Reef Chemistry Question of the Day #6

[Randy Holmes-Farley]

Phosphate has numerous forms that exist in reef aquaria, and the relative proportion of the various forms depends on several factors, especially pH. The actual forms present have a big impact on various processes, such as phosphate binding to calcium carbonate surfaces.

In seawater at pH 8.2, which form of inorganic phosphate predominates?


A. H₃PO₄
B. H[SIZE=-1]₂[/SIZE]PO[SIZE=-1]₄^-[/SIZE]
C. HPO₄^--
D. PO[SIZE=-1]₄^---[/SIZE]


I'd rate this one as medium-high difficulty, except that everyone has at least a 25% chance of success.

Not that it helps any for this question, but my avatar is a phosphate molecule. Specifically, since the double bond is shown, it is H₃PO₄ without any hydrogen atoms shown. If it were really PO4---, all four phosphate atoms would be identical, with none able to be specified as the one with the double bond to the central phosphorus atom.

Good luck.

 

[LetItReef]

The answer is C (HPO4--). If you look at the chart I have below, at pH 8.2, HPO4-- ishighest at around pH 9. It will need a higher pH to get to D.

 

[beaslbob]

I give up.

 

[DRThompson]

Answer is C, about 80% HPO4--

 

[Randy Holmes-Farley]

Just a hint, that graph is not for seawater, which is VERY different than freshwater.

 

[Preyou]

C would be the correct answer....

 

[LetItReef]

Just a hint, that graph is not for seawater, which is VERY different than freshwater.

in fresh water the answer willbe HPO3—which is C. But for saltwater where there is a lot of Calcium andMagnesium ions, the answer is D. PO4--- it shifts faster, starting at pH 8.2.

So PO4--- it is.

 

[hart24601]

Without the goggle machine you got me stumped. I would have guessed c... But sounds like it's wrong.

 

[Randy Holmes-Farley]

Great work. The answer is C.

Very high percentage correct on this one. I should make them harder.


The curves shown in the graph above shift quite a bit to the left for a variety of reasons, but not quite enough to change the answer.


The first is that in the presence of ordinary salts, such as sodium and chloride, it is easier to make more highly charged ions, such as HPO₄^--
and PO₄^---. In effect, the positively charged ions such as sodium surround the highly negatively charged species and stabilize them. So there ends up being a lot more HPO₄^-- and PO₄^--- than in fresh water.

The second effect was mentioned by LetItReef. Calcium and magnesium form soluble ion pairs with HPO₄^-- and, most especially, PO₄^---. Those ion pairs stabilize these forms even more, relative to H₂PO₄^-, and again, we have more of the higher charged forms. Very little H₂PO₄^- exists in seawater at pH 8.2 (less than 1%).
That said, the shift is not enough to make PO₄^--- the dominant species, at least until the pH gets fairly high, around 8.7, where it does dominate.

Here's a blurb from one of my articles:

The "simplest" form of phosphorus in seawater is inorganic orthophosphate (sometimes called Pi by biologists). It consists of a central phosphorus atom surrounded by four oxygen atoms in a tetrahedron (Figures 1 and 2). Three of these oxygen atoms can either have an attached hydrogen atom or carry a negative charge (Figure 2). The ratio of these different forms depends on the pH in seawater. At pH 8.1 seawater contains 0.5% H₂PO₄^-, 79% HPO₄^-- and 20% PO₄^---. At higher pH the equilibrium shifts toward more PO₄^--- and less HPO₄^--. For a variety of reasons, especially including the ion pairing and consequent stabilization of PO₄^--- by calcium and magnesium, there is far more PO4--- in seawater than in freshwater at the same pH. This shift in phosphate species distribution with pH may seem esoteric, but it actually has important implications for such things as the binding of phosphate to calcium carbonate rock and sand, because the different forms bind to different extents.