Controlling and Measuring pH
Acid-Base Titrations
A titration is a laboratory technique used to determine the unknown concentration of a solution (the analyte) by reacting it with a solution of known concentration (the titrant or standard solution).
Procedure: The titrant is slowly added from a buret to a known volume of the analyte in a flask. An indicator is often used to signal the end of the reaction.
Equivalence Point: The point in the titration where the amount of titrant added is stoichiometrically equivalent to the amount of analyte present. For a strong acid-strong base titration, the pH at the equivalence point is exactly 7.
Endpoint: The point where the indicator changes color. A good indicator is one whose endpoint is very close to the equivalence point.
Buffer Solutions
A buffer is a solution that resists changes in pH when small amounts of acid or base are added.
Composition: A buffer is a mixture of a weak acid and its conjugate base (e.g., CH₃COOH and CH₃COONa) or a weak base and its conjugate acid (e.g., NH₃ and NH₄Cl).
Mechanism:
If a strong acid (H⁺) is added, the conjugate base component of the buffer reacts with it: A⁻ + H⁺ → HA.
If a strong base (OH⁻) is added, the weak acid component of the buffer reacts with it: HA + OH⁻ → A⁻ + H₂O.
In both cases, the strong acid or base is converted into a weak component, causing only a small change in pH.
The Henderson-Hasselbalch Equation
This equation relates the pH of a buffer solution to the pKa of the weak acid and the ratio of the concentrations of the conjugate base and acid.
pKa = -log(Ka), where Ka is the acid dissociation constant.
Equation: pH = pKa + log ( [A⁻] / [HA] )
[A⁻] is the concentration of the conjugate base.
[HA] is the concentration of the weak acid.
Key Insight: This equation shows that when the concentrations of the acid and its conjugate base are equal ([A⁻] = [HA]), the ratio is 1, and log(1) = 0. In this case, pH = pKa. This is the point of maximum buffer capacity.