol·L⁻¹.
  • Use diaphragms with high electrolyte outflow (ground-joint, platinum, capillary).
  • Maintain isothermal conditions (±0.5 K) and allow temperature equilibration of electrode and solution.
  • Keep KCl level inside reference electrode at least 5 cm above test solution; keep refill opening open.
  • If measuring at multiple temperatures, perform separate measurements per temperature.

Applications and users

ISO 23497:2019 is directly applicable to:

• Calibration labs and metrology services performing pH calibrations for industrial installations.
• Process control and quality assurance teams in chemical, water treatment, food, pharmaceutical and paint & varnish industries.
• Manufacturers of pH electrodes and pH measuring systems.
• Calibration service providers and instrument validation teams requiring traceable pH standards with higher buffer capacity for rugged environments.

Keywords: ISO 23497:2019, technical buffer solutions, pH calibration, buffer capacity, pH measurement, ISO 23496, DIN 19268.

Related standards

• ISO 23496 - Reference buffer solutions for pH calibration
• DIN 19268 - pH measurement procedures and uncertainty evaluation
• ISO 80000-9 - Quantities and units (physical chemistry)
• ISO 1042 - Volumetric flasks (preparation)
• ISO 19396-1 - pH electrodes with glass membrane

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 23497
First edition
2019-07
Determination of pH value —
Technical buffer solutions for the
calibration of technical measuring
installations
Détermination de la valeur pH — Solutions tampons techniques pour
l'étalonnage des installations techniques de mesure du pH
Reference number
©
ISO 2019
© ISO 2019
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting
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below or ISO’s member body in the country of the requester.
ISO copyright office
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CH-1214 Vernier, Geneva
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Published in Switzerland
ii © ISO 2019 – All rights reserved

Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 pH values and buffer capacities of technical buffer solutions . 2
4.1 General . 2
4.2 Measurement of the pH values of technical buffer solutions . 2
4.3 Measurement of the buffer capacity . 3
Annex A (informative) Examples of technical buffer solutions . 5
Bibliography . 7
Foreword
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This document was prepared by Technical Committee ISO/TC 35, Paints and varnishes, Subcommittee
SC 9, General test methods for paints and varnishes.
Any feedback or questions on this document should be directed to the user’s national standards body. A
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iv © ISO 2019 – All rights reserved

INTERNATIONAL STANDARD ISO 23497:2019(E)
Determination of pH value — Technical buffer solutions for
the calibration of technical measuring installations
1 Scope
This document specifies requirements for technical buffer solutions. These buffer solutions are
preferably used for the calibration and adjustment of technical pH measuring equipment as well as
pH measuring installations in laboratories. Measuring methods are given to determine pH values and
buffer capacities of such buffer solutions based on pH reference buffer solutions in accordance with
ISO 23496.
NOTE Annex A gives examples of technical buffer solutions.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 23496, Determination of pH value — Part 3: Reference buffer solutions for the calibration of pH
measuring equipment
ISO 80000-9, Quantities and units — Part 9: Physical chemistry and molecular physics
DIN 19268:2007, pH measurement — pH measurement of aqueous solutions with pH measuring chains
1)
with pH glass electrodes and evaluation of measurement uncertainty
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 80000-9 and the following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https: //www .iso .org/obp
— IEC Electropedia: available at http: //www .electropedia .org/
3.1
pH
measure for the acidic or basic character of an aqueous solution
Note 1 to entry: Notation of pH: the "p" and the "H" are upright on one line.
Note 2 to entry: The acidic character is determined by the activity of the existing “hydrogen ions”.
[SOURCE: ISO 23496:2019, 3.1]
1)  A measuring chain in the sense of this standard is generally represented by a combined electrode.
3.2
pH value
decadal logarithm of the hydrogen ion activity multiplied with (−1)
m ⋅γ 
a
  ++
+
HHm,
H
 
 
pH=−lg =−lg
 
 
m m
 
 
where
is the activity of the hydrogen ion, in mol/kg;
a
+
H
m is the standard molality (1 mol/kg);
is the activity coefficient of the hydrogen ion;
γ
+
m,H
m is the molality of the hydrogen ion, in mol/kg.
+
H
Note 1 to entry: Molality is understood as moles per kilogram solvent.
Note 2 to entry: The pH value is not measurable as a measure of a single ion activity. Therefore, pH(PR) values of
solutions of primary reference material are determined, which are approximate to it and can be attributed to it.
This is based on a worldwide agreement, see ISO 80000-9:2009, Annex C.
[SOURCE: ISO 23496:2019, 3.2]
4 pH values and buffer capacities of technical buffer solutions
4.1 General
The reference buffer solutions given in ISO 23496 are the basis for practical pH measurement. The pH
values of these reference buffer solutions are traceable to primary standards, the pH values of which
have been determined by national metrology institutes by means of measurements using primary
[1]
methods . The accuracy and reproducibility have been confirmed by international interlaboratory
[2]
tests .
The reference buffer solutions in accordance with ISO 23496 are diluted solutions (ionic strength
≤0,1 mol/kg). Consequently, they have a relative low buffer capacity. This complicates the handling
in practice, since the pH value can change due to the addition of acid or bases (e.g. CO from the air,
carryovers of other solutions). For the calibration of combined pH electrodes in industrial environment,
thus, further buffer solutions are required, which have a higher buffer capacity.
The pH values of these techni
...