In Mohr Salt Titration:
- Acidification: Mohr's salt is poorly soluble in neutral or basic solutions, so adding HCl creates an acidic environment that enhances its solubility and ensures complete dissolution.
- Formation of Fe(II) Ions: In acidic conditions, the Fe(II) ions from Mohr's salt are present in the form of $Fe^{2+}$ ions. These $Fe^{2+}$ ions undergo oxidation by the oxidizing agent (potassium dichromate) in the titration reaction.
- Indicator: In Mohr's salt titration, an indicator such as ferroin or phenanthroline is used. These indicators undergo a color change in the presence of Fe(II) and Fe(III) ions. The color change indicates the endpoint of the titration when all the Fe(II) ions have been oxidized to Fe(III).
In Potassium Dichromate Titration:
- Acidification: Similar to Mohr's salt titration, HCl is added to acidify the solution containing potassium dichromate. Acidic conditions ensure that the dichromate ions ($Cr_2O_7^{2-}$) are protonated, forming the active oxidizing species, hydrogen dichromate ions ($H_2Cr_2O_7$).
- Redox Reaction: In acidic conditions, the $H_2Cr_2O_7$ ions act as the oxidizing agent, accepting electrons from the reducing agent (usually an analyte being analyzed). The dichromate ions are reduced to Cr(III) ions ($Cr^{3+}$).
- Visual Endpoint: During the titration, the orange color of the dichromate ions gradually changes to green as the reduction takes place. This color change indicates the endpoint of the titration.
Therefore, HCl is essential in both Mohr's salt and potassium dichromate titrations to create an acidic environment, ensure complete dissolution of reactants, and facilitate the redox reactions that occur during the titration processes.
