Overview
ISO 1138:2022 specifies two validated laboratory methods for the determination of total sulfur in all types of carbon black used as rubber compounding ingredients. The standard describes:
- Method A - a classical chemical procedure using a combustion furnace with oxygen, collection of combustion products in acid and titration; and
- Method B - an automatic analyser method (preferred for safety and precision).
The document updates and replaces earlier editions, withdraws hazardous oxygen-bomb methods, and aligns Method B with ISO 15671 for automatic analyser procedures.
Key Topics and Requirements
- Scope: Total sulfur determination in carbon black for the rubber industry.
- Preferred method: Method B (automatic analyser) is recommended because of improved safety and precision.
- Combustion conditions (Method A): Sample is dried (105 °C), combusted at ~1 425 °C in an oxygen stream (approx. 1 L/min), and combustion gases are absorbed and titrated.
- Reagents and apparatus (Method A): Use analytical-grade reagents (acid absorber, iodate/iodide titrant, starch indicator), combustion furnace, combustion tube/boats, oxygen supply with purification, and titration apparatus.
- Result expression: Total sulfur S (%) is calculated from titrant volume and sample mass (S = V / (20 · m), where V = mL of titrant, m = grams of sample).
- Quality and safety: The standard emphasizes reagent purity, equipment conditioning, system purging, and documentation of results.
- Traceability: Test report must reference ISO 1138:2022, identify the sample, and present the results.
Applications and Users
ISO 1138:2022 is intended for:
- Quality control and analytical laboratories in the rubber and tyre industry
- Raw material suppliers and carbon black manufacturers validating sulfur content specifications
- Contract testing labs performing regulatory or customer-required compositional testing
- R&D and formulation teams assessing effects of sulfur on vulcanization and compound performance
Practical benefits include consistent sulfur measurement for material specification, process control, and compliance with customer or regulatory requirements. Laboratories without automatic analysers can use Method A as an acceptable alternative.
Related Standards
- ISO 15671 - referenced for detailed requirements and options for automatic analyser methods and detection (including thermal conductivity or other detectors).
- The standard also replaces and updates earlier versions of ISO 1138.
By following ISO 1138:2022, organizations ensure reproducible, safe, and industry‑accepted measurement of sulfur in carbon black - a key parameter affecting rubber compounding and vulcanization.
