ISO 23496:2019

INTERNATIONAL ISO
STANDARD 23496
First edition
2019-07
Determination of pH value —
Reference buffer solutions for
the calibration of pH measuring
equipment
Détermination de la valeur pH — Solutions tampons de référence
pour l'étalonnage des appareils de mesure du pH
Reference number
ISO 23496:2019(E)
©
ISO 2019

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ISO 23496:2019(E)

COPYRIGHT PROTECTED DOCUMENT
© ISO 2019
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
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Published in Switzerland
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ISO 23496:2019(E)

Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Basic information . 2
4.1 General . 2
4.2 pH values of primary reference buffer solutions . 2
4.3 pH values of secondary reference buffer solutions . 3
5 Preparation of primary and secondary reference buffer solutions .7
5.1 General . 7
5.2 Reference buffer solution A, pH(R) = 1,67 (25 °C), Potassium tetraoxalate dihydrate . 7
9
5.3 Reference buffer solution B, pH(R) = 3,55 (25 °C), Potassium hydrogen tartrate . 7
7
5.4 Reference buffer solution C, pH(R) = 4,00 (25 °C), Potassium hydrogen phthalate . 7
5
5.5 Reference buffer solution D, pH(R) = 6,86 (25 °C), Phosphate . 7
5
5.6 Reference buffer solution E, pH(R) = 7,41 (25 °C), Phosphate . 8
3
5.7 Reference buffer solution F, pH(R) = 9,18 (25 °C), Borax . 8
0
5.8 Reference buffer solution G, pH(R) = 12,45 (25 °C), Calcium hydroxide . 8
4
5.9 Reference buffer solution H, pH(R) = 3,77 (25 °C), Potassium dihydrogen citrate . 8
6
5.10 Reference buffer solution I, pH(R) = 10,01 (25 °C), Sodium carbonate/sodium
2
hydrogen carbonate . 8
6 Storage and shelf life . 8
7 Examples of pH(R) values of reference buffer solutions . 8
8 Additional properties of reference buffer solutions . 8
Annex A (informative) Examples of pH(R) values as a function of temperature at
temperatures from 55 °C to 95 °C .10
Annex B (informative) Volumetric method .11
Bibliography .13
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ISO 23496:2019(E)

Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
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ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
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
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
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on the ISO list of patent declarations received (see www .iso .org/patents).
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.org/iso/foreword .html.
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
complete listing of these bodies can be found at www .iso .org/members .html.
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INTERNATIONAL STANDARD ISO 23496:2019(E)
Determination of pH value — Reference buffer solutions
for the calibration of pH measuring equipment
1 Scope
This document specifies reference buffer solutions for the calibration of pH measuring equipment.
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 4793, Laboratory sintered (fritted) filters — Porosity grading, classification and designation
ISO 19396-1, Paints and varnishes — Determination of pH value — Part 1: pH electrodes with glass
membrane
ISO 80000-9, Quantities and units — Part 9: Physical chemistry and molecular physics
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”.
3.2
pH value
decadal logarithm of the hydrogen ion activity multiplied with (−1)
m ⋅γ 
a 
++
+
HHm,
H
 
 
pH=−lg =−lg
00
 
 
m m
 
 
where
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ISO 23496:2019(E)

is the activity of the hydrogen ion, in mol/kg;
a
+
H
0
m is the standard molality (1 mol/kg);
is the activity coefficient of the hydrogen ion;
γ
+
m,H
is the molality of the hydrogen ion, in mol/kg.
m
+
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.
4 Basic information
4.1 General
Reference buffer solutions are prepared using primary or secondary reference materials. The pH values
of the reference buffer solutions are determined using the methods described in 4.2 and 4.3. The pH
value of a reference buffer solution and the associated uncertainty of measurement are documented
in a calibration certificate as the result of calibration. pH values of primary reference buffer solutions
form the basis of pH measurements in practice and are generally determined by national metrology
institutes. Secondary reference buffer solutions are commercially available. The reference buffer
solutions prepared according to the methods described in Clause 5 serve as a material measure of the
pH value.
4.2 pH values of primary reference buffer solutions
The pH value based upon the activity of single ions according to the formula in 3.2 cannot be measured.
The pH value is assigned to reference buffer solutions outlined in Table 1. This is achieved using an
electrochemical method of measurement that is based on the thermodynamic dependence of the
potential of the platinum/hydrogen electrode on the hydrogen ion activity. Through the use of cells
without transference, diffusion voltages arising from liquid junctions do not appear and therefore do
not need to be considered when calculating the voltage of the cell. The cell (I) for this purpose shall
consist of a platinum/hydrogen and a silver/silver chloride electrode, immersed in the reference buffer
solution to which chloride in a low concentration m has been added.
()−
Cl
−
Pt|Ag|AgCl|reference buffer solution, pH(R), Cl m |Pt, H ( p = 101 325 Pa). (I)
()−
2 H
2
Cl
The pH value of the reference buffer solution is calculated according to Formula (1):
0
 
EE−
m 
()
−
lim  
Cl
  +
pH= +lg lgγ (1)
−
m →0
 0 
Cl
−  
k
Cl m
 
 
 
where
E is the voltage of cell (I);
0
E is the standard potential of the Ag/AgCl electrode;
k is the Nernst slope (k = RT(ln10) / F);
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ISO 23496:2019(E)

0
m is the standard molality (1 mol/kg);
is the molality of the chloride ions in the solution;
m
−
Cl
is the activity coefficient of the chloride ion.
γ
−
Cl
Chloride is added to the reference buffer solution in a minimum of three different molalities less than or
equal to 0,02 mol/kg. To calculate the pH value, m →0 is extrapolated according to Formula (1).
−
Cl
The activity coefficient of the chloride ions is estimated according to Formula (2) for solutions with an
[2]
ionic strength of less than or equal to 0,1 mol/kg .
12/
AI
lgγ =− (2)
−
Cl 12/
0
11+ ,/5 Im
()
where
A is the Debye-Hückel constant;
is the activity coefficient of the chloride ion;
γ
−
Cl
0
m is the standard molality (1 mol/kg);
I is the ionic strength.
pH values of primary reference buffer solutions are often referred to as pH(R) (R = Reference). The
expanded uncertainty of measurement U(pH) when determining a pH(R) value has been shown to be
0,002 to 0,005 at 25 °C depending on the primary reference buffer solution. As agreed, this uncertainty
of measurement does not take into consideration the uncertainty of the Bates-Guggenheim convention
[3]
for estimating the ionic activity of the chloride ion [see Formula (2)] .
The expanded uncertainty of measurement of many pH(R) value measurements of primary reference
buffer solutions prepared according to Clause 5 is U(pH) = 0,005 at 25 °C and U(pH) = 0,008 at 50 °C,
due to the method of measurement and the purity and homogeneity of the materials used for different
batches depending on the buffer solution. Accordingly, the pH(R) values of each batch can vary. Therefore,
the pH(R) value is only valid for an individual batch with a corresponding calibration certificate. For
buffer solution G, the expanded uncertainty of measurement of the pH(R) values is U(pH) = 0,006 at
[4]
25 °C and U(pH) = 0,01 for temperatures higher than 25 °C . Contributions to uncertainty are a result
of the method of measurement used as well as the purity and homogeneity of the material used.
Typical pH values of reference buffer solutions for the temperature range of 0 °C to 50 °C are given in
Table 1.
4.3 pH values of secondary reference buffer solutions
[5]
The differential potentiometric method is suitable for determining the pH values of secondary
reference buffer solutions using primary reference buffer solutions with the same chemical composition.
The isothermal cell used for this purpose (II) shall consist of two electrodes separated by a P 40 glass
frit given in ISO 4793, and containing the primary and secondary standard buffer solutions and identical
platinum hydrogen electrodes at exactly the same hydrogen pressure, i.e.
Pt, H p |primary reference buffer, pH(R)||secondary reference buffer, pH|Pt, H p (II)
() ()
2 H 2 H
2 2
The cell voltage E of cell (II) is a measure of the difference in the pH values of the solutions and enables
the
...