C21 - Buffers and Neutralisation Flashcards
What’s a buffer solution?
A solution which can oppose changes in pH when small quantities of acid or base are added.
What’s an acidic buffer?
A buffer consisting of a weak acid and its conjugate base (usually in salt form) e.g. a mix of CH3COOH and CH3COONa.
What’s a basic/alkaline buffer?
A buffer which consists of a weak base and its conjugate acid (usually in salt form).
E.g. A mix of NH3 and NH4Cl
How does the addition of H+ and OH- ions affect an acidic buffer?
E.g. CH3COOH H+ + CH3COO-
Increase in H+ shifts the position of eq to the left
H+ + CH3COO- -> CH3COOH
Increase in OH- forms water so eq shifts to the right to produce more H+
OH- + H+ -> H2O
What are the two ways to make a buffer?
1) By adding a weak acid (HA) and its conjugate base (e.g. NaA which provides A- ions).
OR
2) Partial neutralisation - when a small quantity of a strong base is added to a weak acid.
When s a buffer most effective at removing the acid or alkali added?
When there are equal concentrations of weak acid and its conjugate base.
[HA]=[A-]
The pH of the buffer = the pKa value of HA
It’s operating pH is over approx a unit above and below the pH of the pKa value.
What is the carbonic acid-hydrogencarbonate buffer equilibrium in the body?
H2CO3 H+ + HCO3-
What happens to the carbonic acid-hydrogencarbonate buffer equilibrium in the body when an acid or alkali is added?
Adding acid:
[H+] increases which react with the conjugate base, HCO3-.
Equilibrium position shifts to the left to remove most H+ ions.
Adding alkali:
[OH-] increases and reacts which H+ to form water. Equilibrium shifts to the right to produce more H+.
What do the parts of a pH titration curve demonstrate?
Large horizontal region at low pH - excess of acid
Vertical section - rapid change of pH due to the addition of a small volume of acid or base.
Equivalence point - the centre of the vertical section
Large horizontal region at high pH - excess of alkali
What does the equivalence point of a pH titration curve show?
The volume of one solution that exactly reacts with the volume of the other solution.
What are acid-base indicators?
Weak acids with a distinctly different colour from its conjugate base, e.g. Methyl orange or phenolphthalein.
What’s the end point of a titration?
When the acid-base indicator contains equal concentrations of weak acid, HA, and conjugate base, A-.
What indicator should be used for titration with a high pH and a low pH?
Low pH (approx 3) - methyl orange
High pH (approx 10) - phenolphthalein
