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CADDIS Volume 2: Sources, Stressors & Responses


Ways to Measure Ammonia

The concentration of ammonia in water is measured as total ammonia and is usually reported in mg/L. The concentration of unionized ammonia (NH3) is derived from well-established formulas (see link to calculator below). When differentiating between unionized and ionized ammonia, it is important to take into account the effects of temperature and pH (Eaton et al. 2005).

  • Ammonia probe: A measuring probe is connected to an electronic ion meter that measures and displays the voltage resulting from ammonia, which is then converted to concentration using a standard curve.
  • Colorimeter: One or more reagents are added in timed intervals to the sample, such that the intensity of the color produced is proportional to the ammonia that reacts with the reagent. This color absorbance then is measured using a colorimeter or spectrophotometer. Blanks and standards are used to generate a standard curve from which the sample absorbance reading is converted to ammonia concentration.

The fraction of unionized ammonia (NH3) is not directly measured, but instead is calculated using measures of total ammonia, pH, temperature, and ionic strength (measured either in terms of total dissolved solids or conductivity). The American Fisheries Society has developed an ammonia calculatorExit EPA Disclaimer (Table 9 on the referenced web page) for this purpose. Once the required data are entered, the tool will calculate the amount of unionized ammonia in both mg/L and µg/L.

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