Overview
ISO 740:1976 specifies a titrimetric method for determining the total soluble alkalinity of sodium carbonate (soda ash) for industrial use. The standard describes sample preparation, dissolution, filtration, and titration procedures to quantify alkalinity expressed as percentage by mass of sodium carbonate (Na2CO3). ISO 740:1976 replaces ISO Recommendation R 740-1968.
Key topics and technical requirements
- Scope: Applicable to sodium carbonate for industrial use; measures total soluble alkalinity by acid–base titration.
- Principle: Dissolve a weighed test portion in water, filter, dilute to volume, and titrate an aliquot with a standard hydrochloric acid solution in the presence of methyl orange indicator.
- Reagents: Use analytical-grade reagents. Primary reagents specified:
- Hydrochloric acid, 1 N standard volumetric solution (or corrected if concentration differs).
- Methyl orange indicator (≈0.5 g/L) - endpoint from yellow to orange–pink.
- Apparatus: Ordinary laboratory glassware; 500 mL one-mark volumetric flask; medium-speed filter; conical flask for titration.
- Test portion: Weigh to nearest 0.01 g; typical masses specified are 50.0, 59.0, 110.0, or 135.0 g depending on anhydrous or hydrate form (mono-, hepta-, decahydrate).
- Procedure highlights:
- Dissolve sample in ~200 mL water at ~50 °C, filter and wash insoluble matter, collect filtrate to 500 mL and dilute to mark.
- Pipette 25.0 mL aliquot, add ~75 mL water and methyl orange, titrate with 1 N HCl until endpoint.
- Alternative: back-titration using 1 N HCl and 1 N or 0.1 N NaOH with appropriate calculations.
- Calculation: The result is calculated from the titration volume, sample mass and the factor that 1 mL of exactly 1 N HCl corresponds to 0.0530 g Na2CO3. With the 25 mL aliquot and 500 mL dilution, % Na2CO3 = (106 × V) / m, where V is mL HCl used and m is grams of test portion. Adjust if reagent normality differs.
- Reporting: Include method reference, numerical result and expression, and any deviations or unusual observations.
Applications and users
- Quality control laboratories in chemical manufacturing (soda ash producers)
- Industrial purchasers and users who require specification compliance (glass, detergents, chemical processing)
- Regulatory bodies and inspection labs performing acceptance testing
- Analytical chemists validating batches for total soluble alkalinity and product conformity
Related standards
- ISO 739 - Preparation and storage of test samples
- ISO 741 - Determination of sodium hydrogen carbonate content
- ISO 742–746 - Methods for chloride, sulphate, iron, loss on ignition and insoluble matter
Keywords: ISO 740:1976, sodium carbonate, total soluble alkalinity, titrimetric method, methyl orange, 1 N HCl, soda ash quality control.
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