ISO 6376:2026

• Abstract:

  This document specifies a gravimetric method for the determination of the content of toluene-insoluble material in pitch used for the production of aluminium.

• Status: Published
• Publication Date: 11-Nov-2026
• ICS: 71.100.10 - Materials for aluminium production
• Technical Committee: ISO/TC 226 - Materials for the production of primary aluminium
• Drafting Committee: ISO/TC 226/WG 1 - Carbonaceous binders, including pitch
• Current Stage: 6060 - International Standard published
• Start Date: 12-Nov-2026
• Completion Date: 12-Nov-2026

• Revises: ISO 6376:1980 - Carbonaceous materials for the production of aluminium — Pitch for electrodes — Determination of content of toluene-insoluble material
• Effective Date: 04-Nov-2023

ISO 6376:2026, published by ISO, specifies a gravimetric method for determining the content of toluene-insoluble material in pitch for electrodes used in aluminium production. It is intended for laboratories and technical teams that need a consistent way to measure how much of a pitch sample remains insoluble after boiling in toluene.

What does ISO 6376 specify?

ISO 6376:2026 specifies a test method for pitch for electrodes based on weighing the fraction that does not dissolve in toluene. The document is organized into clauses on scope, normative references, principle, reagents and materials, apparatus, sampling, procedure, expression of results, precision, and test report.

The method is used to compare pitches on the basis of their insoluble content, which is a practical indicator for material assessment in aluminium production.

What are the key requirements of ISO 6376?

ISO 6376:2026 requires a defined laboratory workflow so that different users measure the same property in the same way. The main points are the sample handling, the boiling and filtration step, the drying and weighing step, and the way the result is reported.

Sampling and test portion

Clause 7 says sampling is carried out in accordance with ISO 6257. Hard pitches are crushed and ground so they pass a 212 µm sieve, while soft pitches are used as received. Clause 8.1 then calls for about 1 g of sample, weighed to the nearest 0.001 g. In practice, this means the sample must be prepared in a controlled way before the actual measurement starts.

Reagents, safety, and apparatus

Clause 5 requires reagents of recognized analytical grade. The method uses toluene and acetone, and the document warns that both are highly flammable and harmful by inhalation, so the work has to be done with proper ventilation and fire precautions.

Clause 6 specifies the essential equipment: a 500 ml conical flask, a reflux condenser, a glass filter crucible with sintered glass disc porosity grade P16, a Hartley funnel or equivalent, and an oven controlled between 105 °C and 110 °C. These details matter because the physical set-up affects how well the insoluble fraction is recovered.

Determination and drying

Clause 8.2 directs the analyst to heat the test portion with 100 ml ± 2 ml of toluene under reflux for 30 min ± 2 min, with occasional swirling. The hot mixture is then filtered immediately through a dried and weighed crucible, the flask is rinsed repeatedly with toluene until the filtrate is colourless, and the residue is washed with acetone.

The crucible and residue are then dried for about 1 h at 105 °C to 110 °C, cooled in a desiccator, and weighed again. In practice, this is the core measurement step: the result comes from the mass left on the crucible.

Calculation, precision, and reporting

Clause 9 expresses the result as a percentage by mass from the masses of the test portion, the empty crucible, and the crucible plus insoluble residue. Clause 9.2 gives precision values of 0.5 % repeatability and 1.0 % reproducibility, which help users judge whether results from the same lab or different labs are close enough.

Clause 10 requires the report to identify the sample, cite ISO 6376:2026, give the test date, state the result and calculation reference, and note any deviations or unusual observations.

What terms does ISO 6376 define?

ISO 6376:2026 uses a few practical terms central to the method.

• Gravimetric method - a method that determines a property by weighing material before and after treatment.
• Test portion - the measured portion of sample used for the determination, about 1 g here.
• Toluene-insoluble material - the fraction that remains undissolved after boiling the pitch in toluene and filtering.
• Repeatability - closeness of results obtained under the same conditions in one laboratory.
• Reproducibility - closeness of results obtained by different laboratories.
• Filtrate - the liquid that passes through the filter crucible during the separation step.
• Hard pitches - pitches that are prepared by crushing and grinding before testing.
• Soft pitches - pitches that are tested as received.

Who uses ISO 6376?

ISO 6376:2026 is used by laboratories testing carbonaceous materials for aluminium production, especially pitch for electrodes. It is also relevant to quality managers, process engineers, and buyers who need a standard test result for incoming material control, product comparison, or dispute resolution.

What changed in ISO 6376 from the previous edition?

ISO 6376:2026 is the second edition and replaces ISO 6376:1980. The document states that the main changes are added analysis procedure details to help run the analysis, corrected text to formal SI formatting, and an updated test report description.

Which standards are used with ISO 6376?

ISO 6376:2026 cites ISO 6257, Carbonaceous materials used in the production of aluminium - Pitch for electrodes - Sampling. ISO 6257 provides the sampling procedure that feeds the test method in ISO 6376.

What does the ISO 6376 document contain?

ISO 6376:2026 contains a full test method with the principle, reagent list, apparatus requirements, sampling instructions, detailed procedure, calculation formula, precision statement, and reporting requirements. It also includes Figure 1 with the details of the Hartley funnel used in the filtration step.

The procedure is written as a practical laboratory sequence, from preparing the crucible to refluxing in toluene, filtering, washing with acetone, drying, and weighing. The document also includes a safety warning section for the solvents used, which is important for laboratory handling and ventilation planning.

International
Standard
ISO 6376
Second edition
Carbonaceous materials for the
2026-11
production of aluminium — Pitch
for electrodes — Determination
of content of toluene-insoluble
material
Produits carbonés utilisés pour la production de l'aluminium —
Brais pour électrodes — Détermination du taux des matières
insolubles dans le toluène
Reference number
© ISO 2026
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 1
5 Reagents and materials . 1
6 Apparatus . 1
7 Sampling . 3
8 Procedure . 4
8.1 Test portion .4
8.2 Determination .4
9 Expression of results . 4
9.1 Method of calculation .4
9.2 Precision . . .4
10 Test report . 5

iii
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This second edition can
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