Reef Chemistry Question of the Day #59 Soda water

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #59

If you add soda water (seltzer; carbon dioxide in water) to normal seawater, which of the following will happen?


A. The pH and alkalinity will both drop
B. The pH will rise and the alkalinity will drop
C. The pH will drop and the alkalinity will stay the same
D. The pH will drop and the alkalinity will rise

Good luck!














.

 

[leptang]

If not sure guess C.

 

[Russellaqua]

Looks like choice A to me.

 

[beaslbob]

c.

ph will drop but there will still be the same amount of carbonate.

 

[Cory]

I'm saying C because Co2 is gassed out of water, this lowers ph temporarily but doesn't effect alkalinity. I'd be interested if the answer was different!

 

[DarkTang]

The answer is A. Alk will drop bc you use co2 for calcium reactor. Can't be B bc it's not making water more acidic

 

[saltyhog]

I will say C.

 

[Paintguru]

A. pH drops due to CO2. Alk should drop because you are diluting the seawater.

 

[Randy Holmes-Farley]

Anyone else before I give the answer?

 

[beaslbob]

nope

 

[Randy Holmes-Farley]

OK then.

The answer is...C. The pH will drop and the alkalinity will stay the same

Although Paintguru's answer that the alkalinity drops by dilution is actually correct. I wasn't thinking of the dilution.

One of the reasons that I posted this question is that some folks have persistently believed that adding CO2 reduces alkalinity. This was started (or at least repeated) by a very famous aquarist more than 20 years ago, and it persists to this day.

The pH drops because carbon dioxide forms carbonic acid in water. That's why we use it in calcium carbonate reactors: it lowers the pH to the point where the media dissolves.

This section from an alkalinity article details the carbon dioxide effect on alkalinity:

http://www.advancedaquarist.com/2002/2/chemistry


Alkalinity Facts

There are several facts about total alkalinity that follow directly from the definition. Unfortunately, some of these have been misunderstood by some hobby authors.
One of these facts is termed The Principle of Conservation of Alkalinity by Pankow ("Aquatic Chemistry Concepts", 1991). He shows mathematically that the total alkalinity of a sample CANNOT be changed by adding or subtracting CO₂. Unfortunately, there is an article available on line, which claims otherwise, and encourages people to "lower alkalinity" by adding CO₂ in the form of seltzer water. This is simply incorrect.
Forgetting the math for the moment, it is easy to see how this must be the case. If carbonic acid is added to any aqueous sample with a measurable alkalinity, what can happen?
Well, the carbonic acid can release protons by reversing equations 1 and 2:
(5) H₂CO₃ ==> H⁺ + HCO₃^-
(6) HCO₃^- ==> H⁺ + CO₃^--
These protons can go on to reduce alkalinity by combining with something that is in the sample that provides alkalinity (carbonate, bicarbonate, borate, phosphate, etc). However, for every proton that leaves the carbonic acid and reduces alkalinity, a new bicarbonate or carbonate ion is formed that adds to alkalinity, and the net change in total alkalinity is exactly zero. The pH will change, and the speciation of the things contributing to alkalinity will change, but not the total alkalinity.
This is not true for strong acids, however. If you add hydrochloric, sulfuric or phosphoric acids (or any acid with a pKa lower than the carbonic acid endpoint), there will be a reduction in the alkalinity.
Another interesting result of the Principle of Conservation of Alkalinity is the equation for determining the total alkalinity when two different aqueous solutions are mixed together. If you mix (a) parts of a solution with total alkalinity A with (b) parts of a solution of total alkalinity B, the resulting alkalinity is just the weighted average of the two samples:

TAmix = [a(A) + b(B)]/[a + b]
​

Equation 7 can be used to calculate changes in TA for water changes in a tank, for additions of limewater, for dilution of tank water with pure water, and a host of other situations where you might want to know what the final alkalinity will be. It can also be used for calculating reductions in alkalinity caused by strong acids, where the alkalinity of the acid is just the normal strength of the acid as a negative number.

 

[buzzword]

I'm thinking C.

 

[beaslbob]

OK then.

The answer is...C. The pH will drop and the alkalinity will stay the same

...

could you please recheck the answer? C is what I said for the same reason. :bigsmile: Isn't this like when co2 lowers (algae action) ph rises but alk stays the same?

 

[Randy Holmes-Farley]

could you please recheck the answer? C is what I said for the same reason. :bigsmile: Isn't this like when co2 lowers (algae action) ph rises but alk stays the same?

Well, the C part of your answer was right, but then you added that carbonate was unchanged.

If alkalinity is unchanged and the pH dropped, there must be more bicarbonate and less carbonate.

 

[beaslbob]

Well, the C part of your answer was right, but then you added that carbonate was unchanged.

If alkalinity is unchanged and the pH dropped, there must be more bicarbonate and less carbonate.

gee I need to learn when to stop talking. :wink:

 

[Randy Holmes-Farley]

:lol:

yes.

 

[Paintguru]

Wait, you just said I was right about the dilution....so clearly the answer is (A) .

 

[Randy Holmes-Farley]

Wait, you just said I was right about the dilution....so clearly the answer is (A) .

Yes, but I like C.