Lignins — Determination of carbohydrate composition in kraft lignin, soda lignin and hydrolysis lignin

This document describes a method for the determination of carbohydrate composition in kraft lignin, soda lignin and biorefinery lignin. The method is applicable to lignin isolated from a kraft pulping process, a soda pulping process, or lignin obtained by hydrolysis of biomass.

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INTERNATIONAL ISO
STANDARD 24215
First edition
2022-06
Lignins — Determination of
carbohydrate composition in kraft
lignin, soda lignin and hydrolysis
lignin
Reference number
ISO 24215:2022(E)
© ISO 2022
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ISO 24215:2022(E)
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© ISO 2022

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© ISO 2022 – All rights reserved
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ISO 24215:2022(E)
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 ....................................................................................................................................................................................................................... 4

8 Drying .............................................................................................................................................................................................................................. 4

9 Test Specimens ...................................................................................................................................................................................................... 5

10 Procedure ....................................................................................................................................................................................................................5

10.1 General ........................................................................................................................................................................................................... 5

10.2 Hydrolysis ................................................................................................................................................................................................... 5

10.3 Determination of monosaccharides ..................................................................................................................................... 5

10.3.1 General ........................................................................................................................................................................................ 5

10.3.2 Determination using an IC instrument .......................................................................................................... 5

10.3.3 Determination using a GC instrument ............................................................................................................ 6

11 Calculation .................................................................................................................................................................................................................. 7

12 Precision ....................................................................................................................................................................................................................... 8

13 Test Report ................................................................................................................................................................................................................. 8

Annex A (informative) Precision ..........................................................................................................................................................................10

Annex B (informative) Comparison of GC/FID and IC/PAD analyses of carbohydrates .............................14

Bibliography .............................................................................................................................................................................................................................15

iii
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ISO 24215:2022(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.

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 24215:2022(E)
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 carbohydrate composition - the contents of the five principal, neutral monosaccharides; arabinose,

galactose, glucose, xylose and mannose - provides chemical information about the main polysaccharides

in lignin. The total content and composition of carbohydrates also provide 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]

The methods described in this document are based on those described in other publications .

[3] [4][5][6]

Although the principle is similar to that described in ISO 21437 and other related methods

for the determination of carbohydrates 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.
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INTERNATIONAL STANDARD ISO 24215:2022(E)
Lignins — Determination of carbohydrate composition in
kraft lignin, soda lignin and hydrolysis lignin
1 Scope

This document describes a method for the determination of carbohydrate composition in kraft lignin,

soda lignin and biorefinery 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 play 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
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ISO 24215:2022(E)
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
3.8
carbohydrate composition

amounts of the five principal, neutral monosaccharides; arabinose, galactose, glucose, mannose and

xylose, in a sample
4 Principle

A lignin sample is treated with sulfuric acid in a two-step (primary and secondary) hydrolysis process

to dissolve the carbohydrates. The acid-insoluble lignin is filtered off. The filtrate consists of hydrolysed

carbohydrates and a small amount of acid-soluble lignin. The amounts of the different monosaccharides

in the filtrate are determined using either ion chromatography (IC) or gas chromatography (GC) in

the presence of an internal standard to validate the results. If GC is used, the hydrolysed sample is

reduced and acetylated, and the resulting alditol acetates of the monosaccharides are then separated

and determined by GC.
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.
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ISO 24215:2022(E)
5.6 Equipment specific to the determination method
5.6.1 IC determination

Ion chromatograph (IC) with an anion-exchange column for monosaccharide determination and

pulsed amperometric detector (PAD)
5.6.2 GC determination
5.6.2.1 Water bath, maintained at a temperature of (40 ± 0,5) °C.

5.6.2.2 Gas chromatograph (GC) with a suitable column for monosaccharide determination and

flame ionization detector (FID).
6 Reagents
All chemicals shall be of ACS or reagent grade.
6.1 Water, of high purity, distilled or deionized.
6.2 Monosaccharide standards

Monosaccharide standards, for calibration: arabinose, galactose, glucose, mannose and xylose.

Prepare standard solutions of appropriate concentrations, each standard solution containing all five

monosaccharides.

6.3 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 with the use of a hydrometer by careful

addition of concentrated sulfuric acid or water.
6.4 Petroleum ether.
6.5 Eluent solution (for IC determination)

The composition of this solution depends on the type of IC column to be used. Therefore, follow the

recommendations given by the IC column supplier.
Reagents 6.6. to 6.13 are required only for GC determinations:

NOTE Alternative reagents (to those in 6.6 to 6.13) and procedures (to those in 10.3.3.1, 10.3.3.2 and

[6]

10.3.3.3) for the neutralization, reduction and derivatization steps, such as those described in TAPPI T249 can

also be used, provided that it is indicated in the report.
6.6 Ammonia, NH conc. 25 % (g/100 g) 13 M (moles/l).
6.6.1 Ammonia, 12 M (moles/l). Mix 9 parts ammonia (25 %) with 1 part water.

6.7 Potassium hydroxide, KOH 7,5 M Weigh 105 g KOH pellets into a 250 ml beaker. Add

approximately 150 ml water (6.1) while stirring. Transfer the solution into a 250 ml volumetric flask,

using an additional 20 ml to 30 ml of water to rinse the beaker in order to complete the transfer. Allow

the solution to cool to ambient temperature and dilute it to the mark with water.

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ISO 24215:2022(E)
6.8 Potassium borohydride, KBH solution.

Dissolve 150 mg KBH in 250 µl 13 M NH and 750 µl distilled water in a septum vial (4 ml).

4 3
This solution shall be freshly prepared before use.
6.9 Acetic acid, CH COOH, concentrated.
6.10 Acetic acid anhydride, concentrated.
6.11 1-methylimidazole.
6.12 Ethanol, 95 ml/100 ml to 99 ml/100 ml (95 % to 99 %).
6.13 Sodium sulfate, Na SO , water-free.
2 4
6.14 2-Deoxy-galactose.
6.14.1 Internal standard solution, 2-deoxy-galactose 20 mg/ml.

Weigh 1,00 g 2-deoxy-galactose to the nearest 0,1 mg, transfer it quantitatively to a 50 ml volumetric

flask, and dilute to the mark with distilled water.

Other internal standards than 2-deoxy-galactose, such as fucose or myo-inositol, can also be used.

However, this shall be specified in the report.
7 Sampling

Obtain a representative sample of lignin equivalent to about 2 g to 3 g on an air-dry basis. Report the

origin of the sample and the sampling procedure. In particular, if the sample is first ground to ensure its

homogeneity, and/or a sieving step is required in order to obtain samples with a uniform particle size

or narrow particle size distribution, this shall be reported.

Lignin samples can contain a significant amount of resins. The resins shall be extracted with petroleum

ether before testing. The percentage resin in the sample and the extraction method shall be included in

the test report.

Extraction with petroleum ether should be carried out by a method similar to that described in

[7]
ISO 14453 .

Although acetone is an effective solvent for extracting resin, it cannot be used here since it will also

dissolve part of the lignin.

NOTE Resins, if not extracted from lignin samples prior to analysis, will remain insoluble in acid and be

weighed as acid-insoluble lignin.
8 Drying

Prior to drying to complete dryness, lignin samples shall be air-dried to over 75 % (g/100 g) solids.

This step is necessary in order to minimize the extent of lignin degradation reactions and the drying

period.

Determine the dry matter content of the lignin by drying a 2 g to 3 g specimen in an oven (5.4) at

(105 ± 2) °C initially for 2 h, and then for additional 1-h periods up to a maximum of 7 h, until the

difference in mass between two successive dryings in a desiccator, does not exceed 0,5 % mass fraction

(g/100 g) of the test piece before drying.
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ISO 24215:2022(E)
9 Test Specimens

Weigh (5.5) a test specimen, equivalent to about 100 mg of oven-dried lignin, to the nearest 0,1 mg and

transfer to a 100 ml beaker. Calculate and record the oven-dry mass W of the test portion, in grams.

10 Procedure
10.1 General
Carry out the entire procedure in duplicate.
10.2 Hydrolysis

Add 1,0 ml of 72 % sulfuric acid (6.3) to the test specimen in the beaker. Stir the contents of the beaker

with a glass rod. To avoid losses, ensure that no material is sticking to the glass rod when it is removed.

Place the beaker in a (30 ± 1) °C water bath (5.2) for 1 h. Stir occasionally.

Add 28 ml of water (6.1). Mix, cover the beaker with aluminium foil and place it in autoclave (5.3) at

(120 ± 3) °C for 1 h. Remove the beaker from the autoclave and allow the beaker and its contents to cool

to approximately 80 °C.

NOTE If the lignin is finely dispersed, it can require overnight or longer to settle.

10.3 Determination of monosaccharides
10.3.1 General
Carry out the determination according to either 10.3.2 or 10.3.3.

NOTE The determination of carbohydrate composition can also be carried out by HPLC (High Performance

Liquid Chromatography), as mentioned in Reference [4] for example, provided that the results have been

validated against those obtained with this document.
10.3.2 Determination using an IC instrument
10.3.2.1 Solution prepara
...

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