Question

How do acid base indicators work?

Answer 1

Hint: We need to know what are indicators and accordingly understand the working of acid-base indicators. Indicators are used for the confirmation of the presence or absence of a particular species. This confirmation is indicated by a clearly visible sign such as a change in color or smell. Acid-base indicator is a type of such indicator where substances show a visible sign and confirm the presence of an acid or base.

Complete step by step answer:
We know that acid-base indicators show a visible sign and confirm the presence of an acid or base. Let us consider an acid with formula $HA$ whose conjugate base is ${A^ - }$ and the reaction is:
$HA(aq) + {H_2}O(l) \rightleftarrows {A^ - }(aq) + {H_3}{O^ + }(l)$
Using an acid-base indicator will help us to know if the reaction equilibrium is towards the right or the left. One example of such an indicator is phenolphthalein. Adding phenolphthalein to the above reaction will be colorless if the solution is acidic, i.e., equilibrium is towards the left and if the solution is basic, a magenta color (bright pink) is observed and the equilibrium is towards the right. Due to this phenomenon, the pH of the solution can be detected and hence they are also known as acid-base indicators.
Hence, acid- base indicators work on the basis that they have distinctly different colors when they come in contact with an acid and a base. Examples of such indicators are litmus paper, methyl orange, methyl red, phenolphthalein, thymol blue etc. They are mostly used in titration to signal the completion of acid-base reactions.

Note:
It must be noted that acid- base indicators are also called pH indicators. Another type of acid-base indicators which are not widely used are olfactory indicators. Two examples of olfactory indicators are Onion and vanilla extracts. When either of the two are mixed with a solution containing an acid, the smell of the indicator does not change. Whereas, when mixed with a solution containing a base, the smell of the indicator fades away and cannot be detected. Thus, these two indicators can be used to identify the solution as acid or base and hence also called acid-base indicators.