ISO 20938:2008

Instant coffee - Determination of moisture content - Karl Fischer method (Reference method)

ISO 20938:2008 specifies a method for the determination of moisture content in instant coffee by the Karl Fischer titration method, suitable for use as a reference method.

Café instantané — Détermination de la teneur en eau — Méthode de Karl Fischer (Méthode de référence)

General Information

• Status: Published
• Publication Date: 19-Nov-2008
• ICS: 67.140.20 - Coffee and coffee substitutes
• Technical Committee: ISO/TC 34/SC 15 - Coffee
• Drafting Committee: ISO/TC 34/SC 15 - Coffee
• Current Stage: 9060 - Close of review
• Completion Date: 03-Jun-2028

Overview

ISO 20938:2008 specifies a reference method for determining moisture content in instant coffee using the Karl Fischer titration. The standard defines the principle, required reagents and apparatus, sample preparation, titration procedure and calculation of moisture (expressed as percentage mass fraction). It is intended for use as a laboratory reference method for accurate moisture analysis in spray‑dried, agglomerated (granules) and freeze‑dried instant coffee.

Key topics and technical requirements

• Method principle: Coulometric/volumetric Karl Fischer titration of water released from a weighed test portion dissolved in a water‑free solvent mixture (formamide–methanol–salicylic acid, “FMS”) and titrated with a pyridine‑free one‑component Karl Fischer reagent (KFR).
• Reagents: Pyridine‑free KFR (titre ≈ 5 mg/ml), low‑water methanol and formamide, salicylic acid, disodium tartrate dihydrate for primary titre standard.
• Solvent mixture (FMS): Formamide:methanol ratio 1:2 by volume; salicylic acid dissolved in methanol before adding formamide. Prepare fresh daily.
• Apparatus: Dry titration vessel (~100 ml) with platinum double electrode, magnetic stirring, piston burette for KFR, and drying tubes/receivers to exclude ambient moisture. Analytical balance with 0.1 mg accuracy.
• Titration and titre determination: Condition receiver with FMS and pre‑titrate KFR; determine KFR titre using disodium tartrate dihydrate or weighed water (25–30 mg) in duplicate. Typical test portion for instant coffee solids: 300–500 mg.
• Safety: Formamide handling precautions are required (protective clothing, splash guards, immediate washing after exposure).
• Precision: Interlaboratory results reported for three sample types:
  • Spray dried (mean 4.442 g/100 g): repeatability r = 0.234, reproducibility R = 0.336
  • Agglomerate (mean 4.598 g/100 g): r = 0.193, R = 0.329
  • Freeze dried (mean 1.722 g/100 g): r = 0.129, R = 0.289
  • Overall repeatability and reproducibility limits narrow to ~0.2% and ~0.3% mass fraction respectively.

Applications and users

• Quality control and R&D laboratories in the coffee and beverage industry performing moisture analysis of instant coffee, granules and concentrates.
• Contract testing and certification labs needing a reference method for auditing or method validation.
• Regulatory and standards organizations requiring traceable, reproducible moisture results for specifications, shelf‑life and hygroscopicity assessments.
• Useful for process control (drying, packaging) where accurate moisture content impacts product stability, flowability and rehydration properties.

Related standards

• ISO 4788 - Laboratory glassware - Graduated measuring cylinders (referenced)
• ISO 5725-1 and ISO 5725-2 - Accuracy (trueness and precision) of measurement methods and results (used in precision evaluation)

Keywords: ISO 20938:2008, instant coffee, moisture content, Karl Fischer titration, reference method, FMS solvent, pyridine-free KFR, moisture analysis, coffee quality control.

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 20938
First edition
2008-12-01
Instant coffee — Determination of
moisture content — Karl Fischer method
(Reference method)
Café instantané — Détermination de la teneur en eau — Méthode de
Karl Fischer (Méthode de référence)

Reference number
©
ISO 2008
PDF disclaimer
This PDF file may contain embedded typefaces. In accordance with Adobe's licensing policy, this file may be printed or viewed but
shall not be edited unless the typefaces which are embedded are licensed to and installed on the computer performing the editing. In
downloading this file, parties accept therein the responsibility of not infringing Adobe's licensing policy. The ISO Central Secretariat
accepts no liability in this area.
Adobe is a trademark of Adobe Systems Incorporated.
Details of the software products used to create this PDF file can be found in the General Info relative to the file; the PDF-creation
parameters were optimized for printing. Every care has been taken to ensure that the file is suitable for use by ISO member bodies. In
the unlikely event that a problem relating to it is found, please inform the Central Secretariat at the address given below.

©  ISO 2008
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means,
electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or
ISO's member body in the country of the requester.
ISO copyright office
Case postale 56 • CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail [e-mail]
Web www.iso.org
Published in Switzerland
ii © ISO 2008 – All rights reserved

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.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
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.
ISO 20938 was prepared by Technical Committee ISO/TC 34, Food products, Subcommittee SC 15, Coffee.
INTERNATIONAL STANDARD ISO 20938:2008(E)

Instant coffee — Determination of moisture content —
Karl Fischer method (Reference method)
1 Scope
This International Standard specifies a method for the determination of moisture content in instant coffee by
the Karl Fischer titration method, suitable for use as a reference method.
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 4788, Laboratory glassware — Graduated measuring cylinders
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
moisture content
content of water, determined according to the procedure specified in this International Standard
NOTE The moisture content is expressed as a percentage mass fraction.
4 Principle
4.1 General
In a Karl Fischer apparatus, a test portion of instant coffee is dissolved in a water-free solution of formamide,
methanol, and salicylic acid (FMS). Subsequent titration with a Karl Fischer reagent is applied under
continuous stirring until the endpoint of the titration is reached and electrometrically detected. The volume of
Karl Fischer reagent consumed is used to calculate the moisture content of the test portion.
4.2 Reaction
During the Karl Fischer titration, water present in the sample reacts with iodine and sulfur dioxide in the
presence of an amine and an alcohol (e.g. methanol):
H O++I SO+ ROH+ 3R NH → 2R NH ⋅HI+ R NH ⋅HSO R
22 2 nn3−−nn3 n 3−n 4
where R is an alkyl or alkoxyl group.
The endpoint of the reaction is detectable by a small, surplus amount of iodine and quantified electrometrically
(either amperometrically or voltametrically). The procedure described in this International Standard is
performed only with pyridine-free Karl Fischer (one-component) reagent (KFR).
5 Reagents and materials
Unless otherwise specified, use only reagents of recognized analytical grade, and only distilled or deionized
water or water of equivalent purity.
1)
5.1 Karl Fischer reagent (KFR, one-component system) , titre approximately 5 mg/ml, water mass
concentration, pyridine-free.
5.2 Methanol, with low water mass concentration, preferably < 0,5 mg/ml.
5.3 Formamide, with low water mass concentration.
WARNING — The use of formamide is of outstanding importance with regard to the time of titration
and the clear determination of the endpoint. Operators should be carefully instructed and adequately
trained so as to avoid direct contact with the reagent (use splash guards and protective clothing), and
to wash immediately and thoroughly both in cases of exposure to formamide, as well as at the end of
the workshift.
5.4 Salicylic (2-hydroxybenzoic) acid.
NOTE Salicylic acid is applied for reasons of stabilizing the pH of the solution. Under these conditions, uncontrolled
side reactions do not take place.
5.5 Disodium tartrate (disodium 2,3-dihydroxybutanedioate) dihydrate.
6 Apparatus
Usual laboratory equipment and in particular, the following. All devices used shall be as dry as possible.
6.1 Analytical balance, capable of weighing to an accuracy of 0,1 mg.
6.2 Titration apparatus, for the determination of the moisture content according to the Karl Fischer
method. Complete sets of titration apparatus are commercially available, consisting of the individual
components given in 6.2.1 to 6.2.5.
6.2.1 Karl Fischer titrator, for titrations according to the amperometric or the voltametric procedure.
6.2.2 Platinum double electrode.
6.2.3 Magnetic stirrer equipment, fitted with a polytetrafluoroethylene-coated stirring bar.
6.2.4 Titration vessel, of approximately 100 ml capacity, with at least four basic glass sockets. One of
these is connected to the burette (6.2.5), the second to the platinum double electrode (6.2.2). The third serves
for adding the FMS solution (8.1.1) and the fourth for the introduction of the sample. The latter shall be tightly
closed during titration by a drying tube or receiver (6.2.5).
NOTE Attaching thermostatic mantling to the titration vessel enables an option of slightly raising the temperature,
resulting in reaction acceleration.
6.2.5 Burettes, reservoirs and drying tubes.
A piston burette is used for accurately measuring the volume of KFR (5.1). The reagent bottle, burette and
connecting tubes for transferring KFR should be protected from light. All ventilation sockets of the system shall
be closed by either drying tubes that contain effective drying agents (e.g. silica gel with moisture indicator or
molecular sieves), or with drying receivers filled with KFR in order to exclude any ambient moisture.

1) Suitable products are commercially available.
2 © ISO 2008 – All rights reserved

6.3 Syringe or weighing pipette, suitable for measuring 25 mg to 30 mg water in the determination of the
KFR titre according to 8.2.2.
If determining water content of liquid coffee extract, a syringe can also be used to measure the liquid coffee
sample and to transfer it into the titration vessel.
6.4 Weighing spoon, made of glass with a protective cover, purchased from the titration apparatus
supplier.
6.5 Graduated measuring cylinder, in accordance with ISO 4788, class A, of capacity 500 ml.
7 Sampling
7.1 General
A representative sample should have been sent to the laboratory. It should not have been damaged or
changed during transport or storage.
Sampling is not part of the method specified in this International Standard. The sampling procedure shall be
subject to agreement by the interested parties.
7.2 Sample preparation
The sample should be truly representative and well homogenized, without damage or change during
transportation and storage. To protect the laboratory sample from ambient moisture, it shall be transferred to
an airtight glass jar immediately after opening the original packaging.
NOTE Instant coffee is highly hygroscopic.
8 Preparative steps
8.1 Preparation of solvent mixture and receiver for titration
8.1.1 Preparation of the formamide-methanol-salicylic acid-mixture (FMS)
For better solubility, first dissolve salicylic acid (5.4) in methanol (5.2), after which add formamide (5.3). In the
FMS mixture, the ratio of formamide to methanol shall be 1 + 2 (parts by volume).
EXAMPLE 1 A mass of 60 g salicylic acid (5.4) is dissolved in 200 ml methanol (5.2) in a stoppered cylindrical flask
under continuous stirring. A volume of 100 ml formamide (5.3) is subsequently added, the flask is then closed and the
FMS completely homogenized by further stirring.
Direct preparation of the mixture in the titration vessel is also feasible.
EXAMPLE 2 A volume of 10 ml formamide (5.3) is placed in the titration vessel, then mixed with 20 ml methanol (5.2)
in which 6 g salicylic acid (5.4) have been previously diss
...