Question

Why is HNO3 added to the Fe(NO3)3 solution used in chemical equilibrium experiment? How does the HNO3 accomplish its function?

Answer 1

HNO₃ serves following purposes:

• It maintains concentration of H⁺ at a fixed value and keeps the reaction mixture fairly acidic and equilibrium does not results in increased amount of products.
• It also keeps Fe in ferric (Fe³⁺) state and prevents it reduction to Fe²⁺

HNO₃ accomplishes its function by releasing H⁺ ions which is the product of equilibrium reaction involved i.e.

Fe³⁺ + HSCN <-----> Fe(SCN)²⁺ + 3H⁺

This prevents formation of more Fe(SCN)²⁺ and hence, the absorbance remains in measurable range i.e. between 0.8 to 0.3 absorbance units and hence error is minimized.