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 (NH₃), the gas, and ammonium (NH₄⁺), the ion. At typical neutral groundwater pHs, relatively non-toxic NH₄⁺ is the dominant species, while a significantly lower amount of relatively toxic NH₃ is present.

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 NH₃ and NH₄⁺ can be calculated.

Why is this important? Many states are setting standards for NH₃ in groundwater. Arguably, a total ammonia laboratory result should not be used directly to compare against the standard. For example, in Wisconsin, the NH₃ Enforcement Standard is 9.7 mg/L. If it is assumed that a laboratory test result of 100 mg/L total ammonia is all NH₃, the standard is exceeded, while the actual calculated concentration of NH₃ 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.