Question

What is an indicator? Explain Ostwald theory of indicator. Explain titration between $$HN{O}_3$$ and $$KOH$$ using suitable indicator with the help of a curve.$$$$

Answer 1

Hint: In the given question, we will first discuss indicator, then learn about Ostwald's Indicator Theory in detail with an example for better understanding, and last, we will learn about titration between $$HN{O}_3$$ and $$KOH$$ employing appropriate indicators with the use of a curve.

Complete answer: Indicator:-
A chemical compound that changes colour at the end point is referred to as an indicator. Acid-base indicators can be used to determine the end point of an acid-base titration. Weak organic acids or weak organic bases are acid base indicators. Within a given pH range, they change colour. Two theories have been presented to explain why acid-base indicators change colour as pH changes.
Indicator Theory of Ostwald:-
According to this theory:-
(a) The acid-base indicator ionises, causing the colour shift. The colour of the unionised form differs from that of the ionised form.
(b) Because it is either a weak acid or a weak base, the ionisation of the indication is greatly impacted in acids and bases. If the indicator is a weak acid, its ionisation in acids is relatively low due to ubiquitous $$H^{+}$$ ions, while it is fairly ionised in alkalies. Due to common $$O{H}^{-}$$ ions, if the indicator is a weak base, its ionisation is great in acids and low in alkalies.
Phenolphthalein (a weak acid) and Methyl orange are two significant markers to consider (a weak base). The following is an illustration of the Ostwald theory:
Phenolphthalein : $$HPh$$ is a symbol that can be used to symbolise it. In solution, it ionises to a modest amount as:
$\underset{Colourless}{HPh}\rightleftharpoons H^{+}+\underset{Pink}{P{h}^{-}}$
Using the law of mass action
$K={\displaystyle \frac{\left[H^{+}\right]\left[P{h}^{-}\right]}{\left[HPhs\right]}}$
Phenolphthalein molecules that have not been dissociated are colourless, whereas $$P{h}^{-}$$ ions are pink. The ionisation of $$HPh$$ is almost minimal in the presence of an acid because the equilibrium shifts to the left hand side due to the high concentration of $$H^{+}$$ ions. As a result, the solution would be colourless. Hydrogen ions are eliminated by $$O{H}^{-}$$ ions in the form of water molecules when alkali is added, and the equilibrium moves to the right. As a result, the concentration of $$P{h}^{-}$$ ions in solution rises, giving the solution a pink hue.
Titration between $$HN{O}_3$$ ​(strong acid) and $$KOH$$ (Strong base):-
The base is placed in a burette, while the acid is placed in a beaker for this titration. Strong acid has a very low pH. When base is introduced, the pH changes slowly at first, but by the end, the pH has jumped from 3 to 10. The titration curve is the result of plotting the volume of base used and the pH change. Strong acid and strong base come in a wide variety of strengths. This range has a large number of indicators.
The pH curve for adding a strong acid to a strong base is depicted in the diagram. The pH ranges for methyl orange and phenolphthalein are superimposed on it.
The indicator does not change colour at the equivalence point, as you can see.
However, because the graph is so steep at that point, there will be little variation in the amount of acid added regardless of which indicator you use. With each indicator, though, it would make sense to titrate to the best possible colour.
If you're using phenolphthalein, titrate until it's completely colourless (pH 8.3), which is the closest you can get to the equivalence point.

Note:
Titration is a typical quantitative chemical analysis procedure used in laboratories to quantify the concentration of a specified analyte. The titrant or titrator is a reagent that is made as a standard solution with a known concentration and volume.