ISO 6791:2026

• Abstract:

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

• Status: Published
• Publication Date: 10-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: 11-Nov-2026
• Completion Date: 11-Nov-2026

• Revises: ISO 6791:1981 - Carbonaceous materials for the production of aluminium — Pitch for electrodes — Determination of contents of quinoline-insoluble material
• Effective Date: 04-Nov-2023

ISO 6791 is an ISO International Standard for carbonaceous materials used in aluminium production. It specifies a conventional gravimetric method for determining the content of quinoline-insoluble material in pitch for electrodes, so laboratories can measure how much solid fraction remains after extraction with quinoline.

What does ISO 6791 specify?

ISO 6791 specifies one laboratory method for pitch used in the production of aluminium: determining quinoline-insoluble material by extraction and weighing. The document is organized into 10 clauses covering scope, references, terms, principle, reagents and materials, apparatus, sample preparation, procedure, expression of results, and test report.

The method is intended for routine analytical work on pitch samples. In practice, that means the user prepares the sample, extracts it with quinoline under controlled conditions, filters the insoluble residue, dries it, and weighs it to calculate the result.

What are the key requirements of ISO 6791?

ISO 6791 requires a gravimetric determination based on the fraction of pitch that does not dissolve in quinoline. The practical meaning is simple: the result comes from weighing the remaining residue after a controlled extraction, not from an instrumental reading.

Principle and reagents

Clause 4 states that the sample is extracted with quinoline at a specified temperature and for a specified time, and the insoluble fraction is then weighed. Clause 5 requires reagents of recognized analytical grade and specifies freshly distilled quinoline, toluene, and a diatomaceous-silica filter aid.

This matters because reagent quality and fresh quinoline affect how completely the pitch is dispersed and extracted. Clause 5 also includes safety warnings for quinoline and toluene, so the work needs proper ventilation and care with ignition sources.

Apparatus and temperature control

Clause 6 requires a water bath controlled at 70 °C to 80 °C, a Hartley funnel or equivalent, glass fibre filter pads, and an oven controlled at 105 °C to 110 °C. The funnel and pad must retain fine particles, so the residue is collected without losing material.

In practice, temperature control is central to the method. If the extraction, filtration, or drying conditions vary, the residue mass can change and the result will not be comparable.

Sample preparation

Clause 7 says the sample shall be prepared in accordance with ISO 6257 and prepared immediately before testing. If the pitch is hard, it is ground to a particle size not exceeding 200 µm; if it is too soft, it is melted and mixed without exceeding 150 °C or 10 minutes of melting.

This matters because the sample must be representative and handleable. The document also allows soft pitch to be taken from the molten sample, which helps labs deal with different pitch consistencies.

Procedure and weighing

Clause 8 requires weighing about 1 g of test portion, then mixing it with about 1 g of dried filter aid and 25 ml of quinoline. The mixture is heated in the 70 °C to 80 °C range for 15 to 20 min, then filtered, washed with hot quinoline and toluene, dried, and reweighed to constant mass.

The practical point is that the residue must be transferred and washed carefully so that all insoluble matter stays on the filter pad. The document also warns that all quinoline and toluene operations should be done in an efficiently ventilated fume cupboard.

Calculation, precision, and reporting

Clause 9 gives the calculation from the masses of the test portion, dried filter pad, filter aid, and final residue, and states precision values for repeatability and reproducibility. Clause 10 requires the test report to identify the sample, cite ISO 6791, give the test date and result, and note any deviations or unusual observations.

For users, this means the method is not just about measuring a mass. It also defines how to calculate, judge, and report the result so different laboratories can compare their work.

What terms does ISO 6791 define?

ISO 6791 does not list formal terms and definitions in Clause 3, but it uses several key technical terms throughout the document.

• Quinoline-insoluble material - the fraction of pitch that remains after extraction with quinoline under the specified conditions.
• QI - short for quinoline insoluble, the abbreviation used in the introduction.
• Primary QI - quinoline-insoluble material formed in the coke oven; the introduction says it is beneficial to carbon anode quality.
• Secondary QI - quinoline-insoluble material formed by additional heat treatment of pitch; the introduction says it may be neutral or harmful to anode quality.
• Pitch for electrodes - the pitch material used in aluminium production for electrode manufacture.
• Test portion - the weighed portion of sample used for the determination.
• Hartley funnel - the filtration funnel used with the filter pad to collect the insoluble residue.

Who uses ISO 6791?

ISO 6791 is used by laboratories and technical staff working with carbonaceous materials for aluminium production. It is relevant to quality control, incoming inspection, and process control for pitch used in electrode manufacture.

The introduction also links the method to carbon paste plants, where pitch content is adjusted based on quinoline-insoluble content. That makes the standard useful for plant chemists, laboratory analysts, and production engineers who need a consistent way to judge pitch behavior.

What changed in ISO 6791 from the previous edition?

ISO 6791:2026 is the second edition and cancels and replaces ISO 6791:1981. The document states that the main changes are added analysis procedure details, corrected text to formal SI formatting, and an updated test report description.

Which standards are used with ISO 6791?

ISO 6257, Carbonaceous materials used in the production of aluminium - Pitch for electrodes - Sampling, is the normative reference. It provides the sampling basis that ISO 6791 relies on before the analytical determination begins.

What does the ISO 6791 document contain?

ISO 6791 contains a worked laboratory method built around a controlled extraction, filtration, drying, and weighing sequence. It includes reagent specifications, apparatus requirements, sample preparation instructions, the detailed filtration procedure, a calculation formula, and precision statements.

The document also includes safety warnings for quinoline and toluene, plus a figure showing the Hartley funnel details. For reporting, Clause 10 sets out the information that should accompany the test result so the analysis can be traced and compared.

International
Standard
ISO 6791
Second edition
Carbonaceous materials for the
2026-11
production of aluminium — Pitch
for electrodes — Determination
of contents of quinoline-insoluble
material
Produits carbonés utilisés pour la production de l'aluminium —
Brais pour électrodes — Détermination du taux de matières
insolubles dans la quinoléine
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 . 2
7 Sample preparation . 3
8 Procedure . 4
8.1 Test portion .4
8.2 Filter preparation . .4
8.3 Determination .4
9 Expression of results . 5
9.1 Method of calculation .5
9.2 Precision . . .5
10 Test report . 5

iii
Foreword
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This document was prepared by Technical Committee ISO/TC 226, Materials for the primary production of
aluminium.
This second edition cancels and replaces the first edition (ISO 6791:1981), which has been technically
revised.
The main changes are as follows:
— added analysis procedure details to help run the analysis;
— corrected text to formal SI formatting;
— updated test report description.
Any feedback or questions on this document should be directed to the user’s national standards body. A
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iv
Introduction
In a carbon paste plant, the pitch content needs to be optimized amongst other factors according to the
quinoline insoluble content of the pitch. Two types of quinoline insoluble (QI) exist: primary and secondary.
Primary QI are formed in the coke oven and are beneficial to the carbon anode quality. Secondary QI (or
mesophase) are formed by additional heat treatment of the pitch and are either neutral or harmful to the
anode quality. Primary QI material is very dense and leads to h
...