Aqueous Ammonia Equilibrium System 



In aqueous ammonia solutions, un-ionized ammonia exists in equilibrium 

 with the ammonium ion and the hydroxide ion. The equation expressing this 

 equilibrium can be written as: 



NH 



3(g) +nH 2°U) - NH 3- nH 2°(aq) ' NH 4 + 0H " + ^^ { 



As indicated in this equation, the dissolved ammonia molecule exists in 

 hydrated form; it is hydrogen-bonded to at least three water molecules 

 (Butler, 1964). The dissolved un-ionized ammonia is represented for con- 

 venience as NH3; the ionized form is represented as NH4+; and total 

 ammonia is the sum of these (NH3 + NH4 + ). 



The effect of pH and temperature on the aqueous ammonia equilibrium is 

 significant. For example, a pH increase from 7.0 to 8.0 within the range 

 0-30 °C results in a nearly tenfold increase in the concentration of NH3; 

 a temperature increase of 5 degrees between 0-30 °C at pH 7.0 results in 

 an NH3 concentration increase of 40-50 percent. 



There is no convenient method for measuring the concentration of NH3 

 and NH4 + separately. However, if total ammonia concentration, pH, and 

 temperature are known, the concentration of NH3 may be calculated. Table 

 1 gives values of percent NH3 in aqueous ammonia solutions of zero 

 salinity. 



TABLE 1. PERCENT UN-IONIZED AMMONIA IN AQUEOUS AMMONIA SOLUTIONS* 



* [condensed from Thurston, tt al. (1974)] 



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