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[CHEMENGR2811]

Dear all,

 

Is there anybody knows the equation for to calculate the "Dew point of Phosphoric acid"?

 

Regards

Chemengr

[MrShorty]

I'm not sure there is a single equation that will rigorously describe the dew point of "phosphoric acid" -- at least not without further specification.

 

Can I safely assume that we are actually talking about an aqueous solution of H3PO4 and not some more exotic solution?

 

I'm pretty sure that phosphoric acid is analogous to other oxy-acids like carbonic acid, nitric acid, and sulfuric acid. The "acid" (H3PO4) does not exist in the vapor/gas phase. In the vapor/gas phase, it would exist as the anhydrous P2O5. A rigorous treatment of the dew point/VLE of a phosphoric acid solution will need to account for this chemistry.

 

If we can assume (and this, for me makes it hard to think of a true "dew point") that the partial pressure of P2O5 is negligible, so that we can ignore it, we can probably simplify the problem to a modification of Raoult's law: P(H2O)=x(H2O)*gamma(H2O)*P0(H2O) where P(H2O) is the partial pressure of water over the solution, x(H2O) is the mole fraction water in the "condensate", P0(H2O) is the pure component vapor pressure of water, and gamma(H2O) is the activity coefficient of water in the liquid. The real challenge in this equation is what expression to use for the activity coefficient of water, which will probably have to come from other measurements of the partial pressure of water over phosphoric acid mixtures.

 

Perry's has a couple of graphs showing the partial pressure of water over phosphoric acid solutions. That might give you something to work with.