**Weak Acid: Titration with Strong Base**

The titration curve will be calculated for a weak acid analyte, *HA*,
that has an acid equilibrium constant of *K _{a}*, and an initial
concentration of

*Using the systematic approach to chemical equilibrium problems...*

*Chemistry*:

*Charge Balance*:

*Mass Balance*:

*Equilibrium Equations*:

*Solve for concentration of A ^{-} using mass balance and K_{a}*...

*Substitute* [*A*^{-}], [*OH*^{-}]=*K _{w}*/[

*Solve for *[*H*_{3}*O*^{+}]

**Calculation of Points Along the Weak Acid/Strong Base Titration Curve**

*K _{w}* equilibrium may be neglected

When* K _{w} *equilibrium can be ignored, the cubic equation
reduces to...

This equation serves as the rule we can use to calculate the theoretical titration curve prior to the equivalence point. There are some important points.

**Initial** (*Region 1*), *F _{NaOH}*=0 and…

The *pH* is found from [*H*_{3}*O*^{+}].
An approximation can be found for a weak acid, [*H*_{3}*O*^{+}]<*F _{HA}*...

*Note that when K _{a} is large*...

* 1/2 way to the equivalence point *(

*For K _{a}<<F_{NaOH}* (

*For K _{a}>>F_{NaOH}* (

** At the equivalence point** (

*K _{b}* is the conjugate base equilibrium constant for

Solving for [*OH*^{-}]

Now since *K _{w}* is negligible relative to

The solution to this quadratic equation is the root

** Past the equivalence point** (

**Resulting titration curve**

The titration curve resulting from 40.0 mL of an acid
with *pK _{a}*=6.0, titrated with 0.1 F NaOH is shown here. The diamonds are points calculated using the methods above. The smooth black line
is found by solving the cubic equation using iteration methods. Shown for reference, in
pink, is the titration curve for 0.1 F strong acid.

This page was created by Professor Stephen Bialkowski, Utah State University.

Last Updated Tuesday, August 03, 2004