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Ammonia (NH3), Ammonium Ion (NH4+)

NH3 is very soluble in water. The Henry's law constant for NH3 is 57.5 mol/(L-atm) at 25ºC NH3 can combine with a proton (H+ ion) to form an ammonium ion (NH4+).

The concentrations of NH3 and NH4+ at equilibrium are related by the following equation

where Ka is a constant (5.5x10-10) and square brackets indicate concentrations (mol/L). The following figure shows how NH3 and NH4+ concentrations vary with pH. In the pH range of most natural waters, NH4+ is the predominant form of N in the -3 oxidation state. NH3 is the species that is toxic to fish, not NH4+.


Figure 1. Percentage of NH4+ and NH3 as functions of pH.

NH4+ is a major component of atmospheric aerosols. (see atmosphere)

The NH4+ ion participates in cation exchange reactions in which dissolved ions replace ions bound by negatively charged sites on mineral particles, such as clays, and natural organic matter.

Ion exchange may be an important reaction for NH4+ in soils, sediments, and aquifer systems. For example, the retardation coefficient of NH4+ (rNH4) in sandstone was estimated from ion exchange measurements to be 16<rNH4<80 (Drever 1982). A pulse of NH4+ would move at a rate between 1/80th and 1/16th that of water in the sandstone aquifer.

Calculations of ion exchange equilibria can sometimes be used to qualitatively describe ion exchange behavior of cations. Two useful empirical ion exchange parameters are the cation exchange capacity (CEC) and the selectivity coefficient. The CEC of a material (clay, soil, ...) is measured by saturating the material with one cation (e.g. NH4+), displacing the first cation with another cation (e.g., Mg2+), and measuring the concentration of displaced NH4+. Table 5 presents the CEC values of some clay minerals.
 
 

Suggested Reading:

Cotton, F. A., Wilkinson, G. 1972. Advanced Inorganic Chemistry, 3rd ed. New York:Wiley.

Drever, J. I. 1982. The Geochemistry of Natural Waters. Englewood Cliffs:Prentice-Hall.

Greenwood, N. N., Earnshaw, A. 1984. Chemistry of the Elements. Oxford:Pergamon.

Morel, F. M. M., Hering, J. G. 1993. Principles and Applications of Aquatic Chemistry. New York:Wiley.

Wark, K., Warner, C. F. 1981. Air Pollution: Its Origin and Control. New York:Harper and Row.

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