# What is the function of an indicator in a titration?

Jun 9, 2018

To signal a stoichiometric endpoint....

#### Explanation:

Let us examine a simple acid base reaction....

$N a O H \left(a q\right) + H C l \left(a q\right) \rightarrow N a C l \left(a q\right) + {H}_{2} O \left(l\right)$

And for the purposes of the example we assume we add a solution of sodium hydroxide FROM a burette, to a KNOWN volume of $H C l \left(a q\right)$ of KNOWN concentration.

And let us consider the $p H$ of such a typical strong acid/strong base titration...

(Perhaps I could have used a BIGGER illustration...)

Now the equivalence point is CLEARLY at $p H = 7.0$...and while ideally, we would like the indicator to change colour at this point, as shown in the graph, the $p H$ changes sigmoidally over SIX whole pH units. And if you look at the abscissa of the $\text{x-axis}$, the dramatic $p H$ change occurs over the addition of $0.01 \cdot m L$...and this is about $1 \cdot \text{drop}$ of titrant...and so several indicators are feasible in the given scenario. As always with these titrations, we titrate to the POINT of colour change, NOT the colour change itself.