There's always confusion about the relationship between ammonia and
ammonium and the pH. The equilibrium that interests us
is:
NH4+ <-> NH3 + H+
which has an equlibrium constant (Ka for the ammonium ion) of
about 5.89 x 10^-10. i.e., if the concentrations of everything
are expressed in moles/L, then:
[NH3][H+]/[NH4+] = 5.89 x 10^-10
in water at 25C
[something] = concentration of something
It follows from this that the concentrations of ammonia and
ammonium are equal when the hydrogen ion concentration is equal to
the Ka, or that the pH is equal to the pKa (take the log of the number
and change the sign). The pKa is 9.23, so the pH for equal ammonia
and ammonium is 9.23. The ratio of ammonia to ammonium will change
by a factor of 10 with every unit change in the pH. The lower the pH,
the more ammonium there will be.
To find the ratio of ammonium to ammonia, subtract the pH from
9.23, and get the inverse log of the result. As an example: at pH 7.23
the result of the subtraction is 2.0, and the ratio is thus 10^2.0,
which is 100.
There is _no_ magic cutoff number at either end, though of
course at pH 3.23, the ratio is 1000000!
(with thanks to Paul Sears of Ottawa...)
--Bruce Stallsmith
Huntsville, AL
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