Hb is a good buffer because of
Ref Robbins 9/e p650 Haemoglobin also functions in CO2 and proton transpo from tissues to lungs . Release of O2 from Oxy Hb at the tissue is accompanied by uptake of proton due to lowering of the pKa of histamine residues
Histidine residues
Protein nature
Acidic nature
Iron molecules
Haematology