Reef Chemistry Question of the Day #73 Altitude 2: pH

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #73

As the second in our high altitude series, we explore pH.

As the pressure is changed by increasing altitude, there is expected to be some impact on pH for natural seawater at 7 dKH that is perfectly equilibrated with that air.

Compared to the same seawater at sea level, how would the pH be different if it was kept at 5,000 feet above sea level? 10,000 feet?

A. 0.04 lower at 5,000 feet and 0.08 lower at 10,000 feet
B. 0.08 lower at 5,000 feet and 0.15 lower at 10,000 feet
C. 0.04 higher at 5,000 feet and 0.08 higher at 10,000 feet
D. 0.08 higher at 5,000 feet and 0.15 higher at 10,000 feet
E. Much different than any of these answers

Good luck!











.

 

[Zero Nitrates]

I'm going with B!

 

[dallasg]

My guess is A

 

[Cory]

C haha. But want to know!

Why is the "73" hyperlinked? It doesn't go anywhere. Are you archiving these questions of the day for easy access?

 

[buzzword]

I'll go with B.

 

[tonizzy22]

Guessing answer D. less dense air means less co2 disolved means higher ph

 

[Randy Holmes-Farley]

Anyone else before I give the answer?

 

[Randy Holmes-Farley]

And the answer is...D. 0.08 higher at 5,000 feet and 0.15 higher at 10,000 feet

tonizzy22 wins today!

As mentioned in the previous day's Q of the D, the pressure at 5,000 feet is about 0.84 times the sea level pressure and at 10,000 feet is about 0.70 times the sea level pressure.

So CO2 at these altitudes is similarly reduced.

Lower CO2 means higher pH.

Most people won't be interested in the detailed math, and it can be looked up in tables or figures like the one linked below. But it can also be calculated exactly if someone wants to. In the pH range we are normally interested in, the pH goes by the log of the CO2 level. So a doubling of the CO2 level lowers pH by about 0.3 (=which is log(2)).

A drop in CO2 to 0.84 x normal means a rise in pH of log (0.84) = 0.08.

Similarly, log(0.70) gives 0.15, so the pH rise at 10,000 feet is 0.15 pH units.

Here's a graph of how seawater changes pH as a function of the CO2 level in the air. Look at the bottom (solid) line. Note that doubling CO2 from 250 to 500 ppm in the air lowers pH from about 8.3 to about 8.0 (the 0.3 pH units mentioned above).

[for real science nerds, note that neither line of the graph maintains total alkalinity constant as I would have liked. It is closer to the solid line, but a bit above it (below the dashed line).]

http://dge.stanford.edu/labs/caldeiralab/Caldeira_research/Caldeira_Berner.html