Reef Chemistry Question of the Day #17

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #17

According to both the theoretical definition of alkalinity and the result with a normal aquarium hobby titration, which of the following do not ever contribute to total alkalinity of seawater, regardless of concentration?

1. Borate
2. Carbonate
3. Phosphate
4. Nitrate
5. Bicarbonate
6. Silicate
7. Hydroxide

Good luck!


















.

 

[leptang]

6. silicate

 

[hart24601]

Without looking on the google machine I am stumped...

I would have to say silicate since the others seem like terms I hear when talking about alk!

 

[acurro]

Think they all do, but carbonate. Carbonate is the end result of the calcification reaction .

 

[Randy Holmes-Farley]

Don't be shy.

The only dumb answer is 8. Calcium, and that's because it wasn't a choice!

 

[lhfelice]

4. Nitrate
6. Silicate
7. Hydroxide

 

[lhfelice]

Sorry Hydroxide does contribute, copy / paste mistake.

 

[Mikesmith34]

I'm with #7. Im not sure but that was my reductive answer

 

[kpiotrowski]

nitrate...and if right a lucky guess, to some degree.

 

[acurro]

Ok. Broke out the chemistry textbook. Do computer stuff, but I think my original post missed carbonate. It really depends how you define alkalinity too.....

1. Borate (BOH3, adds OH+ to solution, so yes)
2. Carbonate (CO3, absorbs H+ ions, so yes)
3. Phosphate (PO3, absorbs H+ ions, yes)
4. Nitrate (NO3, Absorbs H+ ion, yes)
5. Bicarbonate (HC03, adds OH ion. yes)
6. Silicate (SiF6, Absorbs H+ ions, yes)
7. Hydroxide (OH, yes)

 

[Randy Holmes-Farley]

Someone has a correct answer...

 

[Randy Holmes-Farley]

And the answer is.... 4. Nitrate. Nice guessing, kpiotrowski!

In short, total alkalinity counts anything that buffers against the addition of acid down to a pH of about 4.5. All of the ions shown, except nitrate, will take up at least one H⁺ as the pH is dropped to 4.2. Nitrate would not become protonated to become nitric acid until the pH is well below 0.


Here's a blurb from one of my articles with a lot more detail on what total alkalinity comprises and why:

Alkalinity is defined in different ways for different applications. In the chemistry of natural waters, there are several types of alkalinity that are encountered. Each of these is a measure of how much acid (H⁺) is required to lower the pH to a specific level. I'll come back to some of the other types of alkalinity later, but for now we will confine our discussion to the "total alkalinity." frequently referred to as TA.

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. These equations show what happens to carbonate and bicarbonate as acid is added:

(1) H⁺ + CO₃^-- ==> HCO₃^-

(2) H⁺ + HCO₃^- ==> H₂CO₃

I say "could be converted" because regardless of the pH, there will always be some bicarbonate and carbonate present, but at some pH there are enough protons (H⁺) in solution that if they were combined with the bicarbonate and carbonate present, it would all be converted to carbonic acid.

The precise endpoint of a total alkalinity titration isn't always the same pH, but rather depends a bit on the nature of the sample (both its ionic strength and its alkalinity). For normal seawater, this endpoint is about pH = 4.2. In freshwater it depends strongly on the alkalinity, with an endpoint of pH = 4.5 for an alkalinity of 2.2 meq/L, and pH = 5.2 for an alkalinity of 0.1 meq/L.

Consequently, total alkalinity tests have been invented that determine how much acid is required to lower the pH into the 4-5 range.

Chemical Nature of Alkalinity

Based on the definition of total alkalinity given above, it is clear that anything that absorbs protons when the pH is dropped from normal levels to about 4-5 will be counted toward alkalinity. In seawater there are a variety of things that contribute, and in reef tanks the list is even longer. Equation 3 is the defining equation for total alkalinity in normal seawater.


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


The reason for the 2 in front of the carbonate and phosphate concentrations is that they take up two protons as the pH is dropped down to pH 4. All of the other ions just take up a single proton (except protons themselves which must be subtracted).

 

[beaslbob]

Yea

but doesn't nitrate return alk as it is reduced by bacteria or plant action?


(thanks this one was interesting)

 

[Randy Holmes-Farley]

Yea

but doesn't nitrate return alk as it is reduced by bacteria or plant action?


(thanks this one was interesting)

Yes, if nitrate is consumed by something and converted into N₂ gas or into organic tissues, there is a net release of alkalinity back to the system.

1. In the absence of O₂, and taking the nitrogen species completely to N₂ (like in a deep sand bed), we have the following overall reaction:


organic + 124 NO₃^- + 2CO₂ → 124HCO₃^- + 70 N₂ + 208 H₂O

where organic stands for a typical organic material ((CH₂O)₈₀(CH₂)₄₂(NH₃)₁₆).

2. In the presence of O2, where nitrate is just used for building organic tissues, we have:

138 CO₂ + 16 NO₃^- + 154 H₂O → organic + 175 O₂ + 16 HCO₃^-


So we can see that in both cases, we get exactly 1 equivalent of alkalinity (as bicarbonate) for each equivalent of nitrate consumed.

 

[beaslbob]

Thanks. always enjoyed that article. It was very surprising to me anyway.

If that all happens can one then brag about having a "mature" tank. whatever that means.