ISO 25121:2026

• Abstract:

  This document specifies a ultraviolet fluorescence method for the determination of trace sulfur in light olefins. This document is applicable to the determination of sulfur in ethylene, propylene, butadiene and other C4 olefins. The detection range for sulfur is from 0,2 mg/kg to 100 mg/kg.

• Status: Published
• Publication Date: 20-Oct-2026
• ICS: 71.040.40 - Chemical analysis
• ICS: 71.080.10 - Aliphatic hydrocarbons
• Technical Committee: ISO/TC 47 - Chemistry
• Drafting Committee: ISO/TC 47/WG 2 - Light Olefins
• Current Stage: 6060 - International Standard published
• Start Date: 21-Oct-2026
• Completion Date: 21-Oct-2026

ISO 25121:2026 is an International Standard from ISO for determining trace sulfur in light olefins by ultraviolet fluorescence. It applies to ethylene, propylene, butadiene and other C4 olefins, and it specifies a measurement range of 0.2 mg/kg to 100 mg/kg.

It is intended for laboratories and production teams that need a consistent way to measure sulfur in light olefin materials. The method is built around combustion of the sample, conversion of sulfur to sulfur dioxide, and fluorescence detection.

What does ISO 25121:2026 specify?

ISO 25121:2026 specifies a test method for trace sulfur in light olefins and gives the full workflow from sampling to reporting. The document is organized into clauses on scope, normative references, terms and definitions, principle, reagents and materials, apparatus, sampling, procedure, calculation, reporting, precision and test report.

Clause 4 explains the measurement principle: the sample is combusted in an oxygen-rich tube, sulfur is converted mainly to sulfur dioxide, water is removed, and the emitted fluorescence is measured. In practice, this means the laboratory needs equipment that can burn the vaporized sample cleanly and detect a very small sulfur signal reliably.

What are the key requirements of ISO 25121:2026?

The main requirements cover sampling, reagent purity, instrument setup, calibration, analysis, calculation and reporting. Together, they define how to obtain a sulfur result that can be compared between runs and between laboratories.

Sampling and sample handling

Clause 7 requires sampling in accordance with ISO 7382 for ethylene and ISO 8563 for propylene and butadiene. It also says samples should be kept as cold as practical, uncovered only briefly, and analysed as soon as possible. This matters because sulfur can be lost or the sample can be contaminated during handling.

Reagents, standards and apparatus

Clause 5 sets the chemicals and gases needed for calibration and analysis, including sulfur compounds for stock solutions, a suitable diluent, inert gas and oxygen. Clause 6 describes the ultraviolet fluorescence analyzer, the syringe for calibration standards, the inlet system for vaporized samples, the vaporization apparatus, the balance and the flask.

The apparatus has to support complete combustion, controlled gas flow, water removal and fluorescence detection. In practice, the laboratory must verify that the instrument can deliver samples reproducibly and that the detector response is suitable for low-level sulfur measurement.

Calibration and sample analysis

Clause 8 requires preparation of calibration standards across the expected sulfur range and daily verification of system performance. It also requires repeated measurements of standards and samples, with the sample injected only after the vaporized sample loop is fully flushed and pressure has stabilized.

This means the method depends on a stable calibration curve and careful syringe and inlet handling. The document also notes that the user should check for possible nitrogen interference in the detector response.

Calculation, reporting and precision

Clause 9 converts the measured signal into sulfur content by using the sulfur mass from the calibration curve together with sample temperature, pressure, volume and molecular mass. Clause 10 requires the average of two measurements to be reported to the nearest 0.1 mg/kg.

Clause 11 gives precision statements for repeatability and reproducibility. For users, this provides a way to judge whether two results are close enough to be considered consistent within the method.

Who uses ISO 25121:2026?

ISO 25121:2026 is used by testing laboratories, plant quality control teams, producers and suppliers of light olefins, and technical staff involved in batch release or incoming inspection. It is relevant wherever sulfur in ethylene, propylene, butadiene or other C4 olefins needs to be checked against internal specifications or trading requirements.

Which standards are used with ISO 25121:2026?

• ISO 7382 - gives the sampling method for ethylene in the liquid and gaseous phase.
• ISO 8563 - gives the sampling method for propylene and butadiene in the liquid phase.
• ISO 17034 - is cited in the bibliography in connection with certified reference materials suitable for preparing calibration standards.

What does the ISO 25121:2026 document contain?

The document contains the combustion-based principle, reagent and apparatus requirements, sampling instructions, procedural steps for calibration and analysis, calculation equations, reporting instructions, precision statements and the test report content. It also includes a calibration table for selecting a suitable curve and a precision table for repeatability and reproducibility.

For practical use, the most important working parts are the apparatus setup, calibration, vaporization of liquid samples and the final conversion of detector response into sulfur content. The test report clause shows what information has to be recorded so the result can be traced back to the sample and the test conditions.

International
Standard
ISO 25121
First edition
Light olefins for industrial use —
2026-10
Determination of trace sulfur —
Ultraviolet fluorescence method
Reference number
© ISO 2026
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Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 1
5 Reagents and Materials . 1
6 Apparatus . 2
6.1 Ultraviolet fluorescence analyzer .2
6.1.1 Pyrolysis Furnace .2
6.1.2 Combustion tube .2
6.1.3 Flow controller .2
6.1.4 Vapour drier .2
6.1.5 Ultraviolet fluorescence (UV) detector.3
6.2 Sampling Syringe for calibration standards .3
6.3 Inlet system for vaporized light olefin sample .3
6.4 Vaporization apparatus .3
6.5 Analytical Balance . .3
6.6 Flask .3
7 Sampling . 3
8 Procedure . 3
8.1 Preparation of Apparatus .3
8.2 Calibration .4
8.3 Vaporization of liquid light olefin sample .4
8.4 Vaporized light olefin sample analysis .4
9 Calculation . 4
10 Reporting . 5
11 Precision . 5
11.1 Repeatability (r) .5
11.2 Reproducibility(R) .5
12 Test report . 5
Bibliography . 7

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Foreword
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International Standard ISO 25121:2026(en)
Light olefins for industrial use — Determination of trace
sulfur — Ultraviolet fluorescence method
1 Scope
This document specifies a ultraviolet fluorescence method for the determination of trace sulfur in light
olefins.
This document is applicable to the determination of sulfur in ethylene, propylene, butadiene and other C4
olefins. The detection range for sulfur is from 0,2 mg/kg to 100 mg/kg.
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 7382, Ethylene for industrial use — Sampling in the liquid and the gaseous phase
ISO 8563, Propylene and butadiene for industrial use — Sampling in the liquid phase
3 Terms and definitions
No terms and definitions are list
...