pH Calculations

Introduction to pH

pH is a measure of the acidity or alkalinity of a solution.
Defined as the negative logarithm (base 10) of the concentration of hydronium ions (H3O+), or traditionally, hydrogen ions (H+).
A pH less than 7 indicates an acid, pH greater than 7 indicates a base, while a pH of 7 is neutral.

pH Scale in the Ocean

The ocean’s pH scale typically ranges from about 7.5 to 8.4, making it mildly basic.
The average ocean surface pH is approximately 8.1 and has decreased by about 0.1 units since the pre-industrial era due to increasing concentrations of CO2 in the atmosphere.
Changes in oceanic pH, a phenomenon known as ocean acidification, can severely impact marine life, particularly organisms building shells or skeletons made of calcium carbonate (such as corals, oysters, and certain plankton).

Calculations involving pH

The pH of a solution can be calculated with the formula pH = -log[H+].
Note that [H+] represents the molar concentration of H+ in solution.
For an acidic solution, if you know the concentration of H+, you can calculate the pH directly.
For a basic solution, you will need to calculate the concentration of OH- (hydroxide ions) first, and then use the formula pOH = -log[OH-] and remember that pH + pOH = 14 at 25°C.

Role of Carbon Dioxide in pH

Carbon dioxide (CO2) reacts with seawater to form carbonic acid (H2CO3), which disassociates to produce bicarbonate (HCO3-) and hydrogen ions (H+), ultimately increasing the acidity and decreasing the pH.
Carbonate (CO32-) can combine with H+ to form bicarbonate, reducing the availability of carbonate for organisms needing it to build shells and skeletons.
The ability of the oceans to absorb CO2 can be quantified using pH buffering calculations, which look at how the concentration of weak acids or bases in the ocean can moderate the pH.

Ocean Acidification

Ocean acidification refers to the ongoing decrease in the pH of the Earth’s oceans, caused by the uptake of anthropogenic CO2 from the atmosphere.
Monitoring oceanic pH levels is critical in understanding the impacts of climate change on marine ecosystems.
There are significant challenges in accurately measuring ocean pH due to the scale of oceans and their natural variation. However, reliable measurements can be made using spectrophotometric pH methods.

Understanding these interactions—between pH, marine life, CO2, and ocean self-buffering characteristics—helps us envisage potential future scenarios under a warming climate with increasing atmospheric CO2 levels.