ISO 21400:2018

INTERNATIONAL ISO
STANDARD 21400
First edition
2018-12
Pulp — Determination of cellulose
nanocrystal sulfur and sulfate half-
ester content
Pâte — Détermination de la teneur en soufre et en demi-ester de
sulfate des nanocristaux de cellulose
Reference number
©
ISO 2018
© ISO 2018
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Published in Switzerland
ii © ISO 2018 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviated terms . 3
5 Total elemental sulfur content — ICP-OES method . 4
5.1 Principle . 4
5.2 Reagents and apparatus . 5
5.3 Sample purification by dialysis . 6
5.4 Microwave-assisted sample digestion and sample preparation . 8
5.5 Preparation of calibration solutions and blanks .10
5.6 Analysis of standards and samples by ICP-OES .11
5.7 Calculation of dry CNC total elemental sulfur content and CNC surface charge .12
5.8 Test report .13
6 Sulfate half-ester content — Conductometric titration method .13
6.1 Principle .13
6.2 Reagents and apparatus .14
6.3 Sample purification by dialysis .15
6.4 Sample protonation by ion exchange .16
6.5 Sample analysis by conductometric titration .17
6.6 Calculation of dry CNC sulfate half-ester content and CNC surface charge .19
6.7 Test report .20
Annex A (normative) Sample digestion by wet ashing .21
Annex B (normative) Sample protonation by batch treatment with ion exchange resin .23
Annex C (informative) Precision .24
Bibliography .26
Foreword
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This document was prepared by Technical Committee ISO/TC 6, Paper, board and pulps.
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iv © ISO 2018 – All rights reserved

Introduction
This document, which establishes testing methodologies for measuring the total elemental sulfur and
sulfate half-ester group contents of cellulose nanocrystals (CNCs), was developed in response to a need
for a simple and rapid method for indirect quantification of CNC surface charge.
The main purpose of the two methods covered (inductively coupled plasma-optical emission
spectroscopy (ICP-OES) and conductometric titration) in this document is to measure the surface
charge of sulfated CNCs. Sulfate half-ester groups (R–OSO H) covalently bound at the nanocrystal
surface are introduced during concentrated sulfuric acid hydrolysis by partial esterification of the
[1]
cellulose hydroxy groups . The anionic sulfate half-ester groups are strong acids, such that at neutral
-
and basic pH values, the protons dissociate and the CNC surface is negatively charged (R–OSO ). The
pKa of the sulfate half-ester groups on CNCs is approximately 2,5 (as determined by potentiometric
titration), implying that at very low pH the surface groups are protonated and CNCs have a net neutral
[2]
charge . This surface charge controls many important properties of CNC suspensions, including the
colloidal stability, self-assembly and rheological behaviour, both in the pure state and in the presence of
salts and other additives. As such, the CNC surface charge is a very important factor in the processing
and development of commercial products containing CNCs. The sulfate half-ester (sulfur) content will
also be a key entry on material specifications sheets which will accompany the commercial product,
enabling different product grades to be distinguished from each other and from other companies’
products.
ICP-OES and conductometric titration are both included in this document as they provide different but
complementary ways of measuring the surface charge. ICP-OES measures elemental sulfur which is
present in a 1:1 ratio with the charged sulfate half-ester groups, and does not depend on the nature of
the counterion. Conductometric titration, on the other hand, measures only protons associated with
-
the anionic R–OSO , but is much less complicated to carry out. The two analysis methods should yield
equivalent results (see 5.1 and 6.1), or within 5 % to 10 % owing to sources of uncertainty/error such
as transfer losses and slight differences in the purification and protonation steps. CNCs derived from
different cellulose sources have shown different levels of agreement between the results from the two
[3]
methods . The objective of this document is to use this information in quantifying the CNC surface
charge arising from the easily ionized sulfate half-ester moieties introduced during hydrolysis or post-
sulfation.
The tests contained herein are based on literature methods and were developed over several years by a
group of industry experts, and were identified as being those which can yield reproducible and accurate
results. The tests are anticipated to be performed in a laboratory setting.
As with any laboratory procedure requiring the use of potentially hazardous chemicals, the user is
expected to have received proper knowledge and training in the use and disposal of these chemicals.
This document contains footnotes giving examples of apparatus, reagents and sometimes the supplier(s)
of those materials that are available commercially. This information is given for the convenience of users
of this document and does not constitute an endorsement by ISO of the products named. Equivalent
products may be used if they can be shown to lead to the same results.
Annex A provides an alternative method of sample digestion for ICP-OES by wet ashing. Annex B
provides an alternative method of sample protonation for conductometric titration by treatment with
batches of ion exchange resin.
INTERNATIONAL STANDARD ISO 21400:2018(E)
Pulp — Determination of cellulose nanocrystal sulfur and
sulfate half-ester content
1 Scope
This document specifies procedures for the laboratory determination of the total elemental sulfur and
the sulfate half-ester content of cellulose nanocrystals (CNCs) by inductively coupled plasma-optical
emission spectroscopy and conductometric titration, respectively, including sample preparation,
measurement methods and data analysis.
This document is applicable to the characterization of CNCs:
a) with all monovalent counterions (particularly hydronium and sodium cations);
b) which are either in the never-dried state in aqueous suspension, or have been redispersed from a
dried form; and
c) which have been extracted from any naturally occurring cellulose source using a range of sulfuric
acid hydrolysis conditions, or have been sulfated post-hydrolysis using sulfuric acid.
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 3696, Water for analytical laboratory use — Specification and test methods
ISO 14644-1, Cleanrooms and associated controlled environments — Part 1: Classification of air cleanliness
by particle concentration
ISO/TS 80004-1, Nanotechnologies — Vocabulary — Part 1: Core terms
ISO/TS 80004-2, Nanotechnologies — Vocabulary — Part 2: Nano-objects
ISO/TS 80004-6, Nanotechnologies — Vocabulary — Part 6: Nano-object characterization
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO/TS 80004-1, ISO/TS 80004-2,
ISO/TS 80004-6 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
nanoscale
length range approximately from 1 nm to 100 nm
Note 1 to entry: Properties that are not extrapolations from larger sizes are predominantly exhibited in this
length range.
[SOURCE: ISO/TS 80004-1:2015, 2.1]
3.2
nano-object
discrete piece of material with one, two or three external dimensions in the nanoscale (3.1)
Note 1 to entry: T
...