Re: calcium phosphate solubility (fwd)

---------- Forwarded message ----------
Date: Fri, 17 Jan 1997 00:28:16 -0800 (PST)
From: Craig Bingman <cbingman at netcom_com>
To: Aquatic-Plants-Owner at ActWin_com
Subject: Re: calcium phosphate solubility

> How soluble is CaPO3? If an aquarium has significant amounts of
> Ca ions will these precipitate the majority of phosphates? How
> much could stay dissolved in ppm of phosphate assuming a surplus
> of Ca ions (at ph of 6.8 ot 7)? If phosphate is in surplus, how
> much Ca will be dissolved?

Under freshwater planted tank conditions, calcium phosphate is freely 
soluble.  Its precipitation is not a significant feature of freshwater tanks.

The only form of phosphate that forms extremely insoluble salts with 
calcium is the trinegative form.  In dilute solution, the third pKa of 
phosphate is around 12.  So you are never going to get there in a soft 
water tank.  In marine tanks, through generally higher ionic strength 
(which increases the ionic shielding around highly charged ions) and 
direct ion paring interactions with magnesium and calcium, the third pKa 
of phosphate is depressed to below nine, which makes it within spitting 
distance of the pH of the system.  So you build some of that form up, it 
precipitates, and if you sweep that out of the system with a foam 
fractionator, you can pull the orthophosphate concentration down to very 
low concentrations.

The only systems that might exhibit some physicochemical limit on 
phosphate concentration are extremely high pH and high hardness rift 
lake cichlid tanks, and even then, the effect is going to be smaller 
than in marine systems.  Moreover, you don't run foam fractionators on 
freshwater systems, and without them, phosphate tends to accumulate in 
the system in marine aquaria.

If calcium phosphate were somewhat insoluble in freshwater tanks, it 
still wouldn't make a lot of difference, because it would still just 
pile up.

To answer your question about bioavailablity, if the calcium phosphate 
is buried deep, in say, a calcium carbonate rock, then it is 
unavailable.  If it is present as a surface film, algae can still access 
it by mass action.  They pump down the phosphate concentration as low as 
they can, and a little more dissolves from the precipitate.  Algae are 
more determined than phosphate is insoluble in freshwater tanks, by an 
extremely wide margin.

I would think that the main export mechanisms for phosphate in freshwater 
tanks are water changes, use of activated carbon and adsorption of 
organic phosphates, and perhaps the use of alumina.  Alumina is somewhat 
effective in freshwater, although I've been a bit disappointed in the 
trial runs that I've done before.  Bioassimilation is really the answer in 
freshwater planted tanks, as it is in soft coral marine systems.