Reef Chemistry Question of the Day #87 Phosphate and water changes

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #87

When reef aquarists do water changes and track phosphate levels, they are often disappointed with the results.

For example, a 25% water change will drop nitrate from 100 ppm to 75 ppm when measured later that same day, but the same water change often will often not drop 0.1 ppm phosphate to 0.075 ppm when measured later that same day, but rather to something closer to the original 0.1 ppm.

Why?

Good luck!




























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[beaslbob]

1) test kits aren't all that accurate
2) phosphate is leaching from rock, sand etc
3) water change shocked the system slowing consumption of the phosphates
4) you turned the lights out during the water change
5) you stirred up the sand uncovering phosphates
6) you did a water changed instead of letting the algae consume the phosphates.
7) replacement water had phosphates and you're lucky phosphates didn't rise
8) beaslbob is crazy

 

[Marquiseo]

The hobbyist is assuming that 25% of the phosphate will be freely sucked out when doing WC without understanding that phosphate binds to the rocks. Also the hobbyist doesn't realize that 25% of the tank WC doesn't equate to 25% of the phosphate count. A higher % of water change may remove a larger amount of phosphates but you won't be able to state amount of phosphate removed based on the WC amount.

 

[RacingTiger03]

limewater top-off, precipatate out phos, THEN WC.. boom.. what do I get? LOL.



phosphate isn't really free floating throughout the water column throughout the normal day is it? Unless the water change is done way after lights out, I would assume it barely touches phosphates, not even counting the phos introduced by the salt used for the water change, as many of those carry some level of phosphates.

 

[baitrunner]

precipitated phosphates dissolving again

 

[tonizzy22]

Solubility equalibium of new water and ca with precipitated po4.

 

[Steven@]

phosphate binds to the rocks

 

[zesty]

How about a reverse answer? Nitrates are bound to water and that makes them easier to export thru WC.

I'm fairly confident I haven't explained that perfectly. This is from a conversation with a chemist a while back over beers... dang you beers!!!

 

[dallasg]

interested to see the answer

 

[Randy Holmes-Farley]

And the answer is, as some of you noted, phosphate can bind to calcium carbonate surfaces.

So even doing a 100% water change will not remove all phosphate because when you add new water back, some of the phosphate still bound to the rock and sand surface can come back into solution.

How much depends entirely on the phosphate concentration that started (more in solution means more bound), the amount of calcium carbonate surface area, and on the pH.

It is discussed in the scientific literature here:

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for example: