Reef Chemistry Question of the Day #296; Weather and Your Reef Tank

[Randy Holmes-Farley]

Weather can impact reef tanks in a variety of ways.

Today, we explore how storms might impact it in one particular way.

Suppose that you live in a very sturdy home (no damage from any hurricane) with a very good ventilation system (inside air has normal level of CO2) and a backup power supply (you never lose power).

You are all set, you assume, to ride out the cat 5 hurricane rolling your way.

You have previously noted that your tank pH at 11 AM averages about 8.10, every day.

The hurricane slowly arrives over a period of hours, and you are remarkably in the eye of the hurricane for about an hour from 10 to 11 AM.

What pH is most likely in your tank at 11 AM? Why?

A. pH 8.30
B. 8.12
C. 8.10
D. 8.08
E. 7.90

Good luck!

Previous question of the day:

Reef Chemistry Question of the Day 295: Loss of elements to the air

Reef Chemistry Question of the Day 295 Which of the following elements might have appreciable loss to the air by evaporation from a reef aquarium? 1. Nitrogen and iodine 2. Nitrogen and phosphorus 3. Boron and iodine 4. Calcium and phosphorus 5. Manganese and iron Good luck! Previous Reef...

www.reef2reef.com

 

[1112-425XL]

My guess is E. The humidity and storm pressure will cause a reduction in PH??

 

[dwest]

I’m guessing A. The eye of the hurricane is a low pressure mass. This will cause the air inside of the home to exchange with the outside air. Outside air will have lower CO2 than the normal inside air. Lower CO2 will inhibit carbonic acid formation increasing pH. I picked A over B because of the relative pressure reduction with the hurricane eye is very large.

 

[danimal1211]

Barometric should slightly decrease ph I’m no chemist though so I’m gonna guess D.

 

[taricha]

Hah.
B. 8.12 maybe?
The strongest hurricanes might have 10% less atmospheric pressure than normal (notable cat 5 hurricanes have been around 900 millibars, and one atmosphere is ~1013 millibars)
If we pretend CO2 is an ideal gas - always sketchy, but maybe ok if we go slow enough...
"That is, the partial pressure of any gas in a mixture is the total pressure multiplied by the mole fraction of that gas. This conclusion is a direct result of the ideal gas law"
CO2 concentration (mole fraction) in the hurricane won't be different, but the total pressure will be 10% lower, so the CO2 over several hours in the water will drop by ~10%.
The CO2 /alk/ pH curves I remember from saltwater make me think that dropping CO2 by 10% is probably only going to raise pH from 8.10 to 8.12.

 

[Dburr1014]

Hmm, going to flat out guess.

D, storm pressure will lower the ph by 0.02

 

[Randy Holmes-Farley]

And the answer is...


What pH is most likely in your tank at 11 AM? Why?

A. pH 8.30
B. 8.12
C. 8.10
D. 8.08
E. 7.90

The average air pressure at sea level is about 1013 mbar.

Lowest typical pressure inside the eye of a cat 5 hurricane is about 900 mbar (varies a lot).

That lower pressure reduce the CO2 in the air. In this case, by a factor of 900/1013 = 0.89, or 11%.

That 11% drop in CO2 would correspond to very roughly a 0.05 pH unit drop if the were fully equilibrated (folks with CO2sys can calculated the exact value).

Of course, it won't do that instantly, and in 1 h, it won't have equilibrated a whole reef tank, so i picked a value between the starting pH (8.10) and 8.10 + 0.05 = 8.15.

Anyway, the exact value is not as important as the idea that low pressure can cause a very slight pH rise in reef tanks, as taricha noted correctly.

Happy reefing!

 

[Dburr1014]

Thanks randy, as always, a learning experience.

 

[Mark Goode]

The last (and first ever recorded) hurricane in merrie England was in 1987.

But I've learned someting nonetheless.

 

[KrisReef]

I'm late to reading this, but I'm blown away by the answer.

My lame understanding of partial pressure had me thinking that all dissolved gasses would decrease in the tank, but from there I have no idea how the lowering of oxygen, nitrogen, ....CO2 is going to impact the pH? I always got overwhelmed with all of the competing equilibrium equations that impact a calculation like this.

But wait, there are more. The algae and primary producers in the water, feeling the pinch of available CO2 in the lowered atmospheric pressure will likely produce slightly less O2 during photosynthesis which could cause a (relative) increase in dissolved CO2 as all of those equilibrium equations are fighting to maintain balance inside the swirling eye of the storm! The fishes, poor little finsters, they hear the roar of the Cat 5 storm, (not outside, but on the TV that Randy's watching to see how much damage the storm is ripping as the storm chasers document the savage winds!) The fish begin racing around the tank, sucking up the available O2 and releasing huge gulps of CO2 from across their flaring gills.

Was Randy really watching TV news during the storm? (Nice job @taricha !) I really don't do well in pH scale questions.

 

[Shon]

@Randy Holmes-Farley

What about all the lightning? Higher ozone, higher ORP, lower pH?

Edit: nevermind:
"
How do weather conditions affect tropospheric ozone formation?


Variations in weather conditions play an important role in determining ozone concentrations. Ozone is more readily formed on warm, sunny days when the air is stagnant. Conversely, ozone production is more limited when it is cloudy, cool, rainy, or windy.
"

 

[Raul-7]

But saltwater is a basic solution, wouldn't the lower pressure decrease the pH?

 

[Randy Holmes-Farley]

But saltwater is a basic solution, wouldn't the lower pressure decrease the pH?

I’m not sure I see a way that lowering pressure will ever lower pH. That would imply some sort of basic gas that would leave the water due to the lower pressure.

CO2 is acidic, and less CO2 driven into the water will raise pH.

 

[hexcolor reef]

That 11% drop in CO2 would correspond to very roughly a 0.05 pH unit drop if the were fully equilibrated (folks with CO2sys can calculated the exact value).

The eye of a hurricane is warmer so wouldn’t this all depend on the levels of the oceans initial CO2 and acidic levels before the hurricane and the eye of the hurricane? Not to mention the temperature of the ocean water as well as the temperature in the aquarium. Warmer waters are more saturated with CO2 and in this climate CO2 levels are at an abundance which would also get absorbed while the hurricane is moving across different areas where temperatures are changing.
That information is missing.

I do believe PH would drop as less O2 would be in the air and more CO2 would be in the center of a hurricane

 

[Randy Holmes-Farley]

The eye of a hurricane is warmer so wouldn’t this all depend on the levels of the oceans initial CO2 and acidic levels before the hurricane and the eye of the hurricane? Not to mention the temperature of the ocean water as well as the temperature in the aquarium. Warmer waters are more saturated with CO2 and in this climate CO2 levels are at an abundance which would also get absorbed while the hurricane is moving across different areas where temperatures are changing.
That information is missing.

I do believe PH would drop as less O2 would be in the air and more CO2 would be in the center of a hurricane

pH rises as the CO2 pressure is reduced and CO2 tends to leave the water.

The question was about a reef tank and temp was not a part of the scenario.

 

[hexcolor reef]

pH rises as the CO2 pressure is reduced and CO2 tends to leave the water.

that’s why I mentioned temperature as heat affects atmospheric pressure. Which would be essential as you mentioned pressure in the hurricane is about 900mbar and you would only get that if assuming temps are above 50F

 

[Randy Holmes-Farley]

that’s why I mentioned temperature as heat affects atmospheric pressure. Which would be essential as you mentioned pressure in the hurricane is about 900mbar and you would only get that if assuming temps are above 50F

I picked an average cat 5 hurricane pressure, which was recorded at whatever the temp was in the eye. Temp doesn’t change that determination if the pressure is measured.