Chapter 18: Aqueous Ionic Equilibria

Buffer Solutions: Definition, Components, and Properties

Buffer solutions are essential in maintaining a relatively constant pH in a variety of chemical and biological systems. They are typically composed of a weak acid and its conjugate base, or a weak base and its conjugate acid, at specific concentrations or ratios.

• Types of Buffers:

  • Acidic Buffer: Mixture of a weak acid (HA) and its conjugate base (A−), e.g., acetic acid and sodium acetate.

  • Basic Buffer: Mixture of a weak base (B) and its conjugate acid (HB+), e.g., ammonia and ammonium chloride.

• Role of Soluble Salts: Soluble salts are often used to provide the conjugate acid or base component in buffer solutions.

• Examples:

  • Acetic acid/sodium acetate (HC2H3O2/NaC2H3O2)

  • Ammonia/ammonium chloride (NH3/NH4Cl)

  • Carbonic acid/sodium bicarbonate (H2CO3/NaHCO3)

  • Sodium bicarbonate/sodium carbonate (NaHCO3/Na2CO3)

• Naturally Occurring Buffers:

  • Blood maintains a constant pH (~7.45) due to buffers such as the H2CO3/HCO3− system.

  • Natural water's resistance to acid rain is determined by its buffer system, primarily carbonic acid and bicarbonate.

Understanding Buffer Action

The fundamental property of a buffer solution is its ability to resist changes in pH upon the addition of small to moderate amounts of strong acid or base, or upon dilution.

• Buffer Equilibrium:

  • For a weak acid buffer:

  • Acid dissociation constant:

• Action Against Added Acid: Added H3O+ reacts with A− to form HA, increasing [HA] and decreasing [A−].

• Action Against Added Base: Added OH− reacts with HA to form A−, decreasing [HA] and increasing [A−].

• Result: The ratio [A−]/[HA] changes only slightly, so pH remains nearly constant.

Henderson-Hasselbalch Equation and pH Calculation

The Henderson-Hasselbalch equation provides a convenient way to calculate the pH of buffer solutions using the concentrations of the acid and base components.

• Derivation:

  • Start from

  • Rearrange and take the negative logarithm:

• Generalized Form:

• For Weak Base/Conjugate Acid Buffers:

• Key Features:

  • If [base] = [acid], then .

  • Each tenfold change in the ratio [base]/[acid] changes pH by 1 unit.

  • Dilution does not affect pH, as the ratio remains constant.

• Diprotic Acid Buffer Systems:

  • First dissociation:

  • Second dissociation:

Example Calculations

• Acidic Buffer Example: 0.30 M acetic acid and 0.20 M sodium acetate,

• Basic Buffer Example: 0.20 M ammonia and 0.50 M ammonium chloride,

  • First, find for :

Limitations of the Henderson-Hasselbalch Equation

• Not valid if or if concentrations are less than M.

• In such cases, a quadratic equation must be used for accurate pH calculation.

Preparation of Buffer Solutions

Buffer solutions are prepared in the laboratory by mixing calculated amounts of the acid and conjugate base (or base and conjugate acid) to achieve a desired pH and total concentration.

• General Steps:

  1. Identify the relevant acid dissociation equation (monoprotic, diprotic, or triprotic).

  2. Select the appropriate acid/conjugate base pair and obtain and molar mass values.

  3. Calculate the required concentrations and masses for each component using the Henderson-Hasselbalch equation and the total buffer concentration.

  4. Weigh the solid components or measure liquid components using their density.

  5. Mix and adjust the pH as needed using a pH meter and small additions of strong acid or base.

• Example Calculation:

  • Assigned buffer: sodium bioxalate/sodium oxalate, target pH = 4.77, total concentration = 0.20 M.

  • Let ,

  • Solve for to find the required concentrations, then calculate masses using molar masses and solution volume.

• Mass Calculations:

  • For 0.050 M NaHC2O4 in 100.0 mL: g

  • For 0.15 M Na2C2O4 in 100.0 mL: g

• Practical Considerations:

  • Measured pH may differ from calculated values due to uncertainties in dissociation constants and calculation simplifications.

  • Final pH adjustment is performed using a pH meter and dropwise addition of strong acid or base, with continuous stirring.

Additional info: The notes emphasize the importance of buffer capacity and the practical steps for buffer preparation, including the use of analytical balances and burets for accurate measurement. The limitations of the Henderson-Hasselbalch equation are also highlighted, ensuring students understand when more advanced calculations are necessary.