Chemical and Ionic Equilibrium 2 Question 28
28. The best indicator for detection of end point in titration of a weak acid and a strong base is
$(1985,1 \mathrm{M})$
(a) methyl orange (3 to 4 )
(b) methyl red (5 to 6)
(c) bromothymol blue (6 to 7.5 )
(d) phenolphthalein (8 to 9.6)
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Solution:
- When a weak acid $(\mathrm{H} X)$ is titrated against a strong base $\mathrm{NaOH}$, basic salt $(\mathrm{Na} X)$ is present at the end point which makes end point slightly basic with $\mathrm{pH}$ around 8 . Hence, phenolphthalein, that changes its colour in this $\mathrm{pH}$ range, would be the best choice of indicator to detect the end point.
