ISO 1138 Test: Carbon Black Sulfur Content | UnitedTest Total Sulphur Test Machine

Information on the most widely used ASTM standards within the materials testing industry

Descriptions of the most prominent lSO standards within the materials testing industry

Standards including EN, DIN, JlS, and more

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ISO 1138 Carbon Black Sulfur Content Tester | Rubber Compounding Testing | UnitedTest

ISO 1138 specifies two test methods to determine total sulfur mass fraction in all rubber-grade carbon black, including 1425°C combustion iodometric titration and ISO 15671 automatic analysis. UnitedTest manufactures precision ISO 1138 compliant sulfur testing machines for rubber compounding ingredient quality control.

ISO 1138 is an international standard dedicated to rubber compounding ingredients, specifically regulating the determination of sulfur content in rubber-grade carbon black materials. The standard establishes two authoritative testing approaches for measuring total sulfur mass fraction across all carbon black grades used in rubber formulation and industrial compounding production, ensuring consistent material purity and stable rubber product performance.

ISO 1138 includes two distinct test procedures: Method A refers to the classical combustion furnace testing technique that operates at a high temperature of 1425 °C paired with iodometric titration for manual sulfur quantification. Method B is the advanced automatic analyzer method implemented in full accordance with the ISO 15671 standard protocol. Due to its higher testing precision, stable repeatability and improved operational safety, Method B is defined as the preferred industrial testing method, while Method A remains a valid alternative for laboratories without automatic analytical equipment.

Accurate sulfur content detection for carbon black is critical to avoid formula deviation, vulcanization instability and performance defects in finished rubber products. UnitedTest designs and manufactures professional ISO 1138 sulfur testing instruments supporting both traditional combustion titration and automatic analysis workflows. Our testing equipment delivers reliable, repeatable total sulfur detection results, fully meeting quality inspection, formulation verification and third-party certification requirements for rubber compounding factories and polymer laboratories.

Test Principle

Method A: A weighed portion of dried carbon black is heated at 1425 °C in a stream of oxygen; the sulfur is oxidised to sulfur oxides/SO₂, swept out of the tube and absorbed in hydrochloric acid solution in a titrating flask. The absorbed sulfur species are titrated with standard volumetric potassium iodate–potassium iodide solution using starch as indicator; sulfur % is calculated from titrant volume.

Method B: The sample is combusted in an automatic analyser (oxygen stream) and all sulfur is converted to SO₂, which is then quantified — by hydrogen‑peroxide absorption + acid–base titration, by iodine‑solution absorption + dual platinum electrode, or by direct SO₂ measurement (IR absorption / thermal conductivity detector) calibrated with sulfur standards.

Two Test Methods

Item	Method A	Method B
Name	Combustion furnace (classical wet‑chemical)	Automatic analyser
Status	Acceptable only if automatic equipment is unavailable	Preferred method (better safety + precision)
Normative base	Fully specified in ISO 1138:2022, Clause 4	Deferred entirely to ISO 15671; three SO₂ detection options + optional thermal‑conductivity detector (per manufacturer)

Both methods determine total sulfur (all forms) in all types of carbon black for the rubber industry, expressed as % by mass.

Test Specimen Requirements

Material: all types of rubber‑grade carbon black (furnace, thermal, acetylene, etc.)

Pre‑treatment: dry an adequate amount 1 h at 105 °C ± 2 °C in the oven

Test portion: about 1 g, weighed to the nearest 1 mg, transferred to a combustion boat

(For Method B, sample preparation follows ISO 15671 and the instrument manufacturer's instructions; ISO 15671:2023 also specifies sampling/preparation rules for rubber and latex.)

Comparison note: ASTM D1619 dries at 125 °C ± 5 °C for 1 h and uses ~0.5 g portions — specimen conditioning differs between standards, so results must be reported with the method used.

Equipment required for ISO 1138 carbon black sulfur content test

Method A (Combustion‑furnace method)

Recommend the UnitedTest UT2113 High-frequency infrared carbon-sulfur analyzer

- Combustion furnace: capable of stable operation at 1425 °C ± 25 °C

- Combustion tube: approx. 75 cm length with tapered end

- Combustion boats: fused alumina (alundum) or porcelain; new boats must be pre‑fired 1 h at 1425 °C ± 25 °C before use

- Pressure regulator and oxygen flow‑meter

- Oxygen‑purification train: gas‑washing bottle (concentrated sulfuric‑acid), absorption bottle filled with soda‑asbestos‑CO₂‑saturated anhydrous‑calcium‑sulfate mixture

- Coarse‑fritted‑glass gas‑dispersion tube

- 500 mL titrating flasks

- Rubber stopper fitted with high‑temperature metal/refractory baffle

- Gravity‑convection laboratory oven, controlled to 105 °C ± 2 °C

For Method B

Commercial automatic total‑sulfur analyser for rubber additives, operated per ISO 15671 and instrument‑manufacturer manuals

Key Test Parameters & Stipulations

Parameter	Value
Combustion temperature	1425 °C ± 25 °C
Oxygen flow rate	1 L/min during combustion
System purge before a series	30 min with O₂ flowing, furnace at temperature
Drying	105 °C ± 2 °C, 1 h
Test portion	~1 g (± 1 mg)
Titrant	KIO₃–KI standard volumetric solution (1.1125 g KIO₃ + 100 g KI + 5 g KOH per litre)
Indicator	Starch solution (2.5 g starch + 5 mg HgI₂ per litre)
Absorbing acid	HCl: 2 volumes conc. HCl (ρ₂₀ 1.19) + 13 volumes water
End point	Blue colour persists without further titrant addition after 3 min of bubbling
Result formula	S = V / (20·m) — S in %, V = mL titrant, m = g sample

Details Test Procedures of ISO 1138 carbon black sulfur content test

For Method A:

1. Dry carbon‑black sample in oven for 1 h at 105 °C ± 2 °C; weigh about 1 g dried sample to 1 mg accuracy and transfer into combustion boat.

2. Prepare titration flask: fill one‑third volume with hydrochloric‑acid absorbing reagent; add starch indicator; add small amount of standard iodate‑iodide titrant while stirring until faint blue colour appears; fit coarse‑fritted gas‑dispersion tube into flask.

3. Pre‑purge combustion system 30 min with oxygen flow 1 L/min at furnace temperature 1425 °C ± 25 °C before each measurement batch.

4. Insert sample‑loaded combustion boat into hot zone of combustion tube, seal tube; combustion gas flows through absorbing liquid. Continuously titrate with standard iodate‑iodide solution to sustain blue colour inside flask.

5. Reach titration end‑point when blue colour persists for 3 min of gas bubbling without adding titrant; record consumed titrant‑volume V.

6. Compute mass‑percentage total‑sulfur content by given formula.

7. Document results into formal test‑report complying with standard requirements.

For Method B: perform sample measurement following the complete procedural specifications of normative reference ISO 15671, operating the automatic analyser according to manufacturer‑operating‑manual guidance

Industrial Application Fields

Carbon black manufacturers – batch release and specification compliance for sulfur content

Tyre and rubber product makers – incoming raw‑material QC; carbon black is a major compounding ingredient, so its intrinsic sulfur shifts the compound's total sulfur balance

Rubber compounders / R&D – formulation control and reconstruction of a rubber composition from analytical data

Environmental monitoring – sulfur material balance around a production process and emissions reporting

Contract / third‑party testing labs and regulatory or customer‑required compositional testing

Related standard:

ISO 1138	Rubber compounding ingredients — Carbon black — Determination of sulfur content
GB/T 3780.14	Carbon black—Part 14：Determination of sulfur content
ISO 1125	Rubber compounding ingredients — Carbon black — Determination of ash
ISO 15671	Rubber and rubber additives — Determination of total sulfur content using an automatic analyser. It defines instrumental‑analysis rules for ISO 1138 Method B
ASTM D1619	US functional equivalent: Method A = high‑temperature combustion (≥1350 °C) with IR absorption detection; Method B = X‑ray fluorescence; reports repeatability r ≈ 0.034 % / reproducibility R ≈ 0.19 % (Method A)

Why sulfur content matters for carbon black

Vulcanization chemistry. Carbon black is not inert toward curing: its sulfur (plus surface‑bound sulfur species) adds to the compound's total sulfur pool and can alter cure rate, crosslink density, scorch safety and state of cure. Without knowing the filler's sulfur, a compounder cannot reproduce a cure system consistently between carbon‑black lots or suppliers.

Specification and batch‑to‑batch control. Sulfur level is a contractual specification item; measuring it is how suppliers certify and how customers accept or reject a lot.

Formulation reconstruction and process balance. Total sulfur data are needed to back‑calculate a rubber compound's composition from analytical data and to close a sulfur mass balance for a production line.

Environmental and corrosion impact. Sulfur in the filler and process streams becomes SO₂/SO₃ on combustion — relevant for emissions reporting, environmental compliance, and equipment corrosion.

Product consistency and safety of the test itself. Because total sulfur is a quality‑critical, low‑level figure, precision matters — which is why ISO now designates the closed, automated Method B as preferred, and why the older oxygen‑bomb method (hazardous chemicals) was withdrawn. Even Method A's starch indicator contains mercury(II) iodide, an additional reason to migrate to automatic analysis.

Practical takeaway: report results as "total sulfur, % by mass — ISO 1138:2022, Method A or Method B", and where possible use Method B with calibration traceable to certified sulfur standards per ISO 15671, since ISO 1138:2022 publishes no own precision data.

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UnitedTest manufactures ISO 1138 compliant high-frequency infrared carbon sulfur analyzers with third-generation high-frequency furnace and all-in-one dust removal system, delivering stable, high-precision carbon and sulfur elemental analysis for industrial and laboratory material testing.

UnitedTest manufactures high-end coulometric fully automatic sulfur analyzers compliant with ISO 1138 and GB/T214-2007 standards, featuring unattended multi-sample testing and upgraded dual electrolytic cell design for stable total sulfur detection in coal, ore and industrial materials.

Electricity hot blast drying oven is mainly used to measure the dimension change of plastic material after heating, suitable for plastic pipe, plastic sheet etc. ,

Q1: What is ISO 1138:2022 for?

A1: ISO 1138:2022 is an international standard to measure total sulfur mass‑percentage in rubber‑grade carbon black. Two test routes are defined: Method A (combustion‑furnace manual titration) and Method B (automatic analyser). Method B is the preferred test method for safety and repeatability; Method A is fallback for labs without automated instruments.

Q2: Why is total‑sulfur testing critical for carbon‑black raw‑material?

A2: Residual sulfur in carbon black interferes rubber vulcanisation behaviour. Uncontrolled sulfur shifts scorch time, cross‑link density, tensile strength, abrasion and tear resistance for tyres, hoses and rubber seals. It also supports batch‑to‑batch quality consistency and environmental risk assessment for SO₂ emissions during high‑temperature processing or incineration.

Q3: What is the difference between Method A and Method B of ISO 1138?

A3:

Method A (Combustion furnace): Manual wet‑chemistry test. Sample burns at 1425 °C under oxygen flow; sulfur‑containing gas is absorbed in HCl solution and titrated manually. Low capital cost but labour‑intensive, uses toxic reagents, higher human error risk.
Method B (Automatic analyser): Preferred instrumental method following ISO 15671. Sample high‑temperature combustion; SO₂ gas is quantified by built‑in detectors (IR or thermal‑conductivity detector). Fast testing, less chemical handling, better precision and repeatability for routine QC production labs.

Q4: What sample preparation rules must I follow for ISO 1138 test?

A4: Dry carbon‑black sample for 1 hour at 105 °C ±2 °C to remove surface moisture. Weigh approximately 1 g dried specimen to ±1 mg accuracy. New combustion boats for Method A must be pre‑fired for 1 h at 1425 °C ±25 °C before first use to eliminate background sulfur contamination.

Q5: Can I use XRF instrument for ISO 1138 compliance test?

A5: No. ISO 1138:2022 only recognises combustion‑furnace titration (Method A) and automatic combustion analyser (Method B). X‑ray fluorescence is not included in this ISO document, though some national standards (like Chinese GB/T 3780.14) add XRF as an optional alternative screening method.

Q6: What common error sources affect ISO 1138 sulfur‑test accuracy?

A6:

Sulfur impurity in oxygen gas supply → must use sulfur‑free oxygen cylinder gas.
Un‑pre‑fired new combustion boats → bring background sulfur bias.
Expired potassium‑iodate‑iodide titrant (recommended fresh preparation every 30 days for Method A).
Insufficient pre‑purge of combustion pipeline system before sample runs.
Ignoring sample‑drying step, residual surface moisture disturbs weighing and combustion efficiency.

Q7. Is ISO 1138 the same as ASTM D1619?

A7: No. ASTM D1619 is the US counterpart: Method A = combustion (≥1350 °C) with IR absorption, Method B = X-ray fluorescence; it dries at 125 °C ± 5 °C with ~0.5 g portions and publishes precision (r ≈ 0.034 %, R ≈ 0.19 % for Method A). ISO 1138 Method A uses titration at 1425 °C with ~1 g portions. GB/T 3780.14 is the Chinese national equivalent (MOD of ISO 1138).

Q8: ISO 1138 test methods at a glance?

A8:

Item	Method A – Combustion furnace	Method B – Automatic analyser
Status	Acceptable if no automatic equipment	Preferred (safety + precision)
Principle	Combustion at 1425 °C in O₂ → absorption in HCl → KIO₃/KI titration	Combustion → SO₂ → H₂O₂ titration / iodine-electrode / IR or TCD detection (ISO 15671)
Detection	Starch–iodine visual end point	Instrumental, automatic
Typical use	Backup / low-budget labs	Production QC, high-throughput labs

Q9: Equipment needed for ISO 1138 testing?

A9: Combustion furnace rated to 1425 °C ± 25 °C with stable hot-zone control

Combustion tube, approx. 75 cm, one end tapered

Combustion boats in fused alumina or porcelain — new boats fired 1 h at 1425 °C before use

Pressure regulator + flowmeter and an oxygen purification train (H₂SO₄ gas-washing bottle + soda asbestos / anhydrous CaSO₄ drying bottle saturated with CO₂)

Coarse-fritted gas-dispersion tube and 500 ml titrating flasks

Rubber stopper with metal or refractory heat reflector/baffle

Drying oven, 105 °C ± 2 °C

For Method B: an automatic sulfur analyser with oxygen supply, calibration standards and the detector specified for your configuration

Q10: Why was the oxygen bomb method removed?

A10: The 2022 (third) edition withdrew the oxygen bomb calorimeter method — formerly Method A — and Amendment 1:2012, because the procedure involved hazardous chemicals. In the same revision, CAS numbers were added to the chemical reagents of the current Method A.

Q11: Why choose ISO 1138 Carbon‑Black Total Sulfur Analyser from UnitedTest?

A11: Beijing United Test Co., Ltd. (UnitedTest) designs and manufactures material testing machines to ASTM, ISO, EN and BS requirements, exported to more than 80 countries. Our carbon black and rubber laboratory range includes high-temperature combustion furnaces and automatic sulfur analysers configured for ISO 1138 / ISO 15671 workflows, plus carbon black content and dispersion testers.

Tell us your standard, sample type and throughput requirement — we will configure the furnace, gas train, detection and calibration package, and provide IQ/OQ documentation and operator training.

UnitedTest supplies high‑performance automatic total‑sulfur analysers fully aligned with **ISO 1138:2022 Method B**, the preferred instrumental test method for measuring total sulfur content in rubber‑compounding carbon‑black raw materials. Our test instruments follow the requirements of referenced standard **ISO 15671**, supporting SO₂ detection and optional thermal‑conductivity detector configuration for carbon‑black quality‑control labs.

For carbon‑black plants, tyre factories, rubber‑product manufacturers and third‑party testing laboratories, accurate sulfur measurement is essential to stabilise rubber vulcanisation performance and guarantee finished‑product mechanical properties. Compared with labour‑intensive manual combustion‑furnace titration Method A, UnitedTest automatic ISO 1138 sulfur analyser minimises manual chemical handling, cuts human error, improves testing repeatability and laboratory safety for high‑volume batch testing.

Our machines support complete sample‑combustion workflow, system auto‑purge functions, data logging and test‑report output that meets ISO 1138:2022 report‑issuing requirements. Whether for incoming‑material inspection, production‑line quality‑monitoring or R&D formula research for rubber‑grade carbon black, UnitedTest provides reliable testing solutions matching international carbon‑black‑testing standards.

We offer customization to meet your specific needs. Our expert team will collaborate with you to develop the perfect product for you