Ask the Chemist Vol. 1 - pH and Ammonia
Why is it necessary to record the pH and temperature of a groundwater sample when ammonia is a target analyte?
In groundwater, ammonia exists in two forms - ammonia (NH
), the gas, and ammonium (NH
), the ion. At typical neutral groundwater pHs, relatively non-toxic NH
is the dominant species, while a significantly lower amount of relatively toxic NH
When a laboratory analyzes for ammonia by any of the EPA methods, the concentration of reported is the total ammonia (ammonia + ammonium) as nitrogen. If pH and temperature are determined at the time of collection, the concentrations of NH
can be calculated.
Why is this important? Many states are setting standards for NH
in groundwater. Arguably, a total ammonia laboratory result should not be used directly to compare against the standard. For example, in Wisconsin, the NH
Enforcement Standard is 9.7 mg/L. If it is assumed that a laboratory test result of 100 mg/L total ammonia is all NH
, the standard is exceeded, while the actual calculated concentration of NH
based on pH and temperature will be far below the Enforcement Standard for most Wisconsin groundwater samples.
ECCS reports calculated ammonia and ammonium values based on field pH and temperature when requested.
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