Reef Chemistry Question of the Day #43 pH and Mixing

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #43

Suppose that the water in a reef aquarium has a pH of 8.4, and a new batch of artificial seawater at the same temperature has a pH of 7.9. If 10% of the water in the aquarium is replaced by the new water, the final pH in the aquarium will be closest to which of the following values?


A. 8.37
B. 8.35
C. 8.33
D. Not enough information is provided to properly answer.

Good luck!



















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[reef lover]

i think there is not enough info,we need the size of the tank....maybe.....

 

[DFW]

D!!!!

 

[leptang]

A. Is my guess.

 

[JFrar]

just was doing this in my math class and ddnt like it lol, isnt it along the lines of 8.4(.9) + 7.9(.1) = x ... so B, 8.35

 

[cagdason]

I would guess D, as we don't know the Kh we don't know the buffering capacity of the tank so it would be hard to calculate.

 

[Nano sapiens]

In theory, the answer would be 'B' (8.35).

 

[Randy Holmes-Farley]

Anyone else want to try before I give the answer?

 

[Randy Holmes-Farley]

And the answer is...D. Not enough information is provided to properly answer.

As cagdason points out, without knowing the ability of each solution to buffer against pH changes, one cannot know the final pH.

A seawater solution with higher alkalinity will resist pH changes more than one with lower alkalinity, so there will be some tendency for the pH to be closer to the higher alk solution's pH.

But there is also more to it than that.

The buffering capacity of seawater changes with pH. Generally, the buffering is higher the higher you go in pH between the mid 7's and the upper 8's. So even mixing two equal alkalinity seawater solutions in equal amounts will not result in a pH exactly between the two, but closer to the higher pH side.

Finally, and most remarkably, the pH of two solutions mixed together does not even need to be in between the pH values of the two solutions!!!

For example, adding magnesium to seawater (or just sodium chloride solutions) makes bicarbonate a stronger acid and tends to lower the pH (by ion pairing to, and stabilizing, carbonate), which could result in a pH lower than either of the two starting solutions.

Just another fun tidbit in the world of chemistry.

Happy Reefing!