Reef Chemistry Question of the Day #90 Baking Soda 2

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #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 (before gas exchange with the air) is:


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



Good luck.

 

[beaslbob]

Wild guess
D. below 8.2

 

[tonizzy22]

Answer is B ((1mL×8.3pH)+(99mL×8.2pH))/100=8.201pH I would think it would increase more after gas exchange

 

[beaslbob]

Pssssssssst
toni:
he's talking pH this time.

 

[tonizzy22]

You can't use the same logic?

 

[Randy Holmes-Farley]

Maybe. Maybe not.

That's the point of the question.

 

[tonizzy22]

Im stumped then.

 

[Skydvr]

pH is logarithmic, so I'm thinking we can use the same logic as last time if we first raise 10 to the power of the pH values, then take the log of that.

Log[(.01 x 10^8.3) + (.99 x 10^8.2)] = 8.20112....

So I'm going with C on this one.

 

[beaslbob]

You can't use the same logic?

that's correct you can't. PH is not a linear but log function.
as the post above shows after I first posted this response. :xd:

 

[Randy Holmes-Farley]

And the answer is...D. Below 8.200

There is no simple way to calculate the pH that you get on mixing two solutions.

In this case, it shows that the final pH is not even between the two values that you start with, but rather lower than both.

The reason in this case is because bicarbonate is a stronger acid in seawater than in freshwater (or in its own concentrated solution). That shifts the above equilibrium to the right, producing H⁺ and lowering pH.

HCO₃^- <--> H⁺ + CO₃^--

That is true for two reasons. One is that the high ionic strength of seawater tends to have ions of opposite charge surrounding each charged ion. That helps stabilize charged species. The right hand side is more highly charged than the left, so it is stabilized and the reaction proceeds to the right on mixing the solutions.

The second reason is that magnesium and calcium actually ion pair to the carbonate, reducing its free concentration and again shifting the reaction to the right and lowering pH.

In practice, I have measured this myself. Adding enough baking soda to add 0.5 meq/L (1.4 dKH) of alkalinity reduced the pH of artificial seawater by about 0.04 pH units. It is not much, but it is real. I show that here:

Chemistry And The Aquarium: The Relationship Between Alkalinity And pH ? Advanced Aquarist | Aquarist Magazine and Blog

An alternative way to look at it is that seawater has an equilibrium between carbonate and bicarbonate:

HCO₃^- <--> H⁺ + CO₃^--

If you add bicarbonate, you shift the equilibrium to the right by Le Chatlier's Principle, and H+ is produced, lowering the pH.

Happy Reefing.

 

[beaslbob]

dern

I was right?

just like your diy two part writeups

 

[Randy Holmes-Farley]

dern

I was right?

Off to a good start in 2015!

 

[tonizzy22]

Thanks Randy I'm always learning something off of your questions. Will you also do question like this with soda ash?

 

[Randy Holmes-Farley]

Sure, that's a good idea.

 

[beaslbob]

Thanks Randy I'm always learning something off of your questions. Will you also do question like this with soda ash?

He'll probably make an ash of me.:squigglemouth: