Buffers

When is a buffer most effective?

When the concentrations of weak acid and conjugate Base are equal = When pH of buffer solution is the same as the pKa of HA Operating pH is typically over about two pH units, centred at the pH of the pKa value

What is the pH of a buffer when [HA] and [A-] are the same?

Ka = [H+] pKa = pH

How can you calculate the ratio of A- to HA?

Ka/[H+] = [A-]/[HA]

What is present in a buffer solution at equilibrium?

Lots of undissociated HA Lots of acid's conjugate base A- Enough H+ to make the solution acidic

How is a buffer made with a strong base?

Mix an excess of weak acid with a limited amount of strong base e.g. NaOH If all NaOH reacts to form a salt there will still be a lot of acid left and so a buffer is made as the solution is a mixture of the weak acid and its conjugate base

How is a buffer made with a salt?

A mixture of a weak acid and its conjugate base - salt of the weak acid Weak acid will only dissociate a little and so provides the weak acid component of buffer Salt will completely dissolve and so dissociation into ions is complete meaning the conjugate Base component of the buffer is provided

What is a buffer?

A solution which minimises pH changes on the addition of small amounts of an acid or a base Will only resist a change

What equilibria are present in the buffer?

A: HA -> H+ + A- B: NaA -> Na+ + A-

What is the overall equation for the blood buffer?

CO2 + H20 -> H2CO3 -> H+ + HCO3-

What happens if the blood pH rises?

Equilibria shifts right OH- ions are removed from the blood as they react with small concentration of [H+] and produce water

What happens if the blood pH falls?

Equilibrium shifts left H+ ions are removed from blood to produce more H2CO3 as they react with the conjugate Base HCO3- H2CO3 conc then increases so equilibria shifts left to produce CO2 and H2O

What happens when you add acid to a buffer solution?

H+ ions react with the large reservoirs of A- to form HA H+ + A- -> HA Equilibrium shifts to the left to use up the H+ so the pH remains the same

The body produces more acidic materials than alkaline, producing more H2CO3, how does the body stop this from building up?

H2CO3 is converted into CO2 which is exhaled by the lungs

What buffer is there is the blood?

Hydrogen carbonate buffer H2CO3 -> H+ + HCO3- CO2 + H2O -> H2CO3 To ensure blood pH stays between 7.35 - 7.45

What would happen without this buffer in the blood?

If pH fell below 7.35, a condition called acidosis would develop causing fatigue, shortness of breath, shock and death If pH rises above 7.45, a condition called alkalosis develops causing muscle spasms, light headed ness, and nausea

What happens when you add a base to a buffer solution?

OH- reacts with H+ to form water H+ + OH- -> H2O Equilibrium A shifts right to replace the H+ ions removed HA -> H+ + A- Overall equation = HA + OH- -> H2O + A- OH- is removed and ph remains

What assumptions are made when calculating the Ka of a buffer solution?

That the salt is fully dissociated Excess A- from equilibrium B pushes equilibrium A so far left that the dissociation of HA is negligible

What happens after adding excess acid or base to a buffer?

The buffer becomes exhausted and it will no longer act as a buffer

How do you find the pH of a buffer?

[H+] = Ka x [HA]/[A-] Ka = acid dissociation constant [HA]/[A-] = ratio of weak acid and conjugate Base

What is the Ka of a buffer?

[H+][A-]/[HA] Where [A-] = original conc of conjugate base [HA] = original conc of weak acid

What is the Henderson-Hasselbalch equation? What is it used for?

pH = pKa + log [A-]/[HA] Used to find pH and allows you to see how pKa and Base/acid ratio control the pH