ISO 24196:2022

INTERNATIONAL ISO
STANDARD 24196
First edition
2022-06
Lignins — Determination of lignin
content in kraft lignin, soda lignin and
hydrolysis lignin
Reference number
© ISO 2022
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Published in Switzerland
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 2
5 Apparatus . 2
6 Reagents . 3
7 Sampling . 3
8 Drying . 3
9 Test specimens . 3
10 Procedure .4
10.1 General . 4
10.2 Hydrolysis . 4
10.3 Filtration . 4
10.4 Acid-insoluble lignin determination . 4
10.5 Acid-soluble lignin determination . 4
11 Calculation . 5
12 Precision . 6
13 Test report . 6
Annex A (informative) Precision . 7
Bibliography .10
iii
Foreword
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iv
Introduction
There is a rapidly-growing worldwide interest in developing novel applications for lignin as a
replacement for fossil-based raw materials in products including carbon fibre, adhesives, thermoplastics,
resins, composites, and various chemicals. In addition, the use of lignin in these and other applications
will offload recovery boilers in pulp mills, allowing more efficient recovery of pulping chemicals and
increased pulp production. These benefits translate into reduced environmental impact and improved
sustainability owing to the use of renewable materials.
In order to ensure harmonization of testing practices among lignin producers and to facilitate trade,
the use of international standard methods is needed to characterize the lignin raw material for a wide
range of properties such as general composition, functional groups, molecular weight distribution,
particle size, structural features, and thermal behaviour and stability.
The total lignin content provides an indication of the purity of the lignin isolated from the kraft pulping
process (kraft lignin) or the soda pulping process (soda lignin), or that obtained by hydrolysis of
biomass (hydrolysis lignin).
[1][2][3]
The method described in this document is based on that described in other publications .
[4] [5][6][7]
Although the principle is similar to that described in ISO 21436 and other related methods for
the determination of lignin in pulp, the properties and end-use applications of lignin, as well as several
steps in the testing procedure, including sampling, sample preparation, and others, are different from
those of pulp.
v
INTERNATIONAL STANDARD ISO 24196:2022(E)
Lignins — Determination of lignin content in kraft lignin,
soda lignin and hydrolysis lignin
WARNING — This method involves the use of hazardous chemicals. Care should be taken to
ensure that the relevant precautions are taken.
1 Scope
This document describes a method for the determination of lignin content in kraft, soda, and hydrolysis
lignin.
The method is applicable to lignin isolated from a kraft pulping process, a soda pulping process, or
lignin obtained by hydrolysis of biomass.
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
lignins
class of complex organic macromolecules, containing aromatic sub-units, that plays a key role in the
formation of cell walls in wood and bark, conferring mechanical strength and rigidity to the cell walls
and to plants as a whole
Note 1 to entry: Lignin is the main non-carbohydrate constituent of wood
3.2
kraft lignin
depolymerized and chemically modified lignin isolated from a kraft pulping process, such as that
originating from kraft black liquor
3.3
soda lignin
depolymerized and chemically modified lignin isolated from a soda pulping process, such as that
originating from soda liquor
3.4
hydrolysis lignin
lignin produced for commercial applications by conversion of biomass, through enzymatic or acid
hydrolysis, into sugars and lignin streams, followed by separation of the lignin fraction
3.5
biomass
biological material derived from living, or previously living organisms, such as wood, agricultural crops
and other plant-based biodegradable material
3.6
acid-insoluble lignin
klason lignin
residue after treating wood or pulp with sulfuric acid in a two-step hydrolysis procedure to solubilize
the carbohydrates into monosaccharides
3.7
acid-soluble lignin
portion of lignin that is soluble during the acid-insoluble lignin determination
4 Principle
A lignin sample is treated with sulfuric acid in a two-step (primary and secondary) hydrolysis process
to dissolve the carbohydrates. The residue after hydrolysis is filtered off, dried, and weighed, and
referred to as acid-insoluble lignin (or Klason lignin). A small amount of lignin is dissolved during acid
hydrolysis. This so-called acid-soluble lignin is determined by measuring the absorbance at 205 nm of
the filtrate from the acid-insoluble lignin determination. The total lignin content is determined as the
sum of the acid-insoluble lignin and acid-soluble lignin.
According to Reference [8], proteins present in plant tissue condense extensively with lignin during the
sulfuric acid treatment, imparting extra mass to the measured Klason lignin. Since the enzymes used to
produce hydrolysis lignin consist largely of proteins, a correction to the measured acid-insoluble lignin
in hydrolysis lignin shall be made to account for this additional material. The amount of co-condensed
protein in acid-insoluble lignin can be estimated by multiplying its nitrogen content by a factor of
[8]
6,25 . Nitrogen analysis can be performed using the traditional Kjeldahl method or with an automated
nitrogen analyser, provided that the results have been properly validated.
NOTE The secondary hydrolysis is performed with 4 % sulfuric acid instead of 3 % as described in other
[5][6]
methods for the determination of lignin in wood and pulp . The use of 4 % sulfuric acid for secondary
hydrolysis instead of 3 % has no impact on the lignin analysis and is accepted when both lignin and carbohydrates
need to be analysed in the same sample.
5 Apparatus
5.1 Filtration equipment
5.1.1 Filtering flask, 250 ml.
5.1.2 Gooch filtering crucible, fritted glass, medium or fine porosity, 30 ml; adapter for the filtering
crucible, siphon tube (optional).
NOTE 1 The choice of fritted glass porosity depends on the rate of filtration of the particular type of sample.
For slow-filtering samples, the use of medium (M) porosity is preferable. Filtration can be facilitated by using a
medium porosity crucible with a disc of fine porosity glass-fibre filter paper fitted over the sintered glass in the
crucible.
NOTE 2 Other types of filtering crucibles such as alundum or porous porcelain crucibles lined with a mat of
fine fibres can also be used.
5.2 Constant temperature water bath, capable of maintaining a temperature of (30 ± 1) °C.
5.3 Autoclave, capable of maintaining a temperature of (120 ± 3) °C.
5.4 Drying oven, conduction type, maintained at (105 ± 2) °C.
A convection oven shall not be used, as this could lead to increased flare-ups and fire hazard, as well as
loss of sample due to material being ejected from the crucible.
5.5 Analytical balance, accurate to 0,1 mg.
5.6 Spectrophotometer, UV-visible, diode array or simple wavelength, with high purity quartz
cuvettes of pathlength 1 cm.
6 Reagents
6.1 Water, of high purity, distilled or deionized.
6.2 Sulfuric acid, 72 % (720 g/kg, specific gravity 1,633 8 at 20 °C). 72 % sulfuric acid is available
commercially. It can also be prepared from concentrated sulfuric acid as follows:
Slowly add 650 ml of concentrated sulfuric acid (H SO sp gr 1,84) to 400 ml of water, while cooling
2 4
under a cold-water tap. When the temperature has reached equilibrium with the ambient temperature,
adjust the specific gravity of the sulfuric acid solution to 1,633 8 w
...