ISO 9390:1990

Overview - ISO 9390:1990 (Water quality - Borate by azomethine‑H)

ISO 9390:1990 defines a spectrometric method for determination of borate (expressed as boron) in water using the reagent azomethine‑H. The procedure forms a yellow azomethine‑H–borate complex that is measured spectrometrically at an absorption maximum between 410 nm and 420 nm. The method is applicable to potable, ground, surface and saline waters that are not heavily polluted, and covers boron concentrations between 0.01 mg and 1 mg B per litre (working range can be extended by dilution).

Key topics and technical requirements

• Analytical principle: condensation reaction of azomethine‑H with borate yielding a coloured complex measured spectrophotometrically (410–420 nm).
• Reagents: azomethine‑H solution (prepared with ascorbic acid), an ammonium‑acetate/phosphate/citric buffer at pH ≈ 5.9, and fresh mixed reagent prepared on the day of use. Azomethine‑H solution is stable up to one week when stored at 2–4 °C.
• Calibration: two calibration ranges - low range (0–0.20 mg B/L) measured in a 50 mm cell, and extended range (0–1.00 mg B/L) measured in a 10 mm cell. Calibration factor is obtained from the reciprocal slope of a linear calibration graph.
• Sample volume and procedure: typical aliquot is 25.0 mL; add reagent (example given: 10 mL azomethine‑H reagent), allow colour development, then measure absorbance versus blank.
• Interferences & quality control: Mg, Zn, Ca, Na, K, phosphate, sulfate and nitrate do not interfere; Mn, Zr, Cr, Ti, Cu, V, Al, Be and Fe may produce high results. Colour, humic acids or particulates may be removed (e.g., activated carbon or other cleanup). Blank determinations and reagent/water checks are required.
• Contamination prevention: avoid borosilicate glass for sampling and routine use (use polyethylene or PTFE); take precautions against borate contamination from detergents, cosmetics, labware and ambient sources.

Applications and users

• Routine and regulatory water quality testing for potable water, environmental monitoring, groundwater and surface water assessments.
• Laboratories performing boron/borate analysis, wastewater monitoring (to detect borate increases from detergents), and research groups studying trace boron levels.
• Useful to environmental agencies, public health labs and industrial monitoring where borate concentrations are of concern.

Related standards

• ISO 5667‑3 (Sampling - Guidance on preservation and handling of samples) is referenced for sample preservation and handling practices.

Keywords: ISO 9390:1990, borate determination, azomethine‑H, spectrometric method, water quality, boron analysis, calibration, contamination prevention.