Reef Chemistry Question of the Day #91 Baking Soda 3

Randy Holmes-Farley

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Reef Chemistry Question of the Day #91


The same mixture is made as in question 90:

A stock solution is made by dissolving 8.401 grams of baking soda (sodium bicarbonate; molecular weight = 84.01 g/mole) into enough pure freshwater at 25° C to make 1 L total volume. This solution has a pH of 8.300 and an alkalinity of 100.00 meq/L (~280 dKH). 1.000 mL of this solution is added to 99.000 mL of natural seawater (pH 8.200; alkalinity = 2.500 meq/L or 7.00 dKH).


The pH of the final solution after complete equilibrium of all gases with normal air is:


A. Exactly 8.200
B. Exactly 8.201
C. Above 8.201
D. Below 8.200

Good luck!














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beaslbob

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So I guess d as this is after passing all the gas.
which means my answer to #90 was probably wrong or both are wrong. LOL
 

Skydvr

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From my understanding, sodium bicarbonate typically causes a temporary drop in pH (if the solution had a lower pH) while sodium carbonate typically causes a temporary rise in pH (again, relative to the pH of the solution). The difference is due to the CO2 bound in the sodium bicarbonate. So in theory, as the CO2 off gasses from the addition of the sodium bicarbonate addition, pH would increase. But this is also assuming that there is good gas transfer at the surface of the 1L solution and that the atmospheric CO2 concentration is lower than the 1L solution.

So I'm going with not enough information.

EDIT: I just saw the line about "normal air". If we assume that this means normal average CO2 levels, then I'd say it will increase slightly after degassing, so staying with C. This also assumes that the sodium bicarbonate solution was fairly fresh and had not degasses before being added to the seawater.
Since this exam is multiple choice and you don't typically get to explain your answer, I'd have to raise my hand and ask for clarification. I always over analyze multiple choice questions, I prefer open answer exams. Well, unless I don't know the answer, because with multiple choice the answer is there in front of you, you just need to rationalize your way to correct answer.
 
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omykiss001

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After equilibrating with "air" that has some CO2 the bicarbonate that is added will have some additional protons added to the solution that will be available to react with the CO2 to form carbonic acid and would lower the pH a little. This is assuming the water you start with is pure and has not equilibrated with the air and the initial pH reading of 8.3 reflects this. My guess would be a very minor reduction in pH.
 
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Randy Holmes-Farley

Randy Holmes-Farley

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And the answer is...C. Above 8.201

Adding sodium bicarbonate reduces the pH initially (as shown in the previous question of the day), but once the excess CO2 blows off, the pH rises. The alk rose by 0.975 meq/L. In this article, I showed that boosting the alkalinity by a smaller amount (0.5 meq/L) with baking soda and allowing the CO2 to blow off boosted the pH by a bit over 0.1 pH unit. So I'd expect almost double that for the larger addition in this question. :)

http://www.advancedaquarist.com/issues/may2002/chem.htm


Happy Reefing. :)

 
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beaslbob

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And the answer is...C. Above 8.201

Adding sodium bicarbonate reduces the pH initially (as shown in the previous question of the day), but once the excess CO2 blows off, the pH rises. The alk rose by 0.975 meq/L. In this article, I showed that boosting the alkalinity by a smaller amount (0.5 meq/L) with baking soda and allowing the CO2 to blow off boosted the pH by a bit over 0.1 pH unit. So I'd expect almost double that for the larger addition in this question. :)

http://www.advancedaquarist.com/issues/may2002/chem.htm

Happy Reefing. :)

Well
one out of two ain't bad.
 
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