By now, you’ve learned about nitrate and nitrite, two of the three nutrients I am helping my brother to analyze. The third nutrient is ammonium (NH4+) and we are studying the rates of ammonium consumption via phytoplankton and through the process of nitrification.
Phytoplankton use ammonium as a nitrogen source to build biomass, and it is then released by zooplankton as waste after grazing on the phytoplankton. Thus, the common thought is that it has a fast turnover. We’re seeing large concentrations of ammonium in surface waters indicating that it’s being released more rapidly than it’s being consumed. Nitrification is a biologically-mediated process by which ammonium is converted to nitrite and then to nitrate through a series of oxidation reactions. Our Chief Scientist, Dr. Bess Ward, is a pre-eminent scholar of nitrification and will tell you all about it during her Spotlight Series interview (so stay tuned!)
We are determining the ammonium concentration of each seawater sample using a fluorometer, which measures the fluorescence of each sample. Fluorescence is a property in which a substance absorbs a shorter wavelength of light and emits a longer wavelength. The fluorometer shines ultraviolet (UV) light through the sample, which then fluoresces, and the fluorometer measures how much light is emitted by the sample. As with the nitrate and nitrite analyses, we first create a standard curve (we measure the fluorescence of known concentration standards) then test the samples to find their concentration.
Ammonium, itself, does not fluoresce so we first need to react it to create a new compound that will fluoresce. To do this, the ammonium samples are reacted with a solution of OPA (o-phthaldialdehyde, an organic compound) and sulfite (SO32–), according to the reaction below.
When these compounds react with ammonia (NH3), they form a fluorescent compound called isoindole – it is this compound that we directly measure with the fluorometer. This reaction is not instantaneous; we let the reaction occur for at least three hours before we start measuring.
So why does this reaction happen with ammonia and not ammonium? Remember that, in a solution, equilibrium exists between ammonium (NH4+) and ammonia (NH3) and their concentrations are dependent on each other and the pH (pH is a measurement of the acidity or alkalinity of a solution, pH<7 is acidic, pH>7 is basic, and pH=7 is completely neutral).
Seawater has a pH of 8.2 (pure water has a pH of 7, for reference) so the ammonium concentration is more dominant than that of ammonia (ammonia takes over at a pH greater than 9.3). However, there is still some ammonia in the seawater that we’re sampling. As that is reacted with the OPA and sulfite (and is removed from the equilibrium, equation with ammonium), the concentration of ammonia decreases. This decrease in ammonia concentration causes a stress to the ammonium-ammonia equilibrium, and that stress must be corrected according to LeChatelier’s Principle (which states that a system at equilibrium will shift to remove an applied stress). The equilibrium shifts to create more ammonia, which is subsequently reacted with the OPA and sulfite. Thus, the concentration of ammonium will determine how much ammonia can react with the OPA and sulfite, which are always in excess.
So now you know how Andrew and I measure each of the three nutrients we are studying! The chemistry behind each is so interesting and uses a variety of concepts my students have been studying this year. We are currently traveling to the second Process Station where we will have another few days of around-the-clock casts, sampling, and analyses. We’re in the homestretch!
Until next time,