CHE 104 Chapter 17 - CHE 104 Chapter 17 Dr Mark Shuman Buffer Solutions A buffer solution is a solution of a certain pH that is relatively immune to the

# CHE 104 Chapter 17 - CHE 104 Chapter 17 Dr Mark Shuman...

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CHE 104 Chapter 17 Dr. Mark Shuman
Buffer Solutions A buffer solution is a solution of a certain pH that is relatively immune to the effect of addition of moderate amounts of acid or base. pH remains relatively constant when acid is added. pH remains relatively constant when base is added.
Demonstration Strong acid addition to : water pH4 buffer pH7 buffer pH10 buffer Strong base addition to: water pH4 buffer pH7 buffer pH10 buffer
Buffer Solutions (cont.) The most important buffer solution in your world is your blood. Naturally buffered to have a pH~7.4. The buffering of blood is vital because if the pH of blood gets “out of whack” by more than 0.1 or 0.2 pH units you will become very ill. Blood pH “out of whack” by 0.4 units will result in death.
Buffer Solutions (cont.) A buffer solution has a reservoir of component that can absorb added acid. A buffer solution has a reservoir of a component that can absorb added base. A buffer solution could contain both a weak acid and its conjugate base as major species in solution. A buffer solution could contain both a weak base and its conjugate acid as major species in solution.
Buffer Solutions (cont.) A buffer solution can contain both a weak acid and its conjugate base as major species in solution. Acetic acid (HAc) plus actetate anion (Ac - ). Note that a solution of HAc (Ka = 1.8x10 -5 ) has a very small concentration of Ac - already. Not enough to buffer against the addition of acid.
Buffer Solutions (cont.) 0.1M HAc containing 0.1M Ac - would function nicely as a buffer solution. This solution can be prepared easily in a couple of ways: Start with 0.1M HAc and add to it enough NaAc to make [AC - ] = 0.1 Start with 0.2M HAc and add enough NaOH to convert half the HAc to Ac - .
Buffer Solutions (cont.) A buffer solution can contain both a weak base and its conjugate acid as major species in solution. – Ammonia (NH 3 ) plus ammonium anion (NH 4 + ). – Note that a solution of NH 3 (K b = 1.8x10 -5 ) has a very small concentration of NH 4 + already. Not enough to buffer against the addition of base. – 0.1M NH 3 augmented by 0.1M NH 4 Cl would function nicely as a buffer solution.
pH of a Buffer Solution The pH of a buffer solution is going to depend on the K a and the concentrations of the weak acid and its conjugate base. (Note: the same statement can be made for a weak base and its conjugate acid and K b .)
pH of a Buffer Solution HA + H 2 O H 3 O + + A - • K a = [H 3 O + ] eq [A - ] eq / [HA] eq -log K a = -log [H 3 O + ] eq – log ([A - ] eq / [HA] eq ) -log [H 3 O + ] = -log K a + log ([A - ] eq / [HA] eq ) pH = pK a + log ([A - ] eq / [HA] eq ) We know for weak acids: [A - ] eq = [A - ] initial + x ~ [A - ] initial [HA] eq = [HA] initial - x ~ [HA] initial pH = pK a + log ([A - ] initial /[HA] initial )
pH of a Buffer Solution pH = pK a + log ([A - ] initial /[HA] initial )

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• Spring '08
• SHUMAN
• Chemistry, pH, Buffer solutions, buffer solution