Calculate unionized ammonia (NH₃) from total ammonia nitrogen (TAN), pH, temperature, and salinity. 4-tier risk assessment with corrective action recommendations for aquatic facilities.
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Most commercial test kits (API, Hach, Nessler) report Total Ammonia Nitrogen (TAN), not unionized ammonia. You must use pH and temperature to calculate the toxic NH₃ fraction. Reporting TAN alone without calculating NH₃ can be misleading.
A simultaneous increase in both pH and temperature dramatically increases the NH₃ fraction. For example, going from pH 7.0/25°C to pH 8.0/30°C increases the toxic fraction by roughly 10-fold. Always measure all three parameters together.
In systems with algae or plants, pH rises during the day due to photosynthetic CO₂ uptake. An afternoon reading at pH 8.5 may show dangerous NH₃ even though the morning reading at pH 7.5 was safe. For worst-case assessment, measure in the afternoon.
New systems without established nitrifying bacteria will accumulate ammonia rapidly. Do not stock at full density until the biofilter is cycled. Monitor TAN daily during the cycling period.
Unionized fraction calculated using Emerson et al. (1975) pKa formula: pKa = 0.09018 + 2729.92 / (T + 273.15). Salinity correction per Khoo et al. (1977). NH₃ fraction = 1 / (1 + 10^(pKa − pH)). Risk thresholds: safe (<0.02 mg/L), caution (0.02–0.05), danger (0.05–0.2), lethal (≥0.2).
Last validated 2026-04-06. Calculations are designed for planning and documentation support; verify procurement decisions against manufacturer specifications or institutional SOPs.
ConductScience Unionized Ammonia Risk Calculator (v1.0). ConductScience, Inc. 2026. Available at: https://conductscience.com/tools/unionized-ammonia-calculator
Emerson K et al. Aqueous ammonia equilibrium calculations: effect of pH and temperature. J Fish Res Board Can. 1975;32:2379–2383.
Khoo KH et al. Thermodynamics of the dissociation of ammonium ion in seawater from 5 to 40°C. J Solution Chem. 1977;6:281–290.
In aqueous solution, ammonia exists in a pH- and temperature-dependent equilibrium:
The fraction of total ammonia that is unionized (NH₃) is determined by the dissociation constant (pKa) and the water pH:
The pKa of ammonia decreases with increasing temperature (Emerson et al., 1975):
where T is temperature in °C. This means that at higher temperatures, a greater fraction of TAN exists as toxic NH₃. Salinity further reduces pKa (Khoo et al., 1977), increasing the NH₃ fraction in brackish or marine systems.
Zebrafish are relatively sensitive to ammonia compared to many other freshwater species. Key toxicity thresholds:
Larvae and embryos are more sensitive than adults. Chronic low-level exposure can confound experimental results by inducing stress-related gene expression changes, making ammonia monitoring essential for reproducible research.
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