How much of a total ammonia reading is the toxic form. pH changes the answer by a factor of ten.
| pH | Free fraction | Free NH₃ at your reading |
|---|---|---|
| 7.6 | 1.74% | 0.0053 mg/L |
| 7.8 | 2.73% | 0.0083 mg/L |
| 8.0 | 4.26% | 0.0130 mg/L |
| 8.2 | 6.59% | 0.0200 mg/L |
| 8.4 | 10.06% | 0.0306 mg/L |
Ammonia sits in equilibrium between NH₃ and NH₄⁺, and only the uncharged NH₃ crosses gill membranes freely. The split depends on pH, temperature, and salinity, which is why the same test result means very different things in a reef tank at 8.3 and a freshwater tank at 7.0.
pKa = 0.0901821 + 2729.92/T + (0.1552 − 0.0003142·t)·I = 9.3514 · fraction = 1 / (1 + 10^(pKa − pH)) = 6.59%
Total ammonia is the sum of NH₃ and NH₄⁺. Only the uncharged NH₃ crosses gill membranes easily, so it does nearly all the damage, and the split between the two is set by pH, temperature, and salinity.
The fraction is 1 / (1 + 10^(pKa − pH)). The pKa comes from Emerson, Russo, Lund and Thurston (1975), pKa = 0.0901821 + 2729.92/T with T in kelvin, plus an ionic strength correction for seawater from the Khoo and Whitfield work: (0.1552 − 0.0003142·t)·I, where I is the ionic strength computed from salinity.
Checked against the Florida DEP standard procedure's own worked example, this reproduces their published table value exactly. At pH 8.2, 25 °C and 35 ppt it gives 6.6 percent free ammonia.
The pH sensitivity is the practical point. Every unit of pH changes the free fraction roughly tenfold. An ammonia reading of 0.25 ppm is a minor concern in a freshwater tank at pH 7.0 and a genuine emergency in a reef tank at 8.3, and the test kit shows the same number in both.
Kits disagree about units. US kits usually report ammonia nitrogen, NH₃-N, while many European kits report NH₃ directly, and the ratio between them is 1.216. Salifert's marine kit states that it measures the sum of both forms without saying which basis, so treat its reading as total and err toward caution.
Reference points: the US EPA saltwater chronic criterion is 0.035 mg/L of free NH₃ as a four day average, and the acute one is 0.233 mg/L over an hour. Those are written for wild communities under regulatory averaging, so treat them as an upper bound rather than a target. Randy Holmes-Farley recommends keeping total ammonia in a reef below 0.1 ppm, which sits about five times below the EPA chronic figure.
The fix for high ammonia is a water change and less food, not a lower pH. Dropping pH does reduce the toxic fraction, and it leaves you with an uncycled tank plus a pH problem. Ammonia in an established reef tank means something died, the biofilter was disturbed, or the tank was overstocked.
Sources: Emerson et al. 1975, aqueous ammonia equilibrium · US EPA aquatic life criteria for ammonia