Strong acids and bases dissociate almost completely in an aqueous solution into respective ions and hence add H+ ion and hydroxyl ions into solution causing a change in pH.
Buffer is any substance which binds H+ ions reversibly and prevents the change in pH in case there is addition of acids or bases into a solution. Weak acids which do not completely ionise when dissolved in water behave as buffers. Fundamentally the weak acid acts as proton donor and its conjugate base acts as a proton acceptor.
Now when can be a buffer most effective and how one buffer differs from other buffers ?
A buffer will be most effective for handling changes in pH on either side when the proton acceptor and the proton donor are in equal concentrations. . This depends on the dissociation characteristics of any acid i.e its tendency to release hydrogen ions into a solution. The dissociation characteristic of the acids is studied using equilibrium constant or dissociation constant. Dissociation constant is equal to the dissociated ions i.e H+ ions and its conjugate base divided by the concentration of the undissociated acid in the solution. This dissociation constant can be expressed as negative logarithm. Its called pKa. Since its negative logarithm, pKa will be larger for weaker acids.
But how this dissociation constants helps us in understanding the characteristics of the buffer and when they are most effective ?
That is better understood with Henderson Hasselbach equation such that we are solving dissocitaion constant equation for H+.
pKa + log [A-] / [HA] = pH
Buffer is most effective for handling changes in pH on either side when the proton acceptor and the proton donor are in equal concentrations. When they are in equal concentration, pKa = pH. So at a pH which is equal to pKa of the buffer, the dissociation is such that the concentrations of proton donor and proton acceptor are equal..so at this pH…the buffer is most effective in either directions…
Physiological buffers
HCO3- buffer system:
pKa of this buffer is 6.i, so at pH 6.1 this buffer will exist equally in dissociated and undissociated form and will be most effective on both sides. But our body pH is 7.38 - 7.42 so that means at this pH which is higher than pKa , it will exist more in dissociated form. Well that’s good for us since bicarbonate ions will be available to bind to acids.
Phosphate buffer.
Its dissociation constant is 6.86. It also stays mostly in dissociated form and is more effective for capturing acids.
Ammonia buffer
pKa of ammonia buffer is 9.25….…this ammonia buffer system mostly exists as undissociated form i.e as ammonium ion…so actually it will not be able to bind with much hydrogen ions..so actually its a very poor buffer per se…however…the production of ammonium ion by the metabolism of glutamine also produces one bicarbonate which is added back into the blood…also in medullary inerstitium…this ammonium buffer exists in equilibrium with ammonia and ammonia diffuses from their into the collecting duct..where it binds with secreted hydrogen ions…..Not only that, it is one buffer whose production is regulated by kidneys….i.e its production increases in acidosis.
