Blood Plasma: Blood plasma needs to be at pH 7.4 due to enzymes. pH controlled by buffers
such as the carbonic acid- hydrogencarbonate buffer. The body produces more acidic materials
than alkaline. Buffers make sure H2CO3 not built up.
Problems with pH: If pH falls below 7.35, develop acidosis condition. If pH rises above 7.45,
develop alkalosis condition.
Carbonic Acid- Hydrogencarbonate Buffer System: Works in the same way as learnt above.
However conjugate base A- is HCO3- and acid is H2CO3.
H2CO3 ⇌ H
+
+ HCO3-
Calculating Ratio of A-/ HA: For example HCO3-/ H2CO3 in solution.
1) Convert pH into [H+ (aq)] if not done already. 10-pH
.
2) Convert pKa into Ka if not done already. 10-pKa = Ka.
3) Rearrange equation above to find pH of buffer solution to get…
and divide Ka and H+ to get a single number n which represents A-/
HA or n:1. Don’t need to sub in HA.
4) OR To get a ratio divide top and bottom of fraction by smallest number.
Here A- is in top so may have to switch H+/ Ka around.
If find out ratio A- : HA, it will be in form n: 1. Which means for every n amount of moles of A-,
there is 1 mole of HA. So if know moles of HA can work out moles of A-, by multiplying by n.
Write an expression for Ka…
How can bubbling CO2 through a mixture alter the equilibrium: CO2 + H2O -> CO3
2- + 2H+ (CO2 +
H2O -> H2CO3). H+ + OH- -> H2O OR H2CO3 (HA) + OH -> H2O + CO3
2
-. CO2 is slightly acidic.
Equilibrium shifts right to restore OH- (as have removed them from reaction with acid).
Swallowing soluble HA, so COO- reacts with H+ in stomach to form HA again. So HA precipitates
out.