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)

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9020 - International Standard under periodical review
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ISO 20938:2008 - Instant coffee -- Determination of moisture content -- Karl Fischer method (Reference method)
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First edition
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 20938:2008(E)
ISO 2008
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ISO 20938:2008(E)
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© ISO 2008

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ii © ISO 2008 – All rights reserved
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ISO 20938:2008(E)

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

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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.

© ISO 2008 – All rights reserved iii
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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.
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).
© ISO 2008 – All rights reserved 1
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ISO 20938:2008(E)
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.

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
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ISO 20938:2008(E)

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


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

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