What does the Pi stand for in the reaction for biological nitrogen fixation? In the reaction for biological nitrogen fixation, what does the Pi stand for in the products? 
$$\ce{N2 + 8H + 8e- + 16 ATP -&gt; 2NH3 + H2 + 16ADP + 16 P_{i}}$$
It stands for inorganic phosphate (Pi). When ATP is broken down into ADP, energy is released along with a phosphate. You can visualise this if you look at the structure of ATP. ADP is then also able to be broken down into AMP and more Pi