Overview
ISO 904:1976 specifies a titrimetric method for determining the total acidity of hydrochloric acid for industrial use, expressed conventionally as percent HCl by mass. The method replaces ISO Recommendation R 904‑1968 and defines the reagents, apparatus, procedure and calculation required to obtain reproducible acidity results using a volumetric sodium hydroxide titration with bromocresol green as indicator.
Key topics and technical requirements
- Principle: Titration of an aliquot of hydrochloric acid with a standard volumetric 1 N sodium hydroxide (NaOH) solution in the presence of bromocresol green indicator.
- Reagents:
- Sodium hydroxide, 1 N standard volumetric solution.
- Bromocresol green, prepared as 1 g/l in 95% (V/V) ethanol.
- Distilled or equivalent water neutral to bromocresol green.
- Apparatus:
- Approx. 500 ml flasks (sampling and titration), conical 500 ml flask for titration.
- Spherical glass ampoule with a capillary end (~50 mm capillary length) for enclosing the weighed test portion.
- Procedure highlights:
- Weigh the ampoule to 0.0001 g to determine the mass of the test portion by difference.
- Place ampoule with sample into 200 ml cold water in the conical flask, break the ampoule while cooling and shake until vapors are absorbed.
- Grind glass fragments, collect all washings, add two drops of bromocresol green and titrate with 1 N NaOH until colour changes from yellow to blue.
- Apply a correction factor if the NaOH concentration deviates from exactly 1 N.
- Result expression:
- Total acidity expressed as % HCl by mass using the formula given in the standard: % HCl = (V × 0.03646 × 100) / m - where V is the volume (ml) of NaOH used and m is the mass (g) of the test portion.
- Reporting: Include method reference, results and expression method, any unusual features and any non‑standard operations.
Applications and users
- Quality control and acceptance testing at hydrochloric acid producers and chemical plants.
- Analytical laboratories performing volumetric analysis and compositional testing of industrial acids.
- Procurement, regulatory and safety teams that require standardised acidity determination for process control, compliance, handling and storage decisions.
Related standards
ISO 904 is part of a series on hydrochloric acid analysis; related publications include:
- ISO 905 (density evaluation), ISO 906 (sulfate content), ISO 907 (sulfated ash), ISO 908 (oxidising/reducing substances) and others referenced in the standard.
Keywords: ISO 904, hydrochloric acid, total acidity, titrimetric method, 1 N sodium hydroxide, bromocresol green, volumetric analysis, industrial acid testing.
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