The amount of sodium hydrogen carbonate, ${\mathrm{NaHCO}}_3$, in an antacid tablet is to be determined by dissolving the tablet in water and titrating the resulting solution with hydrochloric acid. Which indicator is the most appropriate for this titration?

Acid	$K_a$
${\mathrm{H}}_2{\mathrm{CO}}_3$	$2.5\times {10}^{-4}$
${\mathrm{HCO}}_3^{-}$	$2.5\times {10}^{-8}$

Methyl orange is most suitable. Methyl orange is a frequently used pH indicator in titrations because of its straightforward and obvious colour shift. Due to the fact that it changes colour at the pH of a moderate acid, it is frequently employed in acid titrations. Methyl orange changes colour from yellow in a basic medium to red in an acidic one.