Collaborative Research

pHun with pH
by -- February 5th, 2013

For my final post, I thought I would describe in more detail  my research project.  The oceans are becoming more and more acidic, which has huge impacts on organisms and seawater chemistry.  The example most often used are corals, currently experiencing massive diebacks across the globe.  The effects of altered acidity does, however, vary considerably from species to species, so I have been taking measurements to investigate how Prochlorococcus and others in the marine ecosystem will be affected.


SCIENCE! (photo by Sam)


But what IS acidity?  Without getting into the nitty-gritty, the acidity of  a liquid is a measure of how many hydrogen ions there are floating around.  Higher concentrations of hydrogen ions means higher acidity.




The machine I use, a spectrophotometer, can calculate the amount of ions in a solution.  Skipping a few steps for simplification, I fill a glass tube, called a cuvette, with the seawater I want to test.  I then add a small amount of indicator dye, a dark purple liquid, into the cuvette.  The dye is specially designed to react only with the hydrogen ions.  If there are very few ions, the dye barely reacts and the combined solution remains relatively purple.  If the solution is more acidic and thus has more ions present, more of the dye will react, and the solution will become clearer.  It should be easy to imagine that a cuvette with a dark purple color will let less light pass through than a relatively clear cuvette.  The spectrophotometer simply shoots a beam of light through the cuvette and measures how much light passes through to the other side.  With these measurements, the acidity can be calculated.




Below is a common depth profile of acidity, measured in terms of pH.  A lower pH reflects a higher acidity.  The pH clearly decreases overall with depth, but there is a peak at 100m due to biological activity.  As I mentioned, the shape of the profile is not always the same from day to day, and it will be interesting once we are done to compare my profiles to those of other parameters such as temperature, nutrients, and chlorophyll to see how they are all related.


pH with depth. Lower pH values=higher acidity.

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