Nitric acid is generally avoided in titrations because it is itself an oxidising agent.
Understanding the Problem
In many titrations, particularly redox titrations, the acid is used to provide a suitable medium (e.g., H+ ions) for the reaction to occur correctly or to protonate species. The acid itself should ideally be inert to the reactants and products involved in the main titration reaction.
However, nitric acid (HNO₃) is a strong oxidizing agent, especially when concentrated or heated. This oxidizing property causes significant issues in titration:
- Interference with the main reaction: If the analyte or titrant is easily oxidized or reduced, nitric acid can react with them directly. This consumes the analyte or titrant in side reactions, leading to inaccurate titration results.
- Reacting with the titrant: In titrations involving reducing agents as titrants, nitric acid can oxidize the titrant itself. For example, in the titration of iron(II) with a strong oxidant, using nitric acid could oxidize the iron(II) prematurely or react with the titrant used to measure it.
- Reacting with the indicator: The oxidizing nature of nitric acid can also affect redox indicators, potentially causing them to change color prematurely or irreversibly, making it impossible to determine the endpoint accurately.
Contrast with Suitable Acids
Acids like dilute sulfuric acid (H₂SO₄) are often preferred in redox titrations (like those involving potassium permanganate, KMnO₄) because they provide the necessary acidity without acting as significant oxidizing or reducing agents under typical titration conditions. Hydrochloric acid (HCl) is also sometimes avoided, as mentioned in some contexts, because it can be oxidized by strong oxidizers like KMnO₄ to produce chlorine, which is also an interfering oxidizing agent.
In summary, the primary reason nitric acid is not used in titrations, especially redox titrations, is due to its inherent property as an oxidizing agent, which interferes with the desired chemical reaction being measured.
