Weak Acid Equivalence Point
Date: June 2007
Why is the equivalence point for titration of weak acids
above pH 7. I know that it is has something to do with the conjugate
bases attracting protons, but I still do not understand how the
moles of OH- equal the moles of H+ in a basic solution.
It is not. The equivalence point for titration of a weak acid is when
the pH equals the pKa of the acid. The equivalence point is not when
[H+] = [OH-]; it is when [HA] = [A-].
Department of Physics and Astronomy
University of Wyoming
It is important to remember that the salt produced from a weak acid
and a strong base will hydrolyze (react with water) and produce a base.
Let "HA" represent the proton and the counter anion in a weak acid, let
"MOH" represent a strong base (a metal such as Na along with the hydroxide,
OH) and we can represent a general weak acid and strong base reaction as:
HA + MOH = MA + H2O
but the salt (of the weak acid) will fully dissociate to form:
MA = M(+) + A(-)
The M(+) will not hydrolyze (doing so will produce strong, higher energy,
substances), but the A(-) can react with water in an equilibrium as represented
A(-) + H2O = HA + OH(-)
Note that a base, OH(-) is formed in the hydrolysis of the anion of the weak
acid. Thus, although an equivalent amount of base is used to react with the
weak acid (thereby neutralizing the acid and causing your expectation of a
pH = 7 at the equivalence point), a base is produced in the process. Thus at
the equivalence point, the pH is that of a base.
Greg (Roberto Gregorius)
First, let udistinguish between terms that are often confused "dilute
vs.concentrated" and "weak vs.strong".
Dilute and concentrated refer to the AMOUNT of acid present in the solution.
Weak and strong refer to the degree to which the acid is ionized in solution.
A "strong acid" is an acid that ionizes completely in water. HCl is an
example: HCl ---> H(+1) + Cl(-1).
There is essentially NO concentration of acid present in the neutral form
(HCl) dissolved in the water.
A "weak acid" is an acid that only ionizes partially in water. This can
be written in a "generic" form:
HA ---> H(+1) + A(-1). So in addition to H(+1) and A(-1) in solution, there
is also a measurable amount of
neutral, molecular HA present.
The relative amount of the three species is expressed by an "equilibrium
constant", or "ionization constant" --
both terms are used. The equilibrium constant is defined as: Keq = (H+)x(A-) / (HA),
where the terms in parentheses are the molar concentration (mols / liter) of
each of the species. This Keq is constant at a fixed temperature.
You can look at (HA) as a "storage bin" of available (H+). As you begin to
titrate the "weak acid" with base, (BOH), you convert (H+) into H2O. This
removes some (H+). But the Keq, which is a constant, must be maintained.
This ratio is kept fixed by ionizing an equivalent amount of molecular (HA)
into (H+) and (A-).
If you titrated the solution to pH = 7 you would only have consumed the amount
of (H+) present as ions. You have to add more base to react the amount of (H+)
that has been "stored" in the form of molecular, neutral (HA) until that amount
of (H+) has also been titrated. That means adding more base. You have to add the
amount of base until all of the "stored" (H+) has been neutralized. The amount of
base required to do this is controlled by the value of Keq which determines the
relative amount of (H+) and (HA). The result of having to add this "extra" base is
to make the equivalence point greater than pH=7.
To summarize: (H+) in a solution of a "weak" acid is present in two forms: (H+)
and (HA). You have to add sufficient base (BOH) to neutralize BOTH forms in which
(H+) is present, because as you consume (H+) by neutralization, the neutral form
(HA) will ionize more to maintain the ratio of (H+) to (HA) governed by Keq.
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Update: June 2012