Vapor pressure?
 

Arrrgh, this is killing me. I just don't have the right reference books at hand anymore.

I have a bath of 10% phosphoric acid, heated to 160 degF. I'm trying to figure out the rate of phosphoric acid evaporation. I have all the equations, and most of the variables, but I can't figure out the vapor pressure of the phosphoric acid at that temperature. The best I can do is the vapor pressure at 68 degF, and that won't help me.

Anyone know?

Gotta be something about Charles law, nu?

Charles would certainly be a good place to start. I found a copy of the CRC Handbook, but it doesn't have vapor pressure data for phosphoric acid.

Look at the Clausius/Clapeyron equation:

DeltaH = - R* d(ln (Psat))/d(1/T)

it reduces to:

Ln(Psat2/Psat1) = -DeltaH/R*(1/T2 - 1/T1)

finally:

Psat2 = Psat1*exp(-DeltaH/R*(1/T2 - 1/T1))

DeltaH is the heat of evaporation (assume it's constant)
R is the ideal gas constant
T is absolute temperature
Psat is the vapor pressure

make Psat1 and T1 be the vapor pressure you know at the temp you know

Ahh, that ought to do it. Thanks VERY much, kutulu.

NP. You probably already took care of it, but make sure that you estimate the heat of evaporation right. I don't remember how it worked with aqueous solutions, but you can't just use DeltaH for prosphoric acid. Maybe you could assume that it is 0.9*DeltaHwater + 0.1*DelataHphosphoric. It's not right but it should be ok.