Agriculture Reference
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If other acid-base pairs are present the buffer capacity is
b solution = 2 . 303 [H 3 O + ] + [OH ] +
[B ] + ....
( 3 . 12 )
Polyprotic acids can be treated as a mixture of monoprotic acids. For example,
consider the diprotic acid H 2 C which forms the acid-base pairs H 2 C = HC +
H + and HC = C 2 + H + . The acidity constants are K 1 = [H + ][HC ] / [H 2 C]
and K 2 = [H + ][C 2 ] / [HC ], respectively. The total buffer capacity of the solu-
tion is therefore
[HA][A ]
[HA]
[HB][B ]
[HB]
[A ] +
+
+
2 . 303 [H 3 O + ]
[H 2 C][HC ]
[H 2 C] + [HC ] +
[HC ][C 2 ]
[HC ] + [C 2 ]
[OH ]
b solution =
+
+
( 3 . 13 )
For example, for a solution buffered by the CO 2 -H 2 O-H 2 CO 3 -HCO 3 sys-
tem, application of Equation (3.13) gives
b solution = 2 . 303 { [H 3 O + ] + [OH ] +
[ α 1 0 + α 2 ) + 4 α 2 α 0 ] C T }
( 3 . 14 )
where α 0 1 and α 2 are ionization fractions defined in Table 3.3. This equation
predicts that the buffer capacity will pass through minima at pH 4-4.5 where
[H 3 O + ]and[HCO 3 ] are both low, at pH 8.3 where [H 2 CO 3 ]and[CO 3 2 ]are
low, and at pH 10.5-11 where [HCO 3 ]and[OH ] are low. At these pH ranges,
changes in the concentrations of acids or bases in the solution will cause large
pH changes.
3.3 EQUILIBRIUM WITH THE GAS PHASE
The equilibrium distribution of a volatile solute between gas and liquid phases is
described by Henry's law. For the equilibrium A(l) = A(g) in a dilute solution
at low gas pressure,
[A(l)] = K H p A
( 3 . 15 )
where [A(l)] is the concentration of the dissolved gas in solution, p A is the partial
pressure in the gas phase and K H is the Henry's law constant. (At high concentra-
tions or gas pressures, [A(l)] and p A are replaced by the corresponding activities
and fugacities.) The constant is also sometimes expressed in dimensionless form,
H , such that
[A(l)] = H [A(g)]
( 3 . 16 )
where [A(g)] is the concentration in the gas phase. Hence
= RT K H
H
( 3 . 17 )
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