Buffers - AP Chem Unit 8, Topics 8-10

Buffer Solutions

• Definition: Buffers are specialized acid-base mixtures that can resist pH changes when an acid or base is added.

• Composition:

  • Typically consist of a weak acid and its conjugate base (or a weak base and its conjugate acid).

  • Example: In blood, bicarbonate acts as a buffer to maintain pH around 7.4.

Characteristics of Buffers

• Buffers are crucial in laboratory reactions where maintaining a specific pH is necessary for optimal reactions.

• They allow for slight variations in pH without drastic changes.

Example of Buffer Formation

• Identifying Buffers:

  • Mixtures can form buffers if they include a weak acid and its conjugate base.

  • Example Test: Different mixtures were evaluated:

    • Option 1: Sulfuric acid (strong acid) + any base (not a buffer).

    • Option 2: Acetic acid (weak acid) + acetate ion (conjugate base) (this is a buffer).

    • Therefore, option 2 is a viable buffer.

Buffer Functionality

• Mechanism:

  • Adding acid to a buffer reacts with the conjugate base, limiting pH changes.

  • Adding base to a buffer reacts with the weak acid, similarly stabilizing pH.

pH Calculations with Buffers

• Henderson-Hasselbalch Equation:

  • Formula: pH = pKa + log([A-]/[HA])

  • pKa: Negative log of the acid dissociation constant (Ka) of the weak acid.

  • [A-]: Concentration of the conjugate base.

  • [HA]: Concentration of the weak acid.

Example Calculation

• Given: Solution of 0.60 M hydrochloric acid and 0.20 M sodium fluoride:

  • Calculate pH using ka of hydrochloric acid (Ka = 7.2 × 10^-4).

  • Final result: pH = 2.66.

Advanced Buffer Example

• Targeting a buffer with pH 8.0 using 0.50 M hypochlorous acid (Ka = 3.0 × 10^-8):

  • Solve for the concentration of sodium hypochlorite using Henderson-Hasselbalch equation:

    • pKa = 7.52 derived from -log(3.0 × 10^-8).

    • Solve maintaining the desired pH: Concentration of hypochlorite = 1.50 M using the ratio concept while isolating variables.

Buffer Ratios

• Understanding Capacity:

  • Changing buffer concentration while maintaining the same molar ratio (3:1) still results in the same pH.

  • Example illustrates maintaining a 3:1 base to acid ratio with buffer components.

Buffer Capacity

• Definition: The ability of a buffer to withstand changes in pH upon addition of acids or bases.

• More concentrated buffers have higher buffer capacity.

  • Example: A 1.5 M base with a 0.5 M acid greater absorbs added acids, whereas a buffer with less concentration (0.30 M base, 0.10 M acid) has a lower capacity to withstand pH change.

• Base and Acid Interplay:

  • A buffer rich in conjugate base neutralizes added acids.

  • A buffer rich in conjugate acid neutralizes added bases.

Conclusion

• Buffers play a critical role in maintaining pH in various chemical and physiological processes.

• Understanding buffer composition and calculations using the Henderson-Hasselbalch equation is essential for practical applications in chemistry.