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ammonia Equilibrium

I thought I'd just to clear out some confusion about equilibrium, which is an important
concept for people dealing with aqueous solutions...  Equilibrium is a thermodynamic
concept that tells the relative proportions of chemical species involved in a reaction.
Ammonia, for example undergoes the _equilibrium_  reaction:
NH3+H(+)  <--> NH4
The arrow points both ways, ie the reaction can go in both directions.  At a given
temperature, the ratio (usually designated K) of the concentration of products to the
reactants is constant...


Now, if the pH is changed, eg. lowered, the concentration of H(+) in solution is
increased, with the result that the concentration of the ammonium ion must increase  (and
ammonia to decrease) to keep the ratio, K, constant. So, yes, the ammonium ion increases
in concentration as pH is lowered.  There is always ammonia present as well, it's just a
question of _relative_ amounts.  The equilibrium constant for the above reaction is
5.7*10^-10.  At pH 7, the hydrogen ion concentration is 1*10^-7.  This gives a relative
ratio of ammonia to ammonium of 5.7*10^-3, in other words the ammonium ion is present
over ammonia in a 1000 to 1 ratio....

Robert Andersson