The pH of sodium bicarbonate solution depends on

For a salt such as ${\mathrm{NaHCO}}_3$ , pH of the solution is given by 

$\mathrm{pH}=\frac{1}{2}\left(\mathrm{p}{K}_{\mathrm{a}1}^{\circ }+\mathrm{p}{K}_{\mathrm{a}2}^{\circ }\right)$

where $K_{\mathrm{al}}$  and $K_{\mathrm{a}2}$ are the ionization constants of ${\mathrm{H}}_2{\mathrm{CO}}_3$ and${\mathrm{HCO}}_3^{-}$, respectively