Ammonia and ammonium are related but not identical. Ammonia is unionized NH3; ammonium is the positively charged ion NH4+. In water they coexist in an acid-base equilibrium controlled mainly by pH, temperature and ionic strength.

The distinction matters because an instrument may respond to NH4+ while a laboratory reports total ammonia nitrogen (TAN), and aquatic-life risk is often strongly influenced by the unionized NH3 portion. Always keep the species, method and reporting basis beside the number.

What is the chemical difference?

Property Ammonia Ammonium
Formula NH3 NH4+
Charge Neutral +1
Water chemistry Unionized portion of the equilibrium Ionized portion of the equilibrium
Typical sensor distinction Gas-sensitive NH3 methods may use membrane diffusion after conditioning Ammonium ISE responds to NH4+ activity
Reporting caution mg/L NH3 differs from mg/L NH3-N mg/L NH4+ differs from mg/L NH4-N

The equilibrium is:

NH3 + H2O ⇌ NH4+ + OH−

Adding NH3 to water does not make one permanent species. Both forms can be present, and their proportions change with the water conditions.

How do pH and temperature affect the balance?

Higher pH shifts a larger fraction toward NH3. In freshwater at the same pH, a higher temperature generally also increases the NH3 fraction. A TAN result therefore cannot be converted into unionized NH3 without matched pH and temperature and an equation appropriate to the matrix.

For zero/low-salinity freshwater, EPA cites the Emerson relationship:

pKa = 0.09018 + 2729.92 / (273.2 + T)

fNH3 = 1 / (1 + 10^(pKa − pH))

T is °C and fNH3 is the unionized fraction. Salinity/ionic strength affects the equilibrium, so do not use this as a universal seawater formula.

Is ammonia more toxic than ammonium?

Unionized NH3 is generally the more toxic form to aquatic organisms, but “NH4+ is harmless” is too absolute. Toxicity depends on species, life stage, exposure duration and water chemistry. EPA freshwater criteria are expressed as TAN with pH- and temperature-dependent calculations; they are not a universal fish-farm, drinking-water or discharge limit.

Use this sequence for aquatic interpretation:

defined TAN result → matched pH/temperature → appropriate NH3 fraction → species/exposure/jurisdiction guidance

What is TAN?

TAN means total ammonia nitrogen: the nitrogen contributed by NH3 and NH4+ together under the applicable analytical definition. It is commonly reported as mg/L as N.

TAN is not the same as NH3. If TAN is 2.0 mg/L as N, only a condition-dependent fraction is NH3-N; the rest is NH4-N.

NH3, NH4+ and “as N” conversions

  • NH4-N = NH4+ × 14.0067 / 18.03846
  • NH4+ = NH4-N × 18.03846 / 14.0067
  • NH3-N = NH3 × 14.0067 / 17.03052
  • NH3 = NH3-N × 17.03052 / 14.0067

For example, 1.00 mg/L NH4-N equals approximately 1.288 mg/L NH4+. It does not equal 1.00 mg/L NH4+, and this basis conversion does not calculate NH3.

Does ammonia help plants “fix nitrogen”?

No. Nitrogen fixation is the conversion of atmospheric N2 into biologically available forms by specific biological or industrial processes. Plants and microorganisms may assimilate ammonium or other nitrogen forms, but uptake/assimilation is not the same process as nitrogen fixation.

Which result does a sensor report?

Do not infer the output from the words “ammonia sensor.” A PVC-membrane ammonium ISE responds to NH4+ activity; a gas-sensitive ammonia electrode uses a different membrane-diffusion principle; a colorimetric method produces a method-defined analytical result. The display may then use NH4+, NH4-N, NH3-N or TAN depending on configuration and validated calculations.

For APURE’s current measurement and method guide, read Ammonia & Ammonium Sensor.

Key points

  • NH3 is not NH4+.
  • TAN is not NH3 alone.
  • mg/L of the full species is not mg/L as N.
  • Higher pH generally increases the NH3 fraction.
  • A sensor principle and reporting basis must be verified before values are compared.
  • No one ammonia threshold applies to every water use, species or jurisdiction.