ISO 23649:2025

International
Standard
ISO
IULTCS
IUC 444
First edition
Chemicals for the leather tanning
2025-02
industry — Determination of
cyclosiloxanes
Produits chimiques pour l’industrie du tannage du cuir —
Détermination des cyclosiloxanes
Reference numbers
IULTCS/IUC 444:2025(en) © ISO 2025

IULTCS/IUC 444:2025(en)
© ISO 2025
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Published in Switzerland
ii
IULTCS/IUC 444:2025(en)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 1
5 Apparatus . 1
6 Reagents . 2
7 Sampling and sample preparation . 3
8 Procedure . 3
8.1 Sample solution preparation .3
8.2 Liquid-liquid extraction.3
8.3 Instrumental analysis .4
9 Expression of results . 4
10 Precision . 4
11 Test report . 4
Annex A (informative) Chromatographic analysis operating parameters for GC-MS . 6
Annex B (informative) Precision: reliability of the method . 7
Annex C (normative) Sample preparation for non-water-soluble chemicals . 8
Bibliography . 9

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IULTCS/IUC 444:2025(en)
Foreword
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IULTCS, originally formed in 1897, is a world-wide organization of professional leather societies to further
the advancement of leather science and technology. IULTCS has three Commissions, which are responsible
for establishing international methods for the sampling and testing of leather. ISO recognizes IULTCS as an
international standardizing body for the preparation of test methods for leather.
This document was prepared by the Chemical Test Commission of the International Union of Leather
Technologists and Chemists Societies (IUC Commission, IULTCS), in collaboration with the European
Committee for Standardization (CEN) Technical Committee CEN/TC 289, Leather, in accordance with the
Agreement on technical cooperation between ISO and CEN (Vienna Agreement).
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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IULTCS/IUC 444:2025(en)
Introduction
This document includes a procedure for analysing certain cyclosiloxanes using gas chromatography (GC)
with mass spectrometer (MS) equipment. With this analytical method octamethylcyclotetrasiloxane (D4),
decamethylcyclopentasiloxane (D5) and dodecamethylcyclohexasiloxane (D6) can be determined.
In the leather industry cyclosiloxanes (D4), (D5) and (D6) are used in the manufacture of silicone-based
waterproofing fatliquors and can also be used in the production of silicone-based finishing chemicals
especially as handle modifiers.
At present, the official classification recognised in the EU is the following.
[1]
— Octamethylcyclotetrasiloxane (D4) is classified as persistent, bioaccumulative and toxic.
[2]
— Decamethylcyclopentasiloxane (D5) is classified as persistent, bioaccumulative and toxic.
[3]
— Dodecamethylcyclohexasiloxane (D6) is classified as persistent, bioaccumulative and toxic.

v
International Standard
IULTCS/IUC 444:2025(en)
Chemicals for the leather tanning industry — Determination
of cyclosiloxanes
1 Scope
This document specifies a method for determining the total content of the following cyclosiloxanes in
chemicals for the leather tanning industry:
— octamethylcyclotetrasiloxane (D4);
— decamethylcyclopentasiloxane (D5);
— dodecamethylcyclohexasiloxane (D6);
This method requires the use of gas chromatography (GC) equipped with a single quadrupole mass
spectrometer (MS) to identify and quantify the cyclosiloxanes.
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
3 Terms and definitions
No terms and definitions are listed in this document.
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/
4 Principle
After solubilizing the chemical specimen in water, a liquid-liquid extraction with a hexane-ethyl acetate
mixture is performed and then the cyclosiloxanes are analysed by gas-chromatography with single
quadrupole mass spectrometry (GC-MS). Using an external calibration, quantification is performed taking
into account the recovery rate of the internal standard.
5 Apparatus
The usual laboratory apparatus and, in particular, the following shall be used:
5.1 Ultrasonic bath, capable of operating at a starting temperature of 40 °C to 45 °C.
5.2 Horizontal shaker, capable of at least 300 cycles per minute.
5.3 Glass container with a screw cap, e.g. volume of 20 ml.

IULTCS/IUC 444:2025(en)
5.4 Suitable syringe membrane filters, e.g. polyamide with pore size 0,2 µm.
5.5 Volumetric flasks, e.g. volume of 10 ml and 100 ml.
5.6 Plastic test tubes, 50 ml, or suitable vessel that allows separation of organic and aqueous phase, that
can be sealed for vigorous shaking.
5.7 GC vials, with cap, e.g. volume of 2 ml.
5.8 Analytical balance, with a resolution of 1 mg or better.
5.9 Pipettes, various sizes, e.g. volume of 0,1 ml to 10 ml.
5.10 Centrifuge, capable of at least 4 000 r/min.
5.11 Instrumental equipment, GC-MS.
6 Reagents
If not otherwise specified, analytical reagent grade chemicals shall be used.
1)
6.1 n-hexane, CAS Registry Number® (CAS RN) 110-54-3, suitable for GC-MS analysis.
6.2 Ethyl acetate, CAS RN 141-78-6, suitable for GC-MS analysis.
6.3 Water grade 1, according to ISO 3696.
6.4 Sodium chloride, CAS RN 7647-14-5, purity ≥ 99,0 % (mass fraction).
6.5 Magnesium sulphate anhydrous, CAS RN 7487-88-9, purity ≥ 99,0 % (mass fraction).
6.6 Octamethylcyclotetrasiloxane (D4), CAS RN 556-67-2, purity ≥ 99,0 % (mass fraction).
6.7 Decamethylcyclopentasiloxane (D5), CAS RN 541-02-6, purity ≥ 99,0 % (mass fraction).
6.8 Dodecamethylcyclohexasiloxane (D6), CAS RN 540-97-6, purity ≥ 99,0 % (mass fraction).
6.9 n-hexane-ethyl acetate 50:50 (volume fraction) mixture, prepared by mixing equal amounts by
volume of n-hexane and ethyl acetate.
6.10 Stock solutions of a mix of cyclosiloxanes D4, D5 and D6, ρ = 1 mg/l and 20 mg/l.
EXAMPLE 10 mg of each of the respective octamethylcyclotetrasiloxane (D4) (6.6), decamethylcyclopentasiloxane
(D5) (6.7), and dodecamethylcyclohexasiloxane (D6) (6.8), are dissolved together in a 100 ml volumetric flask (5.5)
and filled up with mixed solution of n-hexane and ethyl acetate (6.9). Mixed stock solutions are prepared to obtain
concentrations of 1 mg/l and 20 mg/l.
6.11 Internal standard, ρ = 100 mg/l.
The use of an internal standard is required to avoid matrix effects. ®
1) Chemical Abstracts Service (CAS) Registry Number is a trademark of the American Chemical Society (ACS). This
information is given for the convenience of users of this document and does not constitute an endorsement by ISO of the
product named. Equivalent products may be used if they can be shown to lead to the same results.

IULTCS/IUC 444:2025(en)
Examples of suitable internal standards: ®
Naphthalene-D8, CAS RN 1146-65-2; ®
Tetrakis(trimethylsilyloxy)silane, CAS RN 3555-47-3.
Prepare a 100 mg/l solution of the internal standard by diluting the internal standard with n-hexane-ethyl
acetate 50:50 (volume fraction) mixture (6.9).
6.12 Calibration solutions of cyclosiloxanes.
Prepare at least four calibration solutions of ρ = 0,1 mg/l to ρ = 10 mg/l of the mix of cyclosiloxanes using the
stock solutions (6.10), see Table 1.
Table 1
...