water
Free ammonia calculator
A test kit reports total ammonia. This works out how much of it is the toxic un-ionised form, from your pH and temperature.
0.007 mg/L free ammonia — 1.404% of the reading
Low but not negligible. Change water, stop feeding, and find out why ammonia is present at all.
Working — every step, so you can check it
- Temperature in kelvin
- 25 + 273.15 = 298.15 K
- pKa
- 0.09018 + 2729.92 ÷ 298.15 = 9.246
- Fraction as NH₃
- 1 ÷ (1 + 10^(9.246 − 7.4)) = 0.01404 (1.404%)
- Free ammonia
- 0.5 × 0.01404 = 0.007 mg/L as N
Formula
- pKa = 0.09018 + 2729.92 ÷ T where T is temperature in kelvin
- Fraction as NH₃ = 1 ÷ (1 + 10^(pKa − pH))
- Free ammonia (mg/L as N) = total ammonia nitrogen × fraction
Assumptions
An estimate whose assumptions are invisible is indistinguishable from a guess.
- Fresh water. Ionic strength corrections that matter at brackish and marine salinities are not applied.
- The test kit reports total ammonia nitrogen, which is the usual convention for hobby kits.
- The dissociation relation is the temperature-dependent form published by Emerson and colleagues (1975) and reproduced throughout water-quality practice.
Limitations
Where this calculation stops being reliable. Read these before acting on a result.
- If you have used a conditioner that binds ammonia, your test reading no longer reflects free ammonia and this calculation will over-state the danger.
- There is no single threshold at which free ammonia becomes harmful. Sensitivity varies by species, by life stage and by exposure duration; regulatory criteria are themselves pH and temperature dependent.
- This is an aid to interpreting a reading, not a substitute for acting on one. Any detectable ammonia in a stocked tank needs a water change.
References
- Emerson et al. (1975), aqueous ammonia equilibrium relation
- US EPA aquatic life criteria for ammonia in fresh water