Overview
ISO 9963-2:1994 - "Water quality - Determination of alkalinity - Part 2: Determination of carbonate alkalinity" specifies a titrimetric method for measuring carbonate alkalinity in natural and drinking waters. The method targets samples with carbonate alkalinity between 0.01 mmol/L and 4 mmol/L (as H+ equivalents); for higher alkalinities a smaller test portion is used. The procedure uses acid titration with simultaneous removal of CO2 and sets the analytical endpoint at pH 5.4 to ensure full neutralization of the carbonate system and reduced interference.
Key topics and technical requirements
- Principle: Titrate the sample with standardized hydrochloric acid while purging the vessel with gas free of CO2 to remove evolved CO2; endpoint chosen to neutralize the carbonate system.
- Endpoint detection:
- Preferred: pH‑meter set to detect pH 5.4 (stable for ≥30 s).
- Alternative: mixed indicator (methyl red + bromocresol green) producing a grey/red endpoint (indicator recommended for non‑coloured samples).
- Sample and test portion: Typical test portion 50.0 mL ±0.1 mL; if acid volume consumed >10 mL use smaller aliquot and dilute to 50 mL (minimum titrant consumption ≥3 mL).
- Reagents and standards: Use analytically pure reagents (grade 2 water per ISO 3696). Prepare and standardize HCl (~0.020 mol/L) weekly using a sodium carbonate standard (≈0.025 mol/L).
- Apparatus: burette (preferably 10 mL, 0.02 mL divisions), titration vessel with gas distribution or filter funnel, pH electrode system conforming to IEC/ISO performance requirements.
- Quality control: Perform triplicate titrations; blank titrations required; report results to two significant digits. Annex A provides conversion factors (e.g., to mg/L CaCO3).
- Precision: International interlaboratory data reported (typical coefficients of variation ~2–4% for various concentration ranges).
Applications and users
ISO 9963-2:1994 is used by:
- Environmental and municipal water testing laboratories
- Drinking water compliance and monitoring programs
- Water treatment plants (process control and dosing calculations)
- Research laboratories performing aquatic chemistry and alkalinity balance studies
Practical applications include routine monitoring of natural waters and tap/drinking water, process control where carbonate buffering influences pH management, and baseline data for aquatic chemistry modelling.
Related standards
- ISO 9963-1:1994 - Determination of total and composite alkalinity (Part 1)
- ISO 3696:1987 - Water for analytical laboratory use (grade specifications)
- ISO 5667-2:1991 - Water sampling guidance
- IEC 746‑2 / relevant pH electrode performance standards
Keywords: ISO 9963-2:1994, carbonate alkalinity, water quality, titrimetric determination, pH 5.4, HCl titration, alkalinity testing, drinking water analysis.
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