Reef Chemistry Question of the Day #212 Dissolved Oxygen and Helium

Randy Holmes-Farley · Sep 1, 2017

Reef Chemistry Question of the Day #212

In normal 35 ppt (35 PSU) seawater at 25 degrees Celsius that is perfectly equilibrated with normal air, which of the following is true?

A. There is more O2 but less helium in 1 L of seawater than 1 liter of air
B. There is more O2 and helium in 1 L of seawater than 1 liter of air
C. There is less O2 but more helium in 1 L of seawater than 1 liter of air
D. There is less O2 and helium in 1 L of seawater than 1 liter of air

Good luck!

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Brew12 · Sep 1, 2017

I'm going with C.

sghera64 · Sep 1, 2017

Given that the partial pressures of all the gasses I've ever seen are much less than 1: D is my answer.

Plus, air is about 21% O2 and 79% N2. (Or it was when I was in school. . . Which was before global warming and the apocalyptic abundance of CO2 in our atmosphere). [emoji12]

Alfrareef · Sep 1, 2017

D for sure...

bif24701 · Sep 1, 2017

I'm thinking D

JimWelsh · Sep 1, 2017

D. http://www.mbari.org/science/upper-...roup/periodic-table-of-elements-in-the-ocean/

Randy Holmes-Farley · Sep 7, 2017

And the answer is...

D. There is less O2 and helium in 1 L of seawater than 1 liter of air

Jim's link above gives the actual data for seawater concentrations (~0.8 ng/L for helium and 6-7 mg/L for oxygen) and air has about 6.2 ng/L for helium and 256 mg/L of oxygen.

One important point to understand is that nonpolar gases just don't dissolve well in water, and so important gases such as oxygen are in shorter supply in an aquarium than in air.

The reason is also interesting. It's not that oxygen or helium wouldn't like to be in water. They do.

The issue is that the water doesn't want them there. Water so much wants to interact with itself (through hydrogen bonding and other interactions) that it actually squeezes out molecules that it can't interact with as strongly as it can with itself. It's a very self-centered liquid!

Happy reefing.