The Basics on Alkalinity

Ocean Acidification is not just about pH! In this post, you'll learn how it shifts the
Acidification is about more than just pH! This post will explain how seawater ‘alkalinity’ affects the balance of all of the carbon-containing molecules (aka ‘carbon species’) in this National Resource Council graphic.

If you’ve been following this blog, you’re probably pretty familiar with ocean acidification by now, but figuring out what’s going on with the ocean chemistry is not actually as simple as just measuring the pH. We have to use a variety of different chemical measurements to see the big picture on how the oceans are changing.  One of the ocean’s chemical parameters we are measuring on this research project is alkalinity.  When we measure alkalinity, we are quantifying the ability of a solution—in this case, seawater—to take up positively charged hydrogen ions (H+, commonly referred to as “acidity” en masse) and bind them to a base (a negatively charged ion or molecule), thus neutralizing the acid.  In other words, alkalinity reflects the ability of seawater to resist acidification.  The carbon molecules with negative charges in seawater are bicarbonate (HCO3) and carbonate (CO32-) ions.  The relative abundance of these carbon ‘species’ (i.e. carbon-containing molecules) is in equilibrium with carbonic acid (H2CO3), dissolved gaseous CO2, and other minerals dissolved in seawater.

Image Credit: The NOAA Ship Okeanos Explorer Education Materials Collection 'Off Base' Unit (click on image to view).
Image Credit: The NOAA Ship Okeanos Explorer Education Materials Collection ‘Off Base’ Unit (click on image to explore the lesson).

The proportion of each of the carbon molecules is affected by how much CO2 is being absorbed or emitted by different areas of the ocean, as well as environmental parameters such as temperature and salinity, storms, input from rivers, and biological activity involving photosynthesis and respiration. Other molecules that do not include any carbon atoms are also a part of what we consider alkalinity, including borate (B(OH)4), silicate (SiO(OH)3), magnesium hydroxide (MgOH,) hydroxides (OH) and some nutrients like phosphate (HPO42-), as well as many other molecules.  Some of these molecules we can measure individually; others we cannot.  We measure the ones that we can to better understand the relative contribution to total alkalinity from each of the different bases in the ocean, but ultimately, we group all the bases together as a single measurement called total alkalinity representing the sum of all the bases in a seawater sample.

Remy titrating water samples to determine total alkalinity. Photo Credit: Meghan Shea
Remy Okazaki titrates seawater samples to determine total alkalinity. Photo Credit: Meghan Shea

We measure alkalinity by titrating a carefully collected sample of seawater with a dilute solution of hydrochloric acid.  Titration involves adding a solution with a known concentration of a chemical to a solution in which the concentration of another chemical is unknown in order to measure the concentration of the unknown chemical.  In the case of alkalinity, the titration process takes place by the addition of small increments of precisely measured amounts of hydrochloric acid (the titrant) to the sample until an endpoint is observed in the sample being investigated.  In this case, the endpoint we are looking for is an abrupt change in pH.  Many of you have probably conducted similar acid-base titrations in high school chemistry classes.  Our process is no different, except that we do it with some pretty cool, precise and accurate scientific instrumentation.  Our titrator is computer-controlled and custom-built by a small group of experts in Dr. Andrew Dickson’s lab at the Scripps Institution of Oceanography in San Diego, CA.

Knowing the total alkalinity and the buffering capacity of any one area of the ocean helps us to correlate changes that we see in biology to what is happening with the chemistry in the atmosphere and the ocean.  Since our measurements are very accurate, we can even try to make some predictions about what the marine environments will be like for the organisms that will live in them in the not-too-distant future.

We have four people on our WCOA16 Alkalinity analysis team.  On Leg 1 (San Diego to Baja California, Mexico and then back up to San Francisco) Dr. José Martín Hernandez-Ayon and Julian Herndon measured Total Alkalinity, and on Leg 2, (San Francisco to Canada and back to Seattle) Dr. Remy Okazaki and Morgan Ostendorf took over the titrations.  By the end of the 35-day cruise we will have analyzed more than two thousand 10-minute samples—that’s nearly 14 days of constant titration!

Author: Julian Herndon


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