Reef Chemistry Question of the Day #182 Soda Water and Reef Tanks

[Randy Holmes-Farley]

Reef Chemistry Question of the Day [HASHTAG]#182[/HASHTAG]

Which of the following is NOT an effect of adding a small amount of soda water (seltzer) to a reef aquarium?

A. The absolute amount of carbonate declines.
B. The absolute amount of bicarbonate declines
C. The absolute amount of carbonic acid rises
D. The pH declines
E. The total alkalinity remains the same

Good luck!





























.

 

[todda5914]

Logically, I would have to say E, since A & C would move together, as would C & D.

 

[fishyfishyfishy]

B

 

[JimWelsh]

The pH will decline. This will cause a shift in the speciation of alkalinity away from carbonate and toward carbonic acid, leaving bicarbonate essentially unchanged. Total carbonate alkalinity will remain unchanged. Answer is B.

 

[beaslbob]

I like b also

 

[wpeterson]

B. The absolute amount of bicarbonate declines

 

[Habib(Salifert)]

Perhaps I'm picky but my answer is: B and E

Unlike the "absolute amount" , the total alkalinity is a concentrative parameter. The small increase in volume by the Seltzer water will reduce the total alkalinity by the same small amount (neglecting some much smaller effects).

 

[s2nhle]

D for me.

 

[aaron23]

i like B too

 

[Randy Holmes-Farley]

Perhaps I'm picky but my answer is: B and E

Unlike the "absolute amount" , the total alkalinity is a concentrative parameter. The small increase in volume by the Seltzer water will reduce the total alkalinity by the same small amount (neglecting some much smaller effects).

The volume added was said to be small. Maybe so small that it was exactly offset by evaporation while it was being poured in.

 

[Habib(Salifert)]

LOL!!!

 

[fragit]

E for me!

 

[tdjma]

You would also be introducing co2 to the aquarium.

 

[wpeterson]

I said B. The absolute amount of bicarbonate declines because we're introducing additional CO2 into the water column via the soda water, which interacts with the water producing either bicarbonate or carbonic acid. So I expect the level of bicarbonate would increase when the CO2 level in the water increases.

 

[Randy Holmes-Farley]

You would also be introducing co2 to the aquarium.

That's correct.

So which effect do you believe the CO2 has?

 

[brandon429]

I'd vote E/C for that co2 effect Randy going solely off memory one time six yrs ago in reefcentral thread I asked you and Habib if diurnal pH swings was changing alk each morning and got a resounding no, so that's my vote. If my answer is wrong then I recalled wrong lol

 

[dieselwagon]

C02 dissolved in water would produce carbonic acid, H2CO3. Water carbonic acid dissociates rapidly to form a H+ ion and HCO2 bicarbonate, so it would affect the carbonate equilibrium, and pH values change as a result. there for it would lower the ph but only slightly.

 

[Randy Holmes-Farley]

And the answer is...B. The absolute amount of bicarbonate declines (meaning this does NOT happen, it increases)

Adding seltzer (soda water) adds carbon dioxide.

Adding carbon dioxide to the water products carbonic acid (so it rises, eliminating answer C):

CO2 + H2O <----> H2CO3

That added carbonic acid will tend to ionize a bit, producing some bicarbonate and a smaller amount of carbonate:

H2CO3 <----> HCO3- + H+

HCO3- <----> CO3-- + H+

Both of those reactions release H+, reducing pH and eliminating answer D.

Adding (or subtracting) carbon dioxide to a solution like seawater cannot have any impact on total alkalinity (eliminating answer E, except see below about dilution) because for every H+ produced by dissociation of carbonic acid (which by itself would tend to lower alkalinity), there is an equal amount of bicarbonate produced (which adds to alkalinity and exactly offsets the H+ produced). The same is true for any dissociation of bicarbonate to carbonate.

While the above equations alone are not adequate to know what happens to bicarbonate and carbonate, if you know the pKa values it can be easily calculated. For our purposes, it is perhaps easier to see on the attached graph of the speciation of CO2 in seawater:

http://acmg.seas.harvard.edu/people/faculty/djj/book/bookchap6.html

From it we can see that as the pH is lowered from about pH 8.2 (as when adding CO2 to seawater), the relative amount of carbonate declines and bicarbonate rises. Note that the graph is somewhat misleading as it is showing the fraction of total CO2 species present, not the total. The bicarbonate will never decline no matter how much CO2 you add. It is just that the CO2/Carbonic acid will be increasing and make the bicarbonate drop as a fraction of the total CO2 species, but not in absolute. terms. I could not find a graph of absolution concentrations when CO2 is added to seawater.

FWIW, Habib's comment related to the fact that the seltzer also added a little bit of water, which will dilute the whole system and the total alkalinity will decline. So answer E would also be acceptable if that is the basis for the answer.

Happy reefing!