PH and it's effect on PO4?

[reefbeard]

Would maintaining a PH of 8.3-8.4 in a reef invironment be high enough to cause a continual precipitaion of phosphates into calcium phosphate? Would precipitation only occur where and when the high PH limewater enters the tank? Are higher PH systems synonymous with better PO4 management?

Just curious if I could effectively maintain lower PO4 levels with the assistance of a higher PH system along with skimming and running GFO.

 

[Cory]

The effect is negligible. Randy talks about it in one of his articles but remains uncertain. In a high ph enviroment, like 11, po4 precipitates out.

 

[reefbeard]

The effect is negligible. Randy talks about it in one of his articles but remains uncertain. In a high ph enviroment, like 11, po4 precipitates out.

Thanks for the info. Wasn't too sure how high PH had to be in order to cause precipitation of phosphate.

 

[Randy Holmes-Farley]

Higher pH will encourage precipitation, but how significant it is at pH 8.3-8.4, I do not know.

I discuss it here:
https://www.reef2reef.com/blog/phosphate-in-the-reef-aquarium-by-randy-holmes-farley/

Phosphate Reduction via Calcium Phosphate Precipitation

One mechanism for phosphate reduction in reef aquaria may simply be the precipitation of calcium phosphate, Ca3(PO4)2. The water in many reef aquaria is supersaturated with respect to this material, as its equilibrium saturation concentration in normal seawater is only 0.002 ppm phosphate. As with CaCO3, the precipitation of Ca3(PO4)2 in seawater may be limited more by kinetic factors than by equilibrium factors, so it is impossible to say how much will precipitate under reef-aquarium conditions (without, of course, somehow determining it experimentally). This precipitation may be especially likely where calcium and high pH additives (such as limewater/kalkwasser) enter the aquarium water. The locally high pH converts much of the HPO4– to PO4—. Combined with the locally high calcium level (also from the limewater), the locally high PO4— level may push the supersaturation of Ca3(PO4)2 to unstable levels, causing precipitation. If these calcium phosphate crystals are formed in the water column, e.g., if they form at the local area where limewater hits the aquarium water, then they may become coated with organics and be skimmed out of the aquarium.

Many reefkeepers accept the concept that adding limewater reduces phosphate levels. This may be true, but the mechanism remains to be demonstrated. Craig Bingman has done a variety of experiments related to this hypothesis, and published them in the old Aquarium Frontiers magazine. While many aquarists may not care what the mechanism is, knowing how it occurs will help us understand the limits of this method, and how to best employ it.

One possible mechanism could be through calcium phosphate precipitation, as outlined above. A second mechanism for potential phosphate reduction when using high pH additives is the binding of phosphate to calcium carbonate surfaces. The absorption of phosphate from seawater onto aragonite is pH dependent, with the binding maximized at around pH 8.4 and with less binding occurring at lower and higher pH values. Habib Sekha (owner of Salifert) has pointed out that limewater additions may lead to substantial precipitation of calcium carbonate in reef aquaria. This idea makes perfect sense. After all, it is certainly not the case that large numbers of reef aquaria exactly balance calcification needs by replacing all evaporated water with saturated limewater. And yet, many aquarists find that calcium and alkalinity levels are stable over long time periods with just that scenario. One way this can be true is if the excess calcium and alkalinity, which such additions typically add to the aquarium, are subsequently removed by precipitation of calcium carbonate (such as on heaters, pumps, sand, live rock). It is this ongoing precipitation of calcium carbonate, then, that may reduce the phosphate levels; phosphate binds to these growing surfaces and becomes part of the solid precipitate.

If the calcium carbonate crystal is static (not growing), then this process is reversible, and the aragonite can act as a reservoir for phosphate. This reservoir can inhibit the complete removal of excess phosphate from a reef aquarium that has experienced very high phosphate levels, and may permit algae to continue to thrive despite all external phosphate sources having been cut off. In such extreme cases, removal of the substrate may even be required.

If the calcium carbonate deposits are growing, then phosphate may become buried in the growing crystal, which can act as a sink for phosphate, at least until that CaCO3 is somehow dissolved. Additionally, if these crystals are in the water column, e.g., if they form at the local area where limewater hits the aquarium water, then they may become coated with organics and skimmed out of the aquarium.

If phosphate binds to calcium carbonate surfaces to a significant extent in reef aquaria, then this mechanism may be attained with other high pH additive systems (such as some of the two-part additives, including Recipe #1 of my DIY system). However, this potential precipitation of phosphate on growing calcium carbonate surfaces will not be as readily attained with low pH systems, such as those using calcium carbonate/carbon dioxide reactors or those where the pH is low due to excessive atmospheric carbon dioxide, because the low pH inhibits the precipitation of excess calcium and alkalinity as calcium carbonate, as well as inhibiting the binding of phosphate to calcium carbonate.

 

[reefbeard]

Higher pH will encourage precipitation, but how significant it is at pH 8.3-8.4, I do not know.

I discuss it here:
https://www.reef2reef.com/blog/phosphate-in-the-reef-aquarium-by-randy-holmes-farley/

Phosphate Reduction via Calcium Phosphate Precipitation

One mechanism for phosphate reduction in reef aquaria may simply be the precipitation of calcium phosphate, Ca3(PO4)2. The water in many reef aquaria is supersaturated with respect to this material, as its equilibrium saturation concentration in normal seawater is only 0.002 ppm phosphate. As with CaCO3, the precipitation of Ca3(PO4)2 in seawater may be limited more by kinetic factors than by equilibrium factors, so it is impossible to say how much will precipitate under reef-aquarium conditions (without, of course, somehow determining it experimentally). This precipitation may be especially likely where calcium and high pH additives (such as limewater/kalkwasser) enter the aquarium water. The locally high pH converts much of the HPO4– to PO4—. Combined with the locally high calcium level (also from the limewater), the locally high PO4— level may push the supersaturation of Ca3(PO4)2 to unstable levels, causing precipitation. If these calcium phosphate crystals are formed in the water column, e.g., if they form at the local area where limewater hits the aquarium water, then they may become coated with organics and be skimmed out of the aquarium.

Many reefkeepers accept the concept that adding limewater reduces phosphate levels. This may be true, but the mechanism remains to be demonstrated. Craig Bingman has done a variety of experiments related to this hypothesis, and published them in the old Aquarium Frontiers magazine. While many aquarists may not care what the mechanism is, knowing how it occurs will help us understand the limits of this method, and how to best employ it.

One possible mechanism could be through calcium phosphate precipitation, as outlined above. A second mechanism for potential phosphate reduction when using high pH additives is the binding of phosphate to calcium carbonate surfaces. The absorption of phosphate from seawater onto aragonite is pH dependent, with the binding maximized at around pH 8.4 and with less binding occurring at lower and higher pH values. Habib Sekha (owner of Salifert) has pointed out that limewater additions may lead to substantial precipitation of calcium carbonate in reef aquaria. This idea makes perfect sense. After all, it is certainly not the case that large numbers of reef aquaria exactly balance calcification needs by replacing all evaporated water with saturated limewater. And yet, many aquarists find that calcium and alkalinity levels are stable over long time periods with just that scenario. One way this can be true is if the excess calcium and alkalinity, which such additions typically add to the aquarium, are subsequently removed by precipitation of calcium carbonate (such as on heaters, pumps, sand, live rock). It is this ongoing precipitation of calcium carbonate, then, that may reduce the phosphate levels; phosphate binds to these growing surfaces and becomes part of the solid precipitate.

If the calcium carbonate crystal is static (not growing), then this process is reversible, and the aragonite can act as a reservoir for phosphate. This reservoir can inhibit the complete removal of excess phosphate from a reef aquarium that has experienced very high phosphate levels, and may permit algae to continue to thrive despite all external phosphate sources having been cut off. In such extreme cases, removal of the substrate may even be required.

If the calcium carbonate deposits are growing, then phosphate may become buried in the growing crystal, which can act as a sink for phosphate, at least until that CaCO3 is somehow dissolved. Additionally, if these crystals are in the water column, e.g., if they form at the local area where limewater hits the aquarium water, then they may become coated with organics and skimmed out of the aquarium.

If phosphate binds to calcium carbonate surfaces to a significant extent in reef aquaria, then this mechanism may be attained with other high pH additive systems (such as some of the two-part additives, including Recipe #1 of my DIY system). However, this potential precipitation of phosphate on growing calcium carbonate surfaces will not be as readily attained with low pH systems, such as those using calcium carbonate/carbon dioxide reactors or those where the pH is low due to excessive atmospheric carbon dioxide, because the low pH inhibits the precipitation of excess calcium and alkalinity as calcium carbonate, as well as inhibiting the binding of phosphate to calcium carbonate.

Thanks Randy!

 

[Randy Holmes-Farley]

You're welcome.

Happy Reefing. :)