Log D

Hi,

I am attempting to model partitioning for chemicals in seawater using the log D plugin.  Unfortunately, the limits are 0.25 mol/L for both Cl and Na.  In full strength seawater Na and Cl are both 0.5 M.  Is there a way to modify these input parameters?  Seems odd that this environmental matrix would not be supported. Thanks.

Jim

Dear Jim,

in Instant JChem, the calculator plugins are only embedded and only the options available for the plugins themselves are included. Therefore I am moving the post to a more suitable part of the forum.

Best regards,

Karla

Hi, 

No, you can't set this high ionic strength values for the logD prediction. The reason why this is so is that the logD predictor was designed to predict logD for standard ionic strength used in labs. 

To extend the concentrations we would need logD data measured in seawater. Do you have or know sources of such data?

Thank you, 

Daniel

Thanks Daniel.

The log D plugin allows specification for Na and Cl and the upper limit is 0.25 M for each.  At this limit the calculation is basically for half-strength seawater (0.5 M).  I was just wondering why it didn't go up to 0.5M.

I'm not sure about the data relating Kow to ionic strength, but the salting out effect is well known.  Apparently the solubility of neutral, non-polar organics can decrease by 50% in seawater compared to pure water.  I assumed the log D calculation was based on the physical-chemical properties
of the molecule and not specific data from the literature.

Jim

Hi, 

So our logD model is indeed based on physico-chemical properties of a molecule, but to test its validity we would need experimental data at those high ionic strength (seawater). We have 0.25 as an upper limit because that's what we have validity for based on experimental values. 

Daniel

Daniel,
Thanks for the clarification.  I assume it would take a fairly large database with a variety of compounds to validate such calculations.  I don't know of any such compilations for Kow is seawater.  Seems like an important issue, so perhaps someday this will be done, if it hasn't already.

Hi, 

Yes, it would be nice to extend our model to higher concentrations as well, but as you said, we haven't had data to do that so far. 

Daniel