My measurements

My measurements: carbonate chemistry of the ocean (26 April 2023)

Current position while writing (diamond shape). We’ve nearly reached the equator. The circles are the positions at previous blog posts, the black line is roughly the route we took (base map from ggplot2)

Today, I want to explain a little bit about what I am measuring here on board. In the introduction at the very beginning, it was already mentioned that my overall PhD topic is related to seawater pH measurements. However, the pH is only of four parameters that describe the carbonate system of seawater. The three others are: partial pressure of ₂, alkalinity and dissolved inorganic carbon. From this group of four parameters, I measure three continuously for the surface water. These three are alkalinity, pH and the partial pressure of ₂ As all of this is quite abstract and most of you probably can’t relate to all these words, I’ll try to simplify and explain everything briefly. This whole topic is really complex, so my explanations will be simplified and just an overview that is neither complete nor detailed. (If you want to skip the definitions you can go directly to the section “big picture”.)

Let’s start with the parameter that most of you have probably at least heard about, the pH value.

pH-Wert
Some might remember the pH value from school, something with acids and so on. Generally, the pH value of a solution tells you how acidic or basic the solution is. Completely pure water has a pH value of 7. Something acidic has a pH value below 7 and something basic above 7. But what does „acidic“ and „basic“ mean?

The pH value is defined as the negative decadic logarithm of the hydrogen ion activity. That does not really tell one anything. Simplified it means that the pH value describes the concentration of hydrogen ions (H⁺). So, the pH value tells whether there are many or just a few hydrogen ions in the solution (actually, the ions do not exist on their own in the solution but that gets really complicated). It is calculated as the negative logarithm to make the numbers nicer and easier to use. However, this also means that a lower pH value corresponds to more hydrogen ions than a high pH value.

In seawater, the definition of pH is even more complex because there are a couple of salts in the solution. Some of them are included in the definition of the pH and this is then called the total pH (pHT or pH_T). This pH_T value is currently accepted as the pH value to be used for seawater, however, there are also different pH scales.

In the introduction above, I wrote that we are measuring the carbonate system. However, there doesn’t seem to be any carbon in the pH, right? Actually, there is a relationship between the seawater pH and the CO₂ that is in solution. When CO₂ goes in solution in water, carbonic acid is created (just like in the soda machine that makes sparkling water) and this acid does interact with the pH value of the water. With increasing CO₂ concentration in the air, the global uptake of CO₂ by the ocean also increases and the seawater pH decreases on average. This process is called ocean acidification and among other effects, negatively impacts the growth of corals.

Of course, the whole system is much more complicated than this summary. If you’d like to know more, the Wikipedia page about ocean acidification gives a much broader picture for example.

Alkalinity
In contrast to pH, alkalinity is not a word you’d come across that often. As with the pH, for seawater usually the total alkalinity is measured. The “total”, again, refers to the ions and ion interactions included in the consideration.

The alkalinity describes the potential of a solution to bind hydrogen ions. So, it basically describes how much the pH is influenced by addition of acid. For seawater, a titration with hydrochloric acid can be used to determine the concentration of bases that can accept hydrogen ions. This does sound very similar to the pH value, but it is not the same. The alkalinity of seawater changes proportionally with its salinity (which corresponds to the dilution) but the pH does not.

Don’t worry if this is really confusing. Alkalinity is not the most intuitive concept.

Partial pressure of CO₂

You might know the partial pressure from biology where it is important for the transition of oxygen and carbonate dioxide (CO₂) between air and blood. Very broadly, the partial pressure of CO₂ describes how much CO₂ is dissolved. This value can then, for example, be compared to the partial pressure in the atmosphere. Comparable to having different temperatures, the CO₂ tends to “flow” to areas of lower partial pressure. We might measure a partial pressure of 500 µatm for the seawater and know that the atmosphere has approximately a partial pressure of 400 µatm. This means the sea is on average emitting CO₂. While if we had a partial pressure of 300 µatm for the sea, it would have a higher CO₂ uptake than emission.

Dissolved inorganic carbon
Dissolved inorganic carbon (DIC) summarises all inorganic carbonate species which are dissolved in the seawater. Most of them are different dissociation species of carbonic acid. It does not include organic molecules and particulate matter, not surprising given the name.

Big picture
Now that we know what all of this means, I’ll try to give an overview of the broader relationships. The four carbonate parameters I described are related to each other. If you know two of them, you can calculate the other two (with higher or lower uncertainties depending on the combination). Together they describe the carbonate system of that particular „piece“ of seawater. The carbonate system is of interest because it is key to understanding ocean acidification and its effects as already mentioned but there are more applications like understanding the carbonate cycle better and how regions differ. These regional differences might affect the potential impacts of ocean acidification and climate change and are therefore important to understand global impacts and changing dynamics.

Despite several decades of research, there are still open questions in marine chemistry and currently changes occur that make it even more important to understand the systems. Without knowing how everything is connected today and which changes cause which impacts, no reliable models can be built. As parameters such as the pH value play a crucial role in many biological processes it is often measured.

Measurements
As already mentioned, I am doing underway measurements of pH, partial pressure of CO₂ and alkalinity. If you just paid attention, you might notice that two parameters are enough to define the system. For example, I could calculate the pH value from partial pressure and alkalinity without the need to measure it. However, measuring it has the advantage of being able to compare the results.

At this point, I don’t want to explain the measurement methods in detail. This post is getting long enough as it is. Most of the instruments I am using here are commercially available and measure quite reliably on their own after an initial calibration. I am checking from time to time that they are still happy and measure values in the expected range. The only instrument needing more attention is the one for alkalinity measurements as it is still earlier in its development. I am checking regularly that there aren’t any bubbles in the tubes, all reagents are still full and different measurement values are as expected. If the values should at some point differ or fluctuate too much, I’d have to check for bubbles more carefully and maybe re-do the calibration.

All of this is of course not something you just learn by seeing the instrument. That is why I was at the GEOMAR for a week in January. There I got an introduction to the different instruments, which things I must check and how to get the data from the sensor. Now that we have nine hours time difference compared to Germany, the times in which I could get direct answers to any questions are quite short, so it was important to have everything clear.

With my notes from January, I set up all the sensors and started the measurements. Of course, it’s different to be standing alone in front of the instrument in a different environment with the additional “pressure” that it has to work. I was surprised that most things work quite well. I am especially happy that the alkalinity measurements seem to be running smoothly. As the instrument has so many tubes and potential problems (I got a whole bunch of spare parts), I didn’t really expect it to start smoothly.

Measurements on board. The seawater is coming out of the tube from the top into the sink and leaves it through the overflow. This results in a continuous water exchange and the instruments can measure the current values without being attached to the outside of the hull. The wooden plate reduces the CO₂ exchange between air and water. Additionally to the measurements I mentioned above (green), you can also see a nitrate sensor and a different pH sensor that is not working yet (brown). By the way, the water has approximately 28 °C, quite a pleasant working temperature. (photo: Rieke Schäfer)

Originally, we had planned to measure pH with two different systems. However, one of the sensors is still making problems … Sometimes I feel a bit like on a space station, sending problem descriptions to the “base” and getting back detailed lists of what to try. Despite trying different approaches and asking several people, the sensor does not want to do anything as soon as it is connected to the power supply. The people on board here have a lot of experience with fixing things, but without knowing what is wrong that does not help. Unfortunately, seawater is not the best environment for electronics and it does get into places where it is not supposed to go.

„Troublemaker“: despite lots of effort, the pH sensor just does not want to measure. In the other picture earlier, you can still see the space we left for it. In total, we have three pH sensors: one is measuring, one is measuring unreliably and one does not want to measure at all. (photo: Rieke Schäfer)

I really hope this wasn’t too much. For everyone who made it to the end, here are some pictures as a treat:

Lots of water (Fotos: Rieke Schäfer)

Unfortunately, it is difficult to take pictures of the stars. A moving boat is not the best platform for it. (photo: Rieke Schäfer)

Further readings ;)
In case anyone wants more details:

• Englisch: „Guidelines for Measuring Changes in Seawater pH and Associated Carbonate Chemistry in Coastal Environments of the Eastern United States” by Adam R. Pimenta and Jason S. - a relatively understandable introduction (2018)
• I already linked some Wikipedia pages earlier, they give a quite good overview to start off.
• There is also a long Wikipedia page on the marine carbonate cycle
• And in case anyone is really motivated: most (all?) books on aquatic chemistry and chemical oceanography have chapters on the carbonate system.

For example:

• „Aquatic Chemistry Concepts“ by James F. Pankow (1991)
• “Aquatic Chemistry – Chemical Equilibria and Rates in Natural Waters” by Werner Stumm and James J. Morgan (1996)
• “Chemical Oceanography” by Frank J. Millero (2013)

PTB doctoral student Rieke Schäfer is blogging here directly from the RV "Sonne" on her way west from South America across the Pacific Ocean.