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INTERNATIONAL ISO
STANDARD 6588-2

Third edition
2020-03

Paper, board and pulps —

Determination of pH of aqueous
extracts —
Part 2:
Hot extraction
Papier, carton et pâtes — Détermination du pH des extraits aqueux —
Partie 2: Extraction à chaud

Reference number
ISO 6588-2:2020(E)
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ISO 6588-2:2020(E)


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ISO 6588-2:2020(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
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
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
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www​.iso​.org/​patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to
the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see
www​.iso​.org/​iso/​foreword​.html.
This document was prepared by Technical Committee ISO/TC 6, Paper, board and pulps.
This third edition cancels and replaces the second edition (ISO 6588-2:2012), which has been technically
revised. The main changes compared to the previous edition are as follows:
— addition of the last four paragraphs in the introduction to clarify the differences, in the role and
mode of addition of a salt solution, between this document and ISO 29681[5].
A list of all parts in the ISO 6588 series can be found on the ISO website.
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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ISO 6588-2:2020(E)


Introduction
Kraft fibre is well known to contain ionisable groups that are fixed to or in the fibre wall. In order to
fulfil the electro-neutrality, these groups are balanced by an equivalent number of positive charges,
which can be either protons or various metal ions. Especially in pulp suspensions at low ionic strengths,
this can give rise to a marked uneven distribution of mobile ions between the volume held by the fibre
wall and the bulk suspension liquor. This means that the fibre acts as an ion exchanger. These ion-
exchange phenomena can be modelled very well with the Donnan theory[2][3].
If a relatively clean pulp fibre sample, as for example bleached dried pulp, is diluted in deionised water,
the result will be a pulp suspension with a very low ionic strength. In such a system, most of the cations
present, including protons, will be concentrated in the water volume held by the fibre wall. If the pH
is measured, it will be measured in the bulk suspension liquor. By adding salt to this kind of system,
the ion exchange phenomena will be decreased and the concentration of different cations will be the
same in the water held by the fibre wall and in the bulk suspension liquor. Since the process waters
always contain a certain amount of ions, such a salt addition will give a more realistic environment
when measuring the pH of relatively clean pulp samples.
Based on the foregoing, two ISO Standards, this document and ISO 29681, are available for determination
of the pH of hot aqueous extracts of paper, board, or pulps. In ISO 29681, a salt solution is added prior
to extraction; however, in this document, the extraction is carried out with distilled or deionized water.
ISO 29681 is specifically applicable to bleached pulps from virgin fibres and to pulp samples having a
low ionic strength for which the pH-value will give more realistic results related to mill conditions than
those obtained with this document.
It is necessary to be aware that the results will not be the same when measuring pH according to this
document and to ISO 29681. The difference can be significant especially when measuring pulps having
a low ionic strength.
ISO 6588-1 differs from this document, in relation to the extraction conditions. No general guidance can
be given as to which of the two procedures (hot or cold) is best suited in a particular situation.
It should be noted that in this document, addition of a salt solution is performed after extraction and
filtration only for the purpose of obtaining a more stable and accurate pH measurement.
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INTERNATIONAL STANDARD ISO 6588-2:2020(E)

Paper, board and pulps — Determination of pH of aqueous

extracts —
Part 2:
Hot extraction

1 Scope
This document specifies a method for the determination of the pH-value defined by the electrolytes
extractable by hot water from a sample of paper, board or pulp.
This document is applicable to all kinds of paper, board and pulp.
As the quantity of extractable ionic material approaches zero, as in the case of highly purified pulps,
the precision of the method becomes poor because of the difficulties encountered in making pH
measurements on water containing little electrolytic material.
Since the extraction in this document is performed with distilled or deionised water, the pH-value
measured will sometimes be different (e.g. fully bleached pulp) from the pH-value measured under
mill process conditions in which various types of process waters, e.g. chemically treated river water
containing electrolytes, are used. In such cases, ISO 29681 can be used instead, as it is specifically
applicable to bleached pulps from virgin fibres and to pulp samples having a low ionic strength for
which the pH value will give more realistic results related to mill conditions than those obtained with
this document. For cellulosic papers used for electrical purposes, the method used can be that given in
IEC 60554-2[4].

2 Normative references
The following referenced documents are indispensable for the application 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 186, Paper and board — Sampling to determine average quality
ISO 7213, Pulps — Sampling for testing

3 Terms and definitions

No terms and definitions are listed in this document.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https://​w ww​.iso​.org/​obp
— IEC Electropedia: available at http://​w ww​.electropedia​.org/​

4 Principle
Extraction of a sample of 2 g for 1 h with 100 ml of boiling water of high purity. Filtration of the extract
and addition of a salt solution. Measurement of the pH-value of the extract at a temperature between
20 °C and 25 °C.
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ISO 6588-2:2020(E)


5 Reagents

5.1 Water, distilled or deionised water shall be used throughout the test. The conductivity of the water
shall not exceed 0,1 mS/m after boiling for 1 h and cooling in an acid-free atmosphere (e.g. free of CO2,
SO2, H2S) to a temperature between 20 °C and 25 °C. The pH of the water should be in the range of 6,8 to
7,3. Instructions for the determination of conductivity are specified in ISO 3696.

5.2 Standard buffer solutions, with known pH-values of about 4, 7 and 9. Such buffer solutions are
commercially available. Some examples of suitable buffer solutions are given, and their preparation is
described, in Annex A.

5.3 Potassium chloride solution, 1M. Dissolve 7,4 g of KCl, analytical grade, in 100 ml of freshly
boiled, distilled water. Prepare a fresh solution every week.

6 Apparatus and equipment

Use the following, in addition to ordinary laboratory apparatus and equipment.

6.1 Glassware of chemically resistant glass, flasks with ground-glass joints, stoppers, beakers, reflux
condenser and fritted glass filter. All glassware shall be cleaned with an acid cleaning solution, without the
use of soap or detergent, and they shall be carefully rinsed with water (5.1) and allowed to dry before use.

6.2 pH-meter, fitted with glass and calomel electrodes or with a combined electrode, capable of being
read to at least 0,05 pH-unit.

7 Sampling and preparation of sample

7.1 Sampling
The sampling procedure to be followed depends on the particular circumstances in each case. If the
analysis is being made to evaluate a lot or a consignment of pulp, paper or board, the sample shall be
taken in accordance with ISO 7213 or ISO 186, as relevant. If the analysis is made on another type of
sample, report the origin of the sample and, if possible, the sampling procedure, and ensure that the
specimen taken in 7.1 is representative of the sample received.
Wear clean protective gloves when handling the sample.
NOTE Some gloves are powdered to prevent them from sticking to one another, and this powder can cause
contamination of the sample.

7.2 Preparation of sample

Do not touch the sample with bare hands and ensure that it has been placed only on clean surfaces. Cut
the sample with a knife or cutter, or tear it, into pieces approximately 1 cm2 in size. Split samples of
heavy board.
Mix the pieces thoroughly. Store the pieces in clean, covered containers.

8 Procedure
Run the procedure in duplicate.
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ISO 6588-2:2020(E)


8.1 Weighing
Weigh 2,0 g ± 0,1 g of air-dry sample.
NOTE Since the amount of sample is not critical, there is no need to determine the dry matter content to
adjust for minor differences in moisture content.

8.2 Extraction
Add 100 ml of water (5.1) to a 250 ml flask (6.1) and heat to boiling. Add the weighed sample, attach the
reflux condenser (6.1) fitting the flask, and continue boiling gently for 1 h ± 5 min on an electric heater.
Cool the flask rapidly to a temperature between 20 °C and 25 °C with the condenser in place. Filter the
extract through a coarse, fritted glass filter into a small beaker (6.1). Immediately add 2 ml of potassium
chloride solution (5.3) and continue with the measurement.

8.3 Determination of pH
Operate the pH-meter in accordance with the manufacturer’s instruction. Wash the electrodes with
water (5.1); allow the water to drain from the electrodes, but do not wipe them. Calibrate the pH-meter
(6.2), at a temperature between 20 °C and 25 °C, with two different buffer solutions (5.2) having pH-
values such that the pH of the extract is between the pH-values of the buffer solutions. The first buffer
solution shall be chosen so that the pH-value of the buffer solution is in the same region as the electric
zero point of the pH-meter (usually = 7). The reading for the second buffer solution should agree with
its correct value to within 0,1 pH-unit.
If the pH-meter fails to show the correct pH-value for the second buffer solution, consult the
manufacturer’s manual. A deviation of, or exceeding, 0,2 pH-units, rounded to the nearest 0,1 pH unit,
indicates a faulty electrode. In addition, a slow but continuous increase or decrease in the reading
indicates a faulty electrode.
After calibration, rinse the electrodes several times with water (5.1) and once in a small quantity of the
extract. Check that the temperature of the extract is between 20 °C and 25 °C. Immerse the electrodes
in the extract. Record the pH when there is no measurable drift, within 30 s.
Before measuring the next sample, rinse the electrodes carefully with water (5.1) to remove any traces
of sample or buffer solution.
At the end of a series of measurements, check the electrodes with the same buffer solutions. The results
for both solutions should agree with their correct value to within 0,1 pH unit. Otherwise repeat the
procedure in this subclause (8.3).

9 Calculation
Calculate the mean of the duplicate determinations.
Report the pH-value to the nearest 0,1 pH-unit. The individual results should not differ by more than
0,2 pH-unit; if they do, repeat the determination with two additional extracts, and report the mean and
the range of all measurements.
NOTE See Annex B for precision.

10 Test report
The test report shall include the following information:
a) a reference to this document, i.e. ISO 6588-2;
b) the date and place of testing;
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ISO 6588-2:2020(E)


c) all the information necessary for complete identification of the sample;

d) the result, expressed as indicated in Clause 9;
e) any unusual features observed in the course of the test;
f) any departure from the procedure described in this document, or any other circumstances which
may have affected the result.
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ISO 6588-2:2020(E)


Annex A
(informative)

Preparation of some standard buffer solutions

All the reagents used shall be of recognized reagent grade. The buffer solutions shall be renewed at
least once a month. The anhydrous salts in A.1 and A.2 shall be dried at 120 °C.

A.1 Buffer solution pH 4,0: Potassium hydrogen phthalate, 0,05 mol/l solution
Dissolve 10,21 g of potassium hydrogen phthalate (KHC8H4O4) in water (5.1) in a 1 l volumetric flask
and dilute to the mark.
The pH-value of this solution is 4,00 at 20 °C and 4,01 at 25 °C.

A.2 Buffer solution pH 6,9: Potassium dihydrogen phosphate and disodium

hydrogen phosphate solution
Dissolve 3,39 g of potassium dihydrogen phosphate (KH2PO4) and 3,54 g disodium hydrogen phosphate
(Na2HPO4) in water (5.1) in a 1 l volumetric flask and dilute to the mark.
The pH-value of this solution is 6,87 at 20 °C and 6,86 at 25 °C.

A.3 Buffer solution pH 9,2: Disodium tetraborate solution

Dissolve 3,80 g of disodium tetraborate decahydrate (Na2B4O7⋅10H2O) in water (5.1) in a 1 l volumetric
flask and dilute to the mark.
The pH-value of this solution is 9,23 at 20 °C and 9,18 at 25 °C.
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ISO 6588-2:2020(E)


Annex B
(informative)

Precision

B.1 General
In February 2002, an international round-robin was performed in which 5 laboratories from different
countries participated.
The calculations have been made according to ISO/TR 24498[6].
The repeatability and reproducibility limits reported are estimates of the maximum difference which
should be expected in 19 of 20 instances, when comparing two test results for material similar to those
described under similar test conditions. These estimates may not be valid for different materials or
different test conditions.
NOTE Repeatability and reproducibility limits are calculated by multiplying the repeatability and
reproducibility standard deviations by 2,77, where 2 , 77 = 1 , 96 2 .

B.2 Repeatability
The pH-values of four different samples were determined in one laboratory according to this document.
Ten determinations were made in each case. Mean values, standard deviations and coefficients of
variations as well as repeatability limits for each type of sample are shown in Table B.1.

Table B.1 — Repeatability

Mean value Standard deviation Coefficient of variation Repeatability limit
Sample
pH sr CV,r (%) r
Copy paper 10,0 0,02 0,21 0,06
Paperboard 8,7 0,06 0,64 0,17
Bleached pulp 5,2 0,03 0,50 0,08
Unbleached pulp 8,0 0,04 0,54 0,11

B.3 Reproducibility
The pH-values of four different samples were determined in five different laboratories according to this
document Mean values, standard deviations and coefficients of variations, as well as reproducibility
limits, for each type of sample, based on the results of five laboratories, are shown in Table B.2.

Table B.2 — Reproducibility

Mean value Standard deviation Coefficient of variation Reproducibility limit
Sample
pH sR CV,R (%) R
Copy paper 9,7 0,25 2,6 0,69
Paperboard 8,5 0,15 1,8 0,42
Bleached pulp 5,6 0,54 9,6 1,50
Unbleached pulp 8,0 0,39 4,9 1,08
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ISO 6588-2:2020(E)


Bibliography

[1] ISO 6588‑1, Paper, board and pulps — Determination of pH of aqueous extracts — Part 1: Cold
extraction
[2] Scallan A.M., In: The pH inside the fibre wall. Cellulose Sources and Exploitation. (Kennedy
J.F., Philips G.O., Williams P.A., eds.). Eric Horwood, London, 1990, pp. 211
[3] Scallan A.M., Predicting the Ion-Exchange of Kraft Pulp Using Donnan Theory. Journal of Pulp
and Paper Science. 1996, 22 (9) pp. J332–J337
[4] IEC 60554‑2, Cellulosic papers for electrical purposes — Part 2: Methods of test
[5] ISO 29681, Paper, board and pulps — Determination of pH of salted water extracts
[6] ISO/TR 24498, Paper, board and pulps — Estimation of uncertainty for test methods by
interlaboratory comparisons
[7] ISO 3696, Water for analytical laboratory use — Specification and test methods
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ISO 6588-2:2020(E)


ICS 85.060; 85.040

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