CaSO4 solubility

[IliasBouchkira]

Dear all,

Please can anyone shows me how to transforme this graphe to another graphe with (moles/kg water for CaSO4 and P2O5). Thank you in advance,

[dlparkhurst]

Because you are posting this in the PHREEQC forum, let me first say that most of the concentrations on this graph are outside the range of applicability of the ion-association databases of PHREEQC, and that pitzer.dat, which would be the database of choice for high ionic strength, does not have parameterizations for phosphorus.

Here is the way I would convert to molality, at least for the more dilute end of the graph. Consider a kilogram of solution.

The number of moles of P2O5 in the solution is
moles(P2O5) = Wtp(P2O5)/100*1000(g/kg)/GFW(P2O5)

The first estimate of the mass of water is
kgw' = 1 - Wtp(CaSO4)/100 - Wtp(P2O5)/100

However, if the form of phosphorus in solution is H3PO4, then every mole of P2O5 consumes 3 moles of water
P2O5 + 3H2O = 2H3PO4

So the remaining mass of water is
kgw = kgw' - 3*moles(P2O5)*GFW(H2O)/1000

molality(P) = moles(P2O5) * 2 / kgw