Reef Chemistry Question of the Day #145 Alkalinity Effects of Chemicals

[Randy Holmes-Farley]

Reef Chemistry Question of the Day [HASHTAG]#145[/HASHTAG]

For each of the chemicals listed below, state whether they will, when added to a reef aquarium and allowed to equilibrate for 5 minutes, raise, lower or leave unchanged the carbonate alkalinity?

A. NaHSO4 (sodium bisulfate)
B. Na2O (sodium oxide)
C. NaH2PO4 (monosodium phosphate)
D. NaHCO3 (sodium bicarbonate)


Good luck!
























.

 

[JimWelsh]

Shouldn't that be Na2O?

 

[Randy Holmes-Farley]

Shouldn't that be Na2O?

Yes, sorry.

 

[JimWelsh]

A. NaHSO4 (sodium bisulfate) -- Lower
B. Na2O (sodium oxide) -- Leave unchanged
C. NaH2PO4 (monosodium phosphate) -- Leave unchanged
D. NaHCO3 (sodium bicarbonate) -- Raise

 

[Randy Holmes-Farley]

The answer is that A lowers carbonate alkalinity, while B and D raise it. C leaves total alkalinity unchanged, but reduces carbonate alkalinity (carbonate alkalinity was the question). They key to figuring out such questions is determining what happens to these ions when added to seawater.

Remember for this question there are two types of alkalinity

Total alkalinity = [HCO3-] + 2[CO3--] + [B(OH)4-] + [OH-] + [Si(OH)3O-] + [MgOH+] + [HPO4--] + 2[PO4---] - [H+]
and
Carbonate alkalinity = [HCO3-] + 2[CO3--]

A. Sodium bisulfate is acidic and immediately becomes Na+ and H+ and SO4-- in water at any pH above 2, including seawater. The H+ being added lowers carbonate alkalinity. The SO4-- has no impact on alkalinity, so alk declines.

When H+ is added to seawater, both of these reactions take place:

H+ + HCO3- --> H2CO3 <--> H2O + CO2
H+ + CO3-- --> HCO3-

In both reactions, carbonate alkalinity decreases.

B. Na2O immediately hydrates in water to become sodium hydroxide, releasing Na+ and OH- into the water. The hydroxide ion (OH-) immediately boosts alkalinity the same way limewater (kalkwasser) does:

OH- + HCO3- --> CO3-- ++ H2O
OH- + CO2 --> HCO3-

Both of these reactions raise carbonate alkalinity

C. Monosodium phosphate is by far the trickiest one here. In fact, I think it is one of the conceptually most difficult of the Reef Chemistry Questions of the Day!

The first thing that happens on adding it is that it dissociates into Na+ and H2PO4- (at least we can think of it that way). Then what happens?

Phosphate in seawater at pH 8.1 consists of an equilibrium mix of 0.5% H2PO4-, 79% HPO4--, and 20% PO4---.

The added HPO4- does not itself directly impact alkalinity, but it immediately redistributes into about that same assortment of H2PO4-, HPO--, and PO4---, assuming you did not add so much that the pH dropped a lot:

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

So H+ is released and the HPO4-- and PO4--- rise. The released H+ depletes total alkalinity, but the raised HPO4-- and PO4--- raise total alkalinity. Since the H+ added exactly equals the rise in the amounts of HPO4-- and PO4--- (counting the PO4--- twice since it needed two H+ to be released to form it), then the total alkalinity remains unchanged after this redistribution.

BUT, carbonate alkalinity is depleted by the H+ which is released (by the same equations shown in A) and since the phosphate forms that rise are not part of carbonate alkalinity, the carbonate alkalinity declines.

D. Sodium bicarbonate directly adds HCO3-, so it boosts carbonate alkalinity.