Question

The amount of sodium hydrogen carbonate, $NaH{CO}_3$, 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 $K_a$
$H_2{CO}_3$ $2.5\times {10}^{-4}$
${H{CO}_3}^{-}$ $2.4\times {10}^{-8}$

• A. Methyl orange, $p{K}_{In}=3.7$
• B. Bromothylmol blue, $p{K}_{In}==7.0$
• C. Phenolphtalein, $p{K}_{In}=9.3$
• D. Alizarin yellow, $p{K}_{In}=12.5$

Answer 1

Answer: (A)

Methyl orange, $p{K}_{In}=3.7$

The indicator should be used such a way that it shows change in colour in the same $pH$ range as required around the equivalence point. Now when solution of ${NaHCO}_3$ is titrating against $HCl$ solution just after equivalence point there will be presence of very low amount of $HCl$ and $pH$ will be around $\sim 3.6$.

$pH={pK}_a+log\frac{\left[{HCO}_3^{-}\right]}{\left[H_2{CO}_3\right]}=3.6+log\frac{\left[{HCO}_3^{-}\right]}{\left[H_2{CO}_3\right]}$

$\therefore$ So Methyl orange having $pK$ in if $3.7$ is the most appropriate for this titration.