Why so many customers choose Coulomb Sulfur Analyzer? A Practical Selection Guide between Coulomb and Infrared Sulfur Analyzer for Coal and Coke Laboratories

Every coal export laboratory, third-party inspection body and power-plant quality team eventually faces the same question: should total sulfur be determined by Coulomb titration or by high-temperature combustion with infrared (IR) detection?

Both techniques are mature, covered by international standards, and used successfully around the world. The right answer depends on your sample mix, throughput, reporting requirements and operating budget — not on which method is “newer”. This guide sets out how each technique works, where each one performs best, and which Sundy analyzer best matches the requirements of your laboratory.

1. How the two techniques differ

Both methods share the same first step: a weighed test portion is burned in a stream of oxygen at high temperature, converting the sulfur in the sample into gaseous sulfur dioxide (SO₂). They differ in how that SO₂ is quantified.

1.1 Coulomb titration

The combustion gases are absorbed in an electrolytic cell containing potassium iodide/potassium bromide electrolyte. The SO₂ reacts with iodine generated electrochemically; the instrument measures the electric charge (current integrated over time) required to maintain the titration endpoint. By Faraday’s law, this charge is directly proportional to the mass of sulfur.

In practice this means the measurement is absolute and inherently linear across the working range: low-, medium- and high-sulfur samples are determined on the same setting, with the same sample weight and the same integration logic.

Sundy SDS720 Coulomb Sulfur Analyzer Schematic Diagram

1.2 Infrared detection

The combustion gases pass through a cell where SO₂ absorbs infrared energy at a specific wavelength. The reduction in transmitted energy is proportional to the SO₂ concentration, and sulfur is calculated from the sample weight and a calibration established with certified reference materials.

IR sulfur analyzers are fully automated: Auto loader(high level type IR sulfur analyzer), automatic crucible identification, automatic remind filter-wool replacement and low-drift cells have made them the workhorse of high-volume commercial laboratories, where sample throughput and unattended operation dominate the economics.

Sundy SDIS450 Infrared Sulfur Analyzer Schematic Diagram

2. The standards landscape

Coulomb and IR sulfur determination are separate, equally valid standardized methods. Which one a laboratory adopts often follows from its market: coal traded in Africa、Asia and Europe is frequently specified to ISO, while North American contracts more often call for ASTM procedures.

Sundy’s Coulomb Sulfur Analyzers— SDS720 and SDS-V — are designed to conform to ISO 20336:2025 and GB/T 214-2007. Sundy’s Infrared Sulfur Analyzers — SDIS401, SDIS450 and SDICS450 — are designed to conform to ISO 19579, ASTM D4239, ASTM D5016 and GB/T 25214, so that a single laboratory can satisfy either contract specification or both.

3. Sample preparation and test procedure(Taking coulomb sulfur Analyzer as an example)

Whichever technique is used, sulfur results are only as good as the sample presented to the instrument. For the determination of total sulfur in coal or coke, the general analysis test sample is ground to pass a 212 µm sieve; for materials containing more than 5 % sulfur, a 100 µm grind is used. The sample is spread in a thin layer and allowed to reach approximate moisture equilibrium with the laboratory atmosphere, then thoroughly mixed before the determination.

Where results are to be reported on a basis other than air-dried, the moisture content of a further portion of the same test sample is determined in accordance with ISO 11722 (hard coal), ISO 687 (coke) or ISO 5068-2 (brown coal and lignite).

A typical automated Coulomb determination — shown here for the SDS720 — runs as follows:

1. Sample logging. Sample ID and weight are entered in the instrument software; the weight is transferred automatically, so no manual transcription is required.

2. Weighing and acceleration. A test portion of 0.05 g ± 0.005 g is weighed to the nearest 0.0001 g into a combustion boat and covered with a thin layer of tungsten trioxide (WO₃) as a combustion accelerator. For samples containing more than 5 % sulfur, a test portion of 0.03 g ± 0.005 g is appropriate.

3. Combustion and titration. The boat is placed on the quartz tray and the run is started from the software. The software controls every subsequent step: automatic pushing of the tray into the combustion tube, the residence time at the preliminary decomposition zone (approx. 500 °C) and at the high-temperature zone (1150 ± 10 °C), and the Coulomb evaluation. Residence time at the preliminary decomposition zone is set according to the combustion behaviour and sulfur content of the sample, normally 60-180 s.

4. Data output. Results are available as a printed or electronic report and can be exported to a

4. Typical performance: repeatability on real coal samples

The table below summarizes a repeatability study on seven coal samples covering a total sulfur range of 0.24 % to 4.43 % (dry basis). Each sample was determined three times.

Every result fell within the repeatability limit specified in ISO 20336:2025, Clause 10.1. Across the full range — from 0.24 % to 4.43 % total sulfur — repeatability was maintained without changing the analytical setting: one setting, one sample weight, one integration logic. This is precisely the behaviour that makes the Coulomb technique attractive for coal export, international trade and third-party testing work.

5. Cost of ownership: what actually drives the numbers

The dominant cost is rarely the reagent. In high-volume laboratories it is labor and turnaround time, which favor automation; in smaller or more specialized laboratories it is capital cost and simplicity, which favor the Coulomb approach.

6. Matching the technique to your laboratory

7. Sundy’s sulfur analyzer portfolio

Sundy manufactures both technology lines, so the recommendation you receive from us is based on your laboratory profile rather than on a single available product.

8. Selected reference installations

Coulomb sulfur analyzers from Sundy are in routine service with commercial laboratories, inspection bodies, power generators, cement producers and metallurgical plants across Asia and Africa, including:

9. Summary: no “better” method — only a better fit

Coulomb titration (SDS720, SDS-V) delivers absolute, charge-based quantification in a compact and economical package. Because the result rests on Faraday’s law rather than on a calibration curve, it is the natural choice for coal and coke routine testing, for low- and mid-sulfur work, and for laboratories that value traceability without recalibration.

Infrared detection (SDIS401, SDIS450, SDICS450) delivers automation and speed: under two minutes per determination, 50-sample loading, unattended operation and a broad application range. It is the natural choice for high-volume commercial, inspection and multi-matrix laboratories.

Because Sundy builds both lines, we can compare them against your sample mix, throughput and reporting requirements — and support whichever instrument you choose with installation, training, calibration materials and spare parts.

To discuss which sulfur analyzer best fits your laboratory — and to arrange a demonstration — please contact us by phone or email.

Phone: +86-731-88112150

Email: sales@sundytech.com