ISO 6778:1984 PDF

Overview

ISO 6778:1984 specifies a potentiometric method for the determination of ammonium in raw water, sewage and wastewater using an ammonia‑sensing membrane probe (ammonia ion‑selective electrode). The procedure covers routine laboratory measurement up to 50 mg N/l (as ammonium nitrogen) without dilution and defines the limit of detection as the Nernstian response limit - approximately 0.2 mg N/l under the prescribed conditions. The method converts ammonium to ammonia by alkaline treatment and measures the electrode potential according to the Nernst equation.

Key topics and technical requirements

• Measuring principle: ammonia-permeable membrane electrode measures partial pressure of ammonia after converting NH4+ to NH3 at pH ~12 (alkaline buffer with NaOH + EDTA).
• Range and detection: applicable up to 50 mg N/l; detection limit ~0.2 mg N/l (Nernstian limit).
• Nernstian response / slope: electrode potential varies about ~58.5 ± 2 mV per decade change in ammonium concentration at normal lab temperature.
• Calibration and standards: use chloride‑based ammonium standards (e.g., 1000 mg N/l and 100 mg N/l stock solutions) to prepare calibration points bracketing expected sample concentrations.
• Instrumentation: ammonia membrane probe (ion‑selective electrode), potentiometer with 0.2 mV precision, magnetic stirrer and 100 ml conical flasks.
• Sample handling: analyze promptly or store at 2–5 °C; optional acidification (pH