Why does concentrated nitric acid often have a yellow color even though pure \(\mathrm{HNO}_{3}\) is colorless?

Nitric acid (HNO3), a powerful and versatile reagent, is an important chemical in both industrial and laboratory settings. Pure nitric acid is colorless, and this clarity indicates a lack of impurities.

When discussing concentrated nitric acid, what's actually being referred to is a solution with a high percentage of HNO3, typically above 70%. Concentrated nitric acid is more likely to exhibit a yellow or even orange hue due to the presence of nitrogen oxides, mainly nitrogen dioxide (NO2) and dinitrogen tetroxide (N2O4).

Nitric acid is not only a strong acid but also a powerful oxidizing agent. It can react with a wide range of substances, from metals to organic compounds, often resulting in notable energy release and sometimes explosive outcomes.

Due to its reactivity, nitric acid is carefully handled and stored, as it can cause severe burns upon contact with skin and the vapors can be harmful if inhaled. The acid is also hygroscopic, meaning it readily absorbs water from the air, which can lead to dilution and the further release of nitrogen oxides.

Let's delve deeper into the chemistry behind the discoloration of concentrated nitric acid. The primary culprit behind the color change is nitrogen dioxide (NO2), a reddish-brown gas that forms when nitric acid decomposes.

Decomposition Reaction

The reaction that leads to the formation of nitrogen dioxide can be represented as:
\[\[\begin{align*}2 HNO3 &\rightarrow H2O + 2 NO2 + \frac{1}{2} O2\right.\end{align*}\]\]
In this decomposition reaction, water (H2O) and molecular oxygen (O2) are also produced along with NO2.

The nitrogen dioxide that forms is not only responsible for the yellowing of nitric acid but also serves as an indicator of the strength and purity of the solution. A more pronounced yellow color suggests a larger quantity of nitrogen oxides, which in turn implies a greater concentration or potential degradation of the nitric acid.

A continuation of the processes leading to the discoloration of concentrated nitric acid involves the gas nitrogen dioxide (NO2) combining to form dinitrogen tetroxide (N2O4).

Dimerization Process

The dimerization reaction can be written as:
\[\[\begin{align*}2 NO2 &\leftrightarrow N2O4\right.\end{align*}\]\]
This is an equilibrium reaction, meaning that nitrogen dioxide and dinitrogen tetroxide can convert back and forth between each other. Dinitrogen tetroxide is colorless at low temperatures but can revert to nitrogen dioxide at higher temperatures or when the pressure is reduced, contributing to the typical yellow color of concentrated nitric acid.

Understanding the equilibrium between NO2 and N2O4 is crucial for working with nitric acid, as it affects not only the color but also the vapor pressure and reactivity of the solution. These gaseous oxides of nitrogen are significant as they can impact the health and safety of those handling nitric acid and also have implications in atmospheric chemistry and pollution.