Question

Which of the following statements are correct regarding internal indicator in the following reaction?
$NaOH+HCl\to NaCl+H_{2}O$

• A. It is dissolved in the solution flask where main reaction is taking place
• B. It has reduced form colour in the flask before end point reaches
• C. Addition of drops of $HCl$ post to end point oxydises internal indicator
• D. Oxidised internal indicator has different colour
• E. All of these

Answer 1

Answer: (E)

All of these

An internal indicator is added to the titration flask for detecting the end point of a redox reaction as compared to an external indicator which is taken separately. The indicator has different colours in oxidized and reduced forms.

An acid-base titration comprises of a redox reaction. Suppose a $HCl$ Vs $NaOH$ titration, where $HCl$ is in burette and $NaOH$ in the flask.

Reaction:

$HCl+NaOH\to NaCl+H_{2}O$

Hence, $NaOH$ gets oxidized. So if we use an internal indicator, it will have its reduced form colour in the flask. When the titration reaches its end-point, all NaOH is oxidized. If we add some more drops of HCl, the indicator starts oxidizing and its colour changes. Hence readings aren't too accurate, but close enough.

eg. Methylene Blue: Oxidized form $\to$ Blue Color

Reduced form $\to$ Colorless