Introduction
The carbonate chemistry of lakes and oceans is a key part of our planet’s habitability. Inorganic carbon is largely divided into three dissolved species: carbonate, bicarbonate, and carbon dioxide. The reactions that relate these species to one another involve protons. In most natural waters, the carbon system does most of the heavy lifting when it comes to determining pH.
On a more practical level, carbonate chemistry explains many aspects of the world around us, such as:
Why coral reefs are typically grow near the tropics
Why tiny carbonate shells dissolve in the deep ocean
Why sodas are acidic and why we store them at low temperatures
We will explore these concepts and more using tools from chemical oceanography.
Figure 1: Corals growing in warm, shallow waters in the Bahamas. In contrast, cool water outside the tropics are less favorable for corals. Photo credit: Kelly Hattori
The Bjerrum plot
A Bjerrum plot, named for Danish chemist Niels Bjerrum, is a tool for visualizing aqueous carbon species. The vertical axis usually depicts the concentration of the three primary species—carbonate (CO3), bicarbonate (HCO3), and carbon dioxide (CO2)—on a logarithmic scale based on powers of ten. For example, a tick mark labeled '“-6” would represent one millionth (10^-6) of a mole, also known as a micromole (umol).
The main job of a Bjerrum plot is to show how carbon species vary as a function of pH, which is the horizontal axis. Note that there are many other variables that also affect species concentrations, such as temperature, pressure, salt content, and the total amount of carbon. For a single Bjerrum plot, we treat these other factors as constants—if we want to change them, we have to draw a new plot.
The web app below draws a Bjerrum plot for the temperature—pressure—salinity conditions determined by the sliders. Hovering over the plot will bring up a tooltip which will allow you to read pH and the species concentrations to several decimal places. Use the sliders and tooltips to answer the questions below.