Wednesday, June 3, 2015

Brendan's guide to measuring CO2 (and subforms) in the ocean

Carbon dioxide is a sneaky molecule. It reacts with water to form carbonic acid, which is a pretty weak acid in the scheme of things, but an acid nonetheless. It is only stable in an acidic solution. In seawater, which is basic (i.e. not acidic), it breaks down to bicarbonate and carbonate. This means several things. For my purposes, it means if I want to measure the total amount of inorganic carbon in seawater, then I’ve got my work cut out for me. Carbon could be hiding as any of four different molecules counting dissolved carbon dioxide (and that’s just the inorganic stuff… don’t get me started on how messy things get when biology gets involved).  Luckily, we chemists are molecular bullies, and we have lots of tools at our disposal to flush the carbon out of hiding:         

Step 1:  Kill all the things!  We don’t want to risk biology doing any more damage than it already has to our pristine seawater chemistry, so the first thing we do after collecting water is poison it.  Weird, I used to feel bad for all of the bacteria and other plankton in my 300 milliliter samples... 

Step 2: Set aside a known amount of seawater.  This is what the vast majority of the doodads in the lab with me are for.  There are plastic and glass tubes, twisting all about, venting air, seawater, and acid in a confusing tangle.  Why is this a challenge?  Well, we have two problems.  First, we can’t lose any carbon while we’re setting the seawater aside, which means we have to keep the water isolated from air as much as possible.  Also, we’re on a rocking boat, which means we can’t use a scale to weigh how much water we have.  So, we do it by rinsing and filling a pipette (think a glass bulb, open on both ends) with known volume.  We keep the pipette at a specific temperature so we know the density of the seawater, and so the pipette doesn’t expand or contract too much and change its volume.  This means a lot of temperature control.  The AC says 70 °F, but that ice-furnace―blasting directly on my workstation―is fighting a Hawaiian summer and exhaust from all of the machines in here, so I’m estimating my subjective temperature is somewhere between “I’ll probably survive” and “at least if I die, my body will be well-preserved.”  

Step 3: Next we put our known amount of seawater into the “stripper.”  This is where we’re really chemical bullies… we acidify the heck out of the seawater with phosphoric acid.  MUAHAHAHA.  All that carbon that was hiding as bicarbonate and carbonate is abruptly forced into carbonic acid, which is constantly exchanging with dissolved carbon dioxide.  We’re also pumping large amounts of nitrogen gas through this acidified solution, so the carbon dioxide gets swept up into the bubbles of nitrogen and carried away (this is called sparging).  Within a couple minutes, all of the carbon has been removed from our beleaguered seawater. 

Step 4: We then pass our dissolved gas on to an ethanolamine mixture in a fancy piece of tech called a coulometer.  The solution sucks up the carbon dioxide, letting the nitrogen gas go.  It changes color as it does, so our instrument knows there’s carbon dioxide and starts electrocuting the solution to break it down.  As the carbon breaks down, the solution goes back to the original color.  The coulometer keeps track of how much electricity it dumped into the solution, and that tells us how much carbon was in our known amount of seawater.

Step X: I’m just noting here that there are a bunch of cleaning, checking, and calibration steps I won’t bore you with.  They’re boring and tedious, and I kinda love them.  Check out Zen and the Art of Motorcycle Maintenance if that last sentence made no sense.

So, this poor carbon was poisoned, acidified, sparged, reacted with ethanolamine, and electrocuted. As I said, chemists are bullies… but in the end, we have the information we want… the amount of inorganic carbon, in all of its various forms, in a known volume of seawater. Bingo!

By Brendan Carter