[APD] Re: Triple PO4 and Apatite solubility

["Jim Seidman" <js5 at seidman_net>]

A.Kumar writes:
>        After some more number crunching, I end up
>with a value of .02mg/L, or .02ppm phosphate.
>	This seems to be kind of low. However, the 0.02ppm
>phosphate will be constantly maintained (by
>dissolution) even as the plants uptake phosphate.

Unfortunately, this isn't really true. Remember that the Ca and F ions
aren't going anywhere, at least not quickly. This will inhibit future
dissolution.

Suppose you had saturated a solution with fluorapatite, and then removed
all the phosphate. You already have 5*6.94E-8 M of Ca++, and 6.94E-8 M of
F-. How much more fluorapatite can you dissolve? You can dissolve about
3.23E-8 M more, less than half of what you did on the first iteration,
given you roughly 0.01 ppm phosphate.

Here's a table showing how this works:
              [Ca++]  [PO4--]     [F-]
Starting	3.47E-07 0.00E+00 6.94E-08
Added	     1.615E-07 9.69E-08 3.23E-08
Total	      5.09E-07 9.69E-08 1.02E-07

The first row is how much you had from the initial dissolution, after
consuming all the phosphate. The second line is how much more fluorapatite
you can dissolve then. If you look at the final line, you'll see that with
the extra Ca++ and F-, Ksp is reached with a much lower [PO4--]. (All
numbers are molar concentrations.)

As you get more and more Ca++ and F- in solution, the phosphate
concentrations will just go down and down. Unless those other ions are
being consumed too, the phosphate won't stay at a current concentration.

So it still seems that Triple PO4 is not a good solution for fertilizing.

- Jim

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