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Temperature [°C]  
pH Units
Ammonium total NH4+NH3
Ammonia NH3
Coose unit (NH3-N or NH3 or NH3 molar)
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Range: 0° … 50°C, NH3: pH< 8: ±3%, at pH10:±1%
        Standard dev. for temperature <±0.02; for pH ±0.015

NH4 total:0.2 mg/l, temperature:18°C, pH-Value:7.7 (pKA 9.465) with result: NH3 = 0.0032 mg/l
tolerated value NH3=0.005 mg/l, temperature:13°C, pH:8.5; result: NH4 total:0.077 mg/l

Data base: UBA-BE-076 (1996) Gerhard Hobiger

The constants were freshly estimated out of table values, i.e. slightly modified. Corrections for activity because of presence of other ions are neglected.

Meaning of the values (partially cited from data base):
The unionized ammonia diffuses more easily through biological membranes than the ammonium ion, whereby the high toxicity of ammonia results. The lethal dose for fish at about 1 mg/l ammonia (at 15° C), leading to suffocation with gill necrosis. For fish larvae are already 0.2 - 0.3 mg/l ammonia fatal. 0.03 to 0.05 mg/l ammonia lead to chronic damage, in which trout are particularly sensitive (Hütter 1990).
(The EU - guideline was 1978 ≤0.005 mg/l NH3)
LA Hütter, water and water analysis, Verlag Salle and Sauerland 1990
See also: Wuhrmann, K. and Woker: Schweiz. Z. Hydrol. 1949.