Titration of Weak Acids and Bases

Review of weak acid from base titrations and weak base strong acid titrations from General Chemistry 2.

Titration curves are graphical representations of the pH of a solution as a function of the volume of titrant added.
Key concepts to understand:
- Calibration Curve: Use when needing to find the concentration of an unknown.
- Concentration Determination: If given a titration curve, one can determine the concentration of an unknown weak acid or base.
Types of titration curves based on titrants:
- Weak acid with strong base
- Weak base with strong acid
Identify indicators that suit the titration curve's equivalence point.

pKa and Equivalence Point: At the equivalence point:
- For weak acids, the pH will be greater than 7 when titrated with a strong base.
- You can identify an acid or base in the solution based on the pH halfway to the equivalence.
Indicator Selection: Choose indicators based on the equivalence point's pH. Indicators should change colors in a range slightly above and below the equivalence point.

Example calculation using acetic acid (weak acid with known Ka):
- Given Ka of Acetic Acid: 1.75 x 10^(-5).
- Calculate pH at various volumes of NaOH (strong base) added:
- Before Addition of Base: Start with weak acid equilibrium, ICE chart to solve for pH.
Derive the pH using the formula:
- Use the equilibrium concentrations and reapply the assumptions after every iteration to continue refining the calculation.
- If the assumption fails (greater than 1% deviation), resolve the equation without making the small assumption.

At halfway to the equivalence volume, pH = pKa, hence this is where buffering occurs and significant fluctuations in pH result from minimal changes in volumes of titrant.

When adding OH- to a weak acid solution, analyze concentrations and reactions to arrive at concentrations of the buffer components.
Key steps include:
- Start with initial moles of acid and base reacting.
- Track changes due to stoichiometry and find concentrations post-reaction.

Polyprotic Acids: Understand the handling of polyprotic systems and their titration curves (e.g., H2A→HA-+H+ and HA-→A2-+H+).
For diprotic systems, know that:
- Ka1 will correspond to the first proton donation, while Ka2 corresponds to the second.
- Use Ka1 to find the pH when assessing the initial weak acid solution.

If given Ka2 for H2A, understand Kb1 is related as follows:
$K<em>b1 = \frac{K</em>w}{K_a2}$ .
Be prepared to write ICE charts for equilibrium calculations based on weak acid/base reactions. Understand that the second dissociation does not have substantial effects due to relative magnitudes of Ka.

The main takeaway is to apply these principles to find pKa from titration curves, calculate unknown concentrations, and select suitable indicators based on equivalence points, controlling for the initial conditions of the titration.