Reef Chemistry Question of the Day #140 Total Alkalinity

Randy Holmes-Farley · Sep 3, 2015

Reef Chemistry Question of the Day [HASHTAG]#140[/HASHTAG]

Which equation best describes total alkalinity (TA) in natural seawater (the bracket symbol means the concentration of the ion inside it)?

A. TA = [HCO3-] + 2[CO3--] + [B(OH)4-] + [OH-] + [Si(OH)3O-] + [MgOH+] + [HPO4--] + 2[PO4---] - [H+]

B. TA = [HCO3-] + [CO3--] + [B(OH)4-] + [OH-] + [Si(OH)3O-] + [MgOH+] + [HPO4--] + 2[PO4---] - [H+]

C. TA = [HCO3-] + 2[CO3--] + [B(OH)4-] + [OH-] + 3[Si(OH)3O-] + [MgOH+] + [HPO4--] + 2[PO4---] - [H+]

D. TA = [HCO3-] + [CO3--] + [B(OH)4-] + [OH-] + [Si(OH)3O-] + [MgOH+] + 2[HPO4--] + 3[PO4---] - [H+]

Good luck!

.

ReefMadScientist · Sep 3, 2015

The answer is A.

beaslbob · Sep 4, 2015

I don't even know what a co3 is. LOL

Randy Holmes-Farley · Sep 4, 2015

Good point. Sorry.

[HCO3-]
bicarbonate

[CO3--]
carbonate

[B(OH)4-]
borate

[OH-]
hydroxide

[Si(OH)3O-]
silicate

[MgOH+]
not exactly sure what this is called in words, but maybe magnesium monohydroxide

[HPO4--]
hydrogen phosphate

[PO4---]
phosphate

[H+]
hydrogen ion

Harold Green · Sep 4, 2015

Randy if you want to help others you should explain what each ion does and it's importance otherwise you're giving a chemistry lesson to many hobbyists who don't yet understand how to cycle a tank. You'd be better concerned explaining total alk., how it affects corals, and how to adjust levels to get the most benefit.

Randy Holmes-Farley · Sep 4, 2015

Harold Green said:
Randy if you want to help others you should explain what each ion does and it's importance otherwise you're giving a chemistry lesson to many hobbyists who don't yet understand how to cycle a tank. You'd be better concerned explaining total alk., how it affects corals, and how to adjust levels to get the most benefit.

I have done all that in other articles. This question of the day is just a tidbit that varies from easy to hard. This one was hard. If they are all easy, there's no challenge for the more advanced science types.

Here are my other articles (including one that details exactly what total alkalinity is in great detail and also in simple terms, as well as others that detail why alk is important and how to adjust it):

https://www.reef2reef.com/threads/randys-reef-chemistry-articles.174821

and these are the alkalinity-related articles:

Chemistry And The Aquarium: Solving Calcium And Alkalinity Problems ? Advanced Aquarist | Aquarist Magazine and Blog

Chemistry And The Aquarium: How To Select A Calcium And Alkalinity Supplementation Scheme ? Advanced Aquarist | Aquarist Magazine and Blog

The “How To” Guide to Reef Aquarium Chemistry for Beginners Part 2: What Chemicals Must be Supplemented by Randy Holmes-Farley - Reefkeeping.com

The Many Methods for Supplementing Calcium and Alkalinity - REEFEDITION

A DIY Alkalinity Test: By Randy Holmes-Farley - REEFEDITION

A Simplified Guide to the Relationship Between Calcium, Alkalinity, Magnesium and pH by Randy Holmes-Farley - Reefkeeping.com

When Do Calcium and Alkalinity Demand Not Exactly Balance? by Randy Holmes-Farley - Reefkeeping.com

An Improved Do-it-Yourself Two-Part Calcium and Alkalinity Supplement System by Randy Holmes-Farley - Reefkeeping.com

Aquarium Chemistry: A Homemade Two-Part Calcium And Alkalinity Additive System ? Advanced Aquarist | Aquarist Magazine and Blog

Calcium and Alkalinity - Reefkeeping.com

Chemistry And The Aquarium: Calcium Carbonate As A Supplement ? Advanced Aquarist | Aquarist Magazine and Blog

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

Aquarium Chemistry: The Chemical and Biochemical Mechanisms of Calcification ? Advanced Aquarist | Aquarist Magazine and Blog

Chemistry and the Aquarium: What is Alkalinity? ? Advanced Aquarist | Aquarist Magazine and Blog

JimWelsh · Sep 8, 2015

I'm going with A, because 1) I know that "2[CO3--]" is correct, and 2) I know that "[Si(OH)3O-]" is correct, but I have a question. When the two phosphate species are neutralized to H2PO4-, why doesn't that still contribute to the alkalinity?

Randy Holmes-Farley · Sep 9, 2015

That's a great question, Jim.

The answer is A.

A. TA = [HCO3-] + 2[CO3--] + [B(OH)4-] + [OH-] + [Si(OH)3O-] + [MgOH+] + [HPO4--] + 2[PO4---] - [H+]

Total alkalinity (TA) is defined as the amount of acid required to lower the pH of the sample to the point where all of the bicarbonate [HCO3-] and carbonate [CO3--] could be converted to carbonic acid [H2CO3]. This is called the carbonic acid equivalence point or the carbonic acid endpoint.

The primary reason we care about it is that, from the equation above, it is mostly a measure of bicarbonate and carbonate with the other values being pretty small additions or subtractions. We care about bicarbonate because that is what corals use to make their skeletons, but we don't (at least before the Mindstream comes out) have a way to readily measure bicarbonate itself.

So, the reason the other things are in the TA equation is because they also take up acid as one is dropping the pH to the carbonic acid endpoint. That endpoint actually varies a bit with the alkalinity itself, but the endpoint in seawater at 7 dKH is about pH 4.2.

Bicarbonate takes up one H+ and carbonate takes up 2 H+ when dropping the pH to 4.2, , hence the 2 in front of carbonate in the equation:

HCO3- + H+ --> H2CO3
CO3-- + 2H+ --> H2CO3

So the question is what else (besides bicarbonate and carbonate) takes up acid as you drop the pH from normal reef tank pH to about pH 4.2.

Any borate that is present will become boric acid, taking up one H+:

B(OH)4- + H+ --> B(OH)3 + H2O

Nearly all of the hydroxide present will become H2O:

OH- + H+ --> H2O

Any silicate present will become silicic acid:

Si(OH)3O- + H+ --> Si(OH)4

This is a strange one, but any magnesium monohydroxide will become free magnesium and water:

MgOH+ + H+ --> Mg++ + H2O

And finally to get to Jim's question...

Any form of phosphate present will be taken to H2PO4- (not H3PO4 because the pKa of H3PO4 in seawater is well below 4 and doesn't form appreciably even at pH 4.2).

So we have:

HPO4-- + H+ --> H2PO4-
PO4--- + 2H+ --> H2PO4-

which is why there is a 1 in front of HPO4-- and a two in front of HPO4- (the latter taking up two H+ in the test).

We also need to subtract out the very tiny amount of H+ already present in the seawater, but that is trivially small in seawater at pH 8.