NaOH is a strong base and bicarbonate buffer is made up a weak carbonic acid H₂CO₃ and its conjugate base HCO₃⁻ bicarbonate ion. The hydroxyl ion, OH- reacts with the H⁺ from the bicarbonate to form water.
Dissociation of carbonic acid:
H₂CO₃ → H⁺ + HCO₃⁻
HCO₃⁻ → H⁺ + CO₃²⁻
Dissociation of sodium hydroxide:
NaOH → Na⁺ + OH⁻
Neutralization reaction:
OH⁻ + H⁺ → H₂O