# 1. Acid-Base Chemistry of Natural Aquatic Systems

Natural waters contain a wide variety of solutes that act together to determine the pH, which typically ranges from 6 to 9. Some of the major processes that affect the acid-base balance of natural systems are:

• Contact with atmospheric carbon dioxide
• Input of acidic gases from volcanic and industrial emissions
• Contact with minerals, rocks, and clays
• Presence of buffer systems such as carbonate, phosphate, silicate, and borate
• Presence of acidic anions, such as $$Fe(H_2O)^{3+}_6$$
• Input and removal of $$CO_2$$ through respiration and photosynthesis
• Other biological processes, such as oxidation ($$O_2+ H^+ +e^- \rightarrow H_2O$$), nitrification, denitrification, and sulfate reduction.

In this chapter and also in the next one which deals specifically with the carbonate system, we will consider acid-base equilibria as they apply to natural waters. We will assume that you are already familiar with such fundamentals as the Arrhenius and Brfinsted concepts of acids and bases and the pH scale. You should also have some familiarity with the concepts of free energy and activity. The treatment of equilibrium calculations will likely extend somewhat beyond what you encountered in your General Chemistry course, and considerable emphasis will be placed on graphical methods of estimating equilibrium concentrations of various species.

## Contributors and Attributions

Stephen Lower, Professor Emeritus (Simon Fraser U.) Chem1 Virtual Textbook