Overview
ISO 746:1976 specifies a standardized analytical method for determining the amount of matter insoluble in water at 50 °C in sodium carbonate (soda ash) for industrial use. The method replaces ISO Recommendation R 746‑1968 and defines the principle, apparatus, reagents, procedure and expression of results required for consistent, comparable test data in quality control and compliance testing.
Key topics and technical requirements
- Principle: Dissolve a defined test portion in water at approximately 50 °C, filter the solution through a tared sintered glass filter, wash the insoluble residue at ~50 °C until washings are no longer alkaline, dry the residue and weigh it.
- Test portion: Representative sample mass is specified in the standard (see ISO 739 for sample preparation and storage).
- Apparatus:
- Glass filter crucible with sintered disk porosity grade P16 (pore size ~10–16 μm).
- Electric oven controllable at 110 ± 5 °C.
- Vacuum/filter pump for reduced-pressure filtration.
- Reagents:
- Use analytical‑grade reagents and distilled or equivalent purity water.
- Phenolphthalein 10 g/L ethanolic solution for endpoint testing of alkalinity of washings.
- Procedure highlights:
- Dissolve the test portion by adding small quantities to ~200 mL water heated to ~50 °C (beaker capacity ~600 mL).
- Filter under reduced pressure and wash the insoluble matter with ~50 °C water until 20 mL of filtrate shows no pink colour with 2 drops phenolphthalein.
- Dry the filter crucible and residue at 110 ± 5 °C for 1 hour, cool in a desiccator and weigh to 0.0001 g precision.
- Expression of results: Report the insoluble matter as a percentage by mass using the mass of dried residue divided by the mass of the test portion × 100. The standard notes how to convert results to a basis of non‑volatile matter at 250 °C using ISO 745 if required.
- Reporting: Include method reference, results and expression used, any unusual observations, and any optional operations not covered by the standard.
Applications and users
- Quality control laboratories in chemical manufacturing (soda ash producers).
- Incoming inspection and acceptance testing for industrial users of sodium carbonate (glass, detergents, pulp & paper, chemical intermediates).
- Regulatory and standards compliance testing to ensure lot-to-lot consistency and product specification conformance.
- Research and analytical labs requiring a reproducible method for measuring insoluble contamination in sodium carbonate.
Related standards
- ISO 739 - Preparation and storage of test samples.
- ISO 745 - Determination of loss of mass and of non‑volatile matter at 250 °C (conversion basis).
- Other ISO methods for sodium carbonate analysis (total soluble alkalinity, chloride, sulfate, iron, etc.) referenced in the annex of ISO 746.
Keywords: ISO 746, sodium carbonate, insoluble matter, determination, water at 50 °C, industrial use, soda ash quality control, analytical method.