Need vapor pressure test for organics in aqueous mixtures

[mdszj]

Hello

I am trying to find a test method to determine the vapor pressure of organic compounds (such as toluene, xylenes, and ethylbenzene) in an aqueous solution. The application is at a pump and treat plant for treating contaminated groundwater at a Superfund site I am working on. The contaminants are present at relatively low concentrations of up to about 20 mg/L. The water treatment system has a NJDEP air permit that requires regular measurements of vapor pressure in various tanks within the system.

I have already checked out ASTM and found several V.P. tests which are all geared to pure products, (including D6377, D6378, D2879, E1194, E1719, and E1782), and been in contact with several ASTM people in committees, etc with no luck.

SO now I am getting desperate - any info on any type of method that can be used to measure vapor pressure of an organic compound in aqueous mixture would be appreciated.

 

[siretb]

Why not pump a known volume of gas (from the tank ceiling) through an activated carbon trap (or other adsorbents) , and send the trap to a laboratory.
A VOC / FID online-analyser will also give total VOC.

 

[mdszj]

Siretb

We regularly collect water samples and have them analyzed for VOC levels. We need to also measure vapor pressure because it it a permit requirement. thx

 

[dcasto]

use the reid vapour pressure test. It is an ASTM standard test.

 

[moltenmetal]

Is there any free product in the tank? If so, you'll be able to measure a total vapour pressure that differs significantly from that of water. Otherwise, total vapour pressure of components won't differ very much from that of water

What you're really after is how much VOC you're emitting via the "breathing" of the tank, correct?

Know the concentrations in the liquid? Know the temperature? Know the solubility of each component? Then you know the Henry's constant, and hence the vapour pressure of each component above the solution, assuming the headspace is in equilibrium with the liquid. The real concentration in the headspace can only be LOWER than this, and that's a conservative assumption for the air permit.