$A$ $0.1 \ N$ solution of $Na_2CO_3$ is titrated with $0.1 \ N$ $HCl$ solution. The best indicator to be used is

If a very small amount of $HPh$ (phenolphthalein) is added to a $0.1 \ M$ aqueous solution of $CH_3COONa$,the fraction of the indicator that exists in the colourless form is:
$[K_b(CH_3COO^{-}) = 10^{-9}, pK_a(HPh) = 9.6, \log 2 = 0.3]$

Indicator for the titration of $HCl$ and $Na_2CO_3$ would be

Which indicator can be used in the titration of a strong acid and a strong base?

The rapid change of $pH$ near the stoichiometric point of an acid-base titration is the basis of indicator detection. The $pH$ of the solution is related to the ratio of the concentrations of the conjugate acid $(HIn)$ and base $(In^{-})$ forms of the indicator by the expression:

The $pH$ indicators are