Natural Sources of Nitric Acid

Nitrogen undergoes a complicated cycle throughout the biosphere. Beginning with molecular nitrogen N, which constitutes about four-fifths of the atmosphere, nitrogen can be converted into other forms that include such species as NO, NO, NO 2, NO3, NjO, NH3, and NH 4. Eventually, some nitric acid will be formed, for example, by the following set of reactions  

Both NO and NO are toxic. Nitric acid (NO) is odorless and colorless, but nitrogen dioxide (NOj) has a pungent odor and reddish-brown color that is easily visible in polluted air. As in the oxidation of SO to SO3, the first and second reactions are both more complicated than shown, but they give an idea of how N O is formed as the important precursor to HNO3. 

Notice the oxidation states of nitrogen in the various species shown in these equations. 

The oxidation state of nitrogen in HNO3 is -i-5. The oxidation state of nitrogen in the other species are as follows  

Just as with the formation of H SO, the formation of HNO3 is an acid-base reaction, which, again by definition, must occur with no change in nitrogens oxidation state. Therefore, by the reactions shown, nitrogen is slowly oxidized first to the +4 state in NOj, and then to the +5 state in N O, which is soluble in water, forming HNO3. 

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