Buffers and Henderson-Hasselbalch Eq

Buffers

They are solutions that resist changes in pH when an acid or base is added

How do buffers act

They act by neutralizing acid or base that is added to the buffered solution

Composition of Buffers

Buffers are made by mixing a solution of a weak acid with a solution of soluble salt containing its conjugate base anion
- weak acid + conjugate base salt

Buffer composition of blood

Mixture of H2CO3 (Carbonic Acid) and HCO3- (Bicarbonate)

How Acid Buffers work: Addition of Base

If a strong base is added to a buffer, the weak acid will give up its H+ in order to transform the base (OH-) into water (H2O) and the conjugate base
HA + OH- → A- + H2O.

What happens if a strong base is added to a buffer solution

The amount of the weak acid decreases while the amount of the conjugate base increases

How Acid Buffers work: Addition of Acid

If a strong acid is added to a buffer, the weak base will react with the H+ from the strong acid to form the weak acid
HA: H+ + A- → HA
- The H+ gets absorbed by the A- instead of reacting with water to form H3O+ (H+), so the pH changes only slightly.

Common Ion Effect

HA(aq) + H2O → A-(aq) + H3O+ (aq)

Adding a salt containing the anion HA (conjugate base of the acid- the common ion), shifts the equilibrium to the left

What happens because of the Common Ion Effect

It causes the pH to be higher than the pH of the acid solution
- lowers the H3O+ ion concentration

Use of Henderson-Hasselbalch Equation

It calculates the pH of a buffer from the pKa and initial concentrations of the weak acid and salt of conjugate base
(as long as x is small, approxiation is valid)

Henderson-Hasselbalch Equation

When to use the Henderson-Hasselbalch Equation

When the "x is small" approximation is applicable

When will the "x is small" approximation work

Both should be true
- The initial concentrations of acid and salt are not very dilute (should be over 100 to 1000x larger than Ka value)
- Ka is fairly small

Basic Buffers

B:(aq) + H2O(l) → H:B+ (aq) + OH-(aq)
Buffers made by mixing a weak base (B:) with a soluble salt of its conjugate acid (H:B+Cl-)

Henderson-Hasselbalch Equation for Basic Buffers

pH is baliktad, its log ([B:]/[H:B+])

Buffering Capacity

amount of acid or base a buffer can neutralize

Buffering Range

pH range the buffer can be effective

Effectiveness of Buffers depends on

1. Relative amounts of acid and base
2. Absolute concentrations of acid and base

Conditions where buffers are most effective

1. MOST EFFECTIVE: equal concentrations of acid and base
2. EFFECTIVE: 0.1 < [base]:[acid] < 10
3. MOST EFFECTIVE: [acid] and [base] are large

Calculation of pH range (highest and lowest ph)

- highest: pKa +1
- lowest: pKa -1

When choosing an acid to make a buffer, choose ...

Choose an acid whose pKa is closest to the pH of the butter

how to find the solution that has the highest buffer capacity

one that has the most components available to react with the added base/acid to resist change, highest concentration or M

Weak bases

NH4OH, N(CH3)3, C5H5N, NH4OH, H2O, C6H5NH2, NH3, PH3

Weak Acids

HClO2, HF, HNO2, HC2H3O2, HClO, HBrO, HCN, HIO, CH3COOH, H2S, H2CO3

Buffer solutions must contain

1. Relatively large concentration of acid to react with any OH ions to it
2. A similar concentration of base to react with any added H+ ions

What should not happen during the neutralization reaction

Buffer's acid and base components must not consume each other

If the ratio of \[HA\]=\[A-\] then

pH = pKa

If \[HA\] > \[A-\] then

pH < pKa

If \[A-\] > \[HA\]

pH > pKa

What phenomenon accounts the mechanism of buffers

Common Ion Effect

What is Common Ion Effect

happens when an ion is added to a mixture (already in equilibrium) that already contains the same ion causes the position of the equilibrium to shift away from forming more of it.

Buffer capacity depends on what

amount of acid and base used to prepare a buffer

* the larger the amount of acid and base, the greater its buffer capacity