Reef Chemistry Question of the Day #262 Oxygen in a Shipping Bag

Randy Holmes-Farley · Apr 23, 2018

Reef Chemistry Question of the Day #262

This is a two part question...

Part 1.
You are off to the best local store in your area (Jay's Top Notch Fish) to buy a fish. They have a 24 h alive guarantee on fish. Once you finally select one, the sales guy bags it up, adds some compressed air to the bag, and ties it off. The air pressure inside the bag after tying is 1 atmosphere.

Assuming he is using normal (outside) air in the compressor, which scenario provides the most total oxygen to the fish over your two hour drive home?

A. The bag filled with water from a reef tank that is saturated with oxygen from normal air.
B. The bag 75% filled with water and 25% air from the compressor.
C. The bag 50% filled with water and 50% air from the compressor.
D. The bag 25% filled with water and 75% air from the compressor.

Part 2.

You are off to the cheapest local store in your area (Al's Double Discount Fish) to buy a fish. They guarantee the fish is alive when you buy it, and have the motto "No one is cheaper".

Once you finally select a fish, the sales guy bags it up, blows into the bag to inflate it, and ties it off. The air pressure inside the bag after tying is 1 atmosphere.

Which scenario provides the most total oxygen to the fish over your two hour drive home?

A. The bag filled with water from a reef tank that is saturated with oxygen from normal air.
B. The bag 75% filled with water and 25% air from the Al.
C. The bag 50% filled with water and 50% air from Al.
D. The bag 25% filled with water and 75% air from Al.

Good luck!

.

Brew12 · Apr 23, 2018

I'm going with D on both.

I'm not convinced that exhaled breath is oxygen depleted enough to make that substantial of a difference.

bluewill · Apr 23, 2018

D . on both cases

Empress · Apr 23, 2018

I say c on both.

Chuk · Apr 23, 2018

Most oxygen in the bag is D but that wasn't the question asked.
Saltwater is ~6.5 ppm O2 at saturation, fresh air is ~200,000 ppm, and exhaled air is ~160,000 ppm.
I think I'm going to go with B for both. There isn't enough oxygen usage to deplete either bag from its level starting level over 100,000 ppm to below 7 ppm in 2 hrs. My quick estimate puts the usage at something on the order of 20,000 ppm assuming 100 "breaths per minute" and 20% effective oxygen removal from the water.

Brew12 · Apr 23, 2018

Chuk said:
Most oxygen in the bag is D but that wasn't the question asked.

Good catch! I didn't notice that.

I guess that means we have to use partial pressures to calculate air exchange with the water?

I think I'll leave that to Randy.

SDReefer · Apr 23, 2018

I'll say D on both.

dimo · Apr 23, 2018

I'm going to go out on a limb here and say B for both, maybe even A. The amount of oxygen exchange between air in bag and water will be limited by time (in this case 2 hours). So the bags with more water than air will provide more oxygen given the short travel time in the bag.

lewisriverfisherman · Apr 23, 2018

I'm going with b....my lfs actually uses bottled oxygen

Kscope · Apr 23, 2018

Question 1=D
Questio 2. It really doesnt matter a b c or d would all be about the same.

swensos · Apr 23, 2018

I’m going to go out on a limb and say it’s a trick question, they will all be the same available oxygen to the fish. Dissolved oxygen is determined by salinity, temperature and pressure, which are all equal in the options. Plus at equilibrium the water will absorb the gasses in the same percentage as available in the atmosphere, so having more fish-guy breath vs less wont change the available oxygen. Two hours isn’t enough to deplete the available oxygen in the quality bag or the “cheap” bag, I think.

SteadyC · Apr 23, 2018

Outside air is 79% nitrogen, 21% oxygen. Don’t count on it for adding oxygen to the water, it’s mostly nitrogen. I’d go with A.

James Kanouff · Apr 24, 2018

B on both from my practical knowledge. I'm no scientist thats for sure. But I'm following along to see the math.

Randy, I use some of these ?"s with our local reef club as talking points at the meetings.i hope you don't mind. I give you credit of course. Thanks for your work on these things and thanks to R2R for giving us all a place to talk about it.

Randy Holmes-Farley · Apr 24, 2018

James Kanouff said:
B on both from my practical knowledge. I'm no scientist thats for sure. But I'm following along to see the math.

Randy, I use some of these ?"s with our local reef club as talking points at the meetings.i hope you don't mind. I give you credit of course. Thanks for your work on these things and thanks to R2R for giving us all a place to talk about it.

That's great! I'm glad you find them useful.

Sleepydoc · Apr 24, 2018

Randy Holmes-Farley said:
Reef Chemistry Question of the Day #262

This is a two part question...

Part 1.
You are off to the best local store in your area (Jay's Top Notch Fish) to buy a fish. They have a 24 h alive guarantee on fish. Once you finally select one, the sales guy bags it up, adds some compressed air to the bag, and ties it off. The air pressure inside the bag after tying is 1 atmosphere.

Assuming he is using normal (outside) air in the compressor, which scenario provides the most total oxygen to the fish over your two hour drive home?

A. The bag filled with water from a reef tank that is saturated with oxygen from normal air.
B. The bag 75% filled with water and 25% air from the compressor.
C. The bag 50% filled with water and 50% air from the compressor.
D. The bag 25% filled with water and 75% air from the compressor.

Part 2.

You are off to the cheapest local store in your area (Al's Double Discount Fish) to buy a fish. They guarantee the fish is alive when you buy it, and have the motto "No one is cheaper".

Once you finally select a fish, the sales guy bags it up, blows into the bag to inflate it, and ties it off. The air pressure inside the bag after tying is 1 atmosphere.

Which scenario provides the most total oxygen to the fish over your two hour drive home?

A. The bag filled with water from a reef tank that is saturated with oxygen from normal air.
B. The bag 75% filled with water and 25% air from the Al.
C. The bag 50% filled with water and 50% air from Al.
D. The bag 25% filled with water and 75% air from Al.

Good luck!

Room Air
Percentage oxygen: 21%
Partial Pressure of oxygen: 21% * 760 mmHg = ~160 mmHg

Exhaled air:
Percentage Oxygen: ~15%
Partial Pressure of oxygen: 15% * 760 mmHg = ~114 mmHg

oxygen in salt water @ 25ºC ~6.6 mg/l or 5 ml/l @ 1 atm.

For part 1, there is 5 ml/l oxygen in the water but 210 ml/l oxygen in the air. Assuming the partial pressures stay in equilibrium, the most oxygen will be provided by D, since there is more oxygen in the air than in the water.

For part 2, there is still more oxygen in the air (150 ml/l) than in the water, so again, answer D is still correct in terms of 'total oxygen.' The problem here is that the partial pressure is lower, so oxygen will diffuse out of the water to equilibrate with the air in the bag. When partial pressures are equal (114 mmHg,) the diffusion will stop. After that point, as the fish consume oxygen, it will diffuse and equilibrate with the air in the bag, so I still say D is correct.

MnFish1 · Apr 25, 2018

Randy Holmes-Farley said:
Reef Chemistry Question of the Day #262

This is a two part question...

Part 1.
You are off to the best local store in your area (Jay's Top Notch Fish) to buy a fish. They have a 24 h alive guarantee on fish. Once you finally select one, the sales guy bags it up, adds some compressed air to the bag, and ties it off. The air pressure inside the bag after tying is 1 atmosphere.

Assuming he is using normal (outside) air in the compressor, which scenario provides the most total oxygen to the fish over your two hour drive home?

A. The bag filled with water from a reef tank that is saturated with oxygen from normal air.
B. The bag 75% filled with water and 25% air from the compressor.
C. The bag 50% filled with water and 50% air from the compressor.
D. The bag 25% filled with water and 75% air from the compressor.

Part 2.

You are off to the cheapest local store in your area (Al's Double Discount Fish) to buy a fish. They guarantee the fish is alive when you buy it, and have the motto "No one is cheaper".

Once you finally select a fish, the sales guy bags it up, blows into the bag to inflate it, and ties it off. The air pressure inside the bag after tying is 1 atmosphere.

Which scenario provides the most total oxygen to the fish over your two hour drive home?

A. The bag filled with water from a reef tank that is saturated with oxygen from normal air.
B. The bag 75% filled with water and 25% air from the Al.
C. The bag 50% filled with water and 50% air from Al.
D. The bag 25% filled with water and 75% air from Al.

Good luck!

.

Air at 1 ATM contains 21 percent O2. Expired O2 from a person contains about 14%. SO - I would suggest D and A

JimWelsh · Apr 25, 2018

I assume "total oxygen" is a measure of mass.

As @Sleepydoc mentions above, seawater equilibrated with air at 25C has 6.6 mg/L of O2.

The oft-cited 21% O2 in air is true, but it is important to note that this is a percentage of the number of O2 molecules relative to the total number of molecules, and not a mass proportion. The mass fraction of O2 in air is actually 23.14% at 25C.

But, that is 23.14% of how much mass? We need to look to the density of air. At 25C and 100% relative humidity, air has a density of 1.17 g/L. So, each liter of air contains 23.14% * 1.17 = 0.27 g, or 270 mg of O2.

So, air has 270 / 6.6 = 40.9 times as much oxygen per liter as seawater at 25C.

Assuming exhaled air has only about 70% of the amount of oxygen as normal air, the amount of oxygen per liter in the air is still 28.6 times as much as is in seawater. The answer is D in both cases.

BTW, I really liked @Sleepydoc's answer, and think it is more correct in terms of the way to think about the problem than my answer. After googling the chemical shown in his avatar, and then considering once again his username, I'm inclined to think he knows a little bit about how O2 moves between liquids and gasses.

SDchris · Apr 26, 2018

Part 1. D
Part 2: A. The amount consumed by the fish in two hours is likely less than the amount lost from the water to air until equilibrium is reached.