Reef Chemistry Question of the Day #14

[hart24601]

2! I hope.

 

[hart24601]

The question of the day is fun, but if/when you get sick of doing it everyday you could make Randy's question of the week and sticky it until the next question comes out!

 

[Randy Holmes-Farley]

The question of the day is fun, but if/when you get sick of doing it everyday you could make Randy's question of the week and sticky it until the next question comes out!

That's a good idea. They will get harder and harder to come up with, without getting too repetitive.

 

[Randy Holmes-Farley]

As most of you answered correctly, the answer is

2. Carbonic acid (H₂CO₃)

Good job!

All of the others will result in a decline in alkalinity. They will break up into H+ and (Cl^-, Br^-, etc.).

The H⁺ will combine with bicarbonate (HCO₃^-) and form carbonic acid, which separates into water and CO₂, and the CO₂ gets blown off to the air:

H⁺ + HCO₃^- --> H₂CO₃ --> H₂O + CO₂


Carbonic acid obviously cannot do this, because as it initially ionizes, the result is another bicarbonate. So the process can happen, but there is no net loss of bicarbonate (alkalinity):

H₂CO₃ --> H⁺ + HCO₃^-
Here's a blurb on it from one of my articles:

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:


(7) TA_mix = [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.