Chapter 6: Air-Organic Solvent and Air-Water Partitioning in other words Henry s Law

1. Chapter 6:Air-Organic Solvent and Air-Water Partitioningin other wordsHenry�s Law equilibrium partitioning between air and water

3. Ranges of KH

4. Partitioning between air and any solvent

5. VP/solubility

7. Factors influencing HLC Temperature Salinity Cosolvents

8. Temperature dependance of HLC

10. Effect of salinity and cosolvents on HLC

11. LFERs relating partition constant in different air-solvent systems Once again, partitioning depends on size, polarity/polarizability, and H-bonding IF these interactions are similar in both solvents, then a simple LFER is sufficient:

12. A familiar estimation technique

13. Table 6.2

14. For water:

15. Measurement of Henry�s Law Relatively few measured values available. Hard to measure when solubility is low. Two approaches: static and dynamic

16. Static determination Static equilibration between air and water in a vessel such as a gas-tight syringe See problem 6.5

17. Dynamic determination batch air or gas stripping first must generate an aqueous solution containing a relatively high concentration of analyte first order process:

18. Henry�s Law Constants of Polychlorinated Biphenyl Congeners andTheir Variation with TemperatureHolly A. Bamford, � Dianne L. Poster, � and Joel E. Baker* ,�

19. HLC of all 209 PCB congeners

20. Does HLC go up or down with MW? Bamford shows HLC going up with MW. Solubility and VP both go down with MW. Which goes down faster? VP pretty well known, soly much less certain Bond contribution methods such as Hine and Mookerjee, Meylan and Howard suggest Kh goes down as MW goes up (based on limited data) Brunner (the only other good experimental data set) shows Kh goes down as MW goes up other VP/soly estimates show that Kh goes down as MW goes up for congeners with same number of ortho chlorines.

22. Old vs. new measurements

23. DS vs DH for air-water eqbm

24. Enthalpy questions Enthalpy of HLC is bigger than DHvap for many congeners. Environmental data shows that the slope of logP vs. 1/T plots increases as MW of PCBs increases. This is true at sites over land AND near water (such as Sandy Hook). If HLC relative to other temperatures is wrong, how can HLC at any given temp be correct?

25. Entropy questions DS of melting and vaporization are pretty much constant at 56.5 and 88 J/molK, respectively. DS of solubilization rises slowly with MW (from ~47 to ~62 J/molK) for PAHs Holly�s DS range from 49 J/molK (reasonable) to 500 J/molK (not reasonable!) How is an entropy change of 500 J/molK possible? DSfus for water = 22 J/molK Holly�s DS are a strong function of DH (R2 = 0.999)

26. Estimation technique Vapor Pressure/Solubility how good is either?

27. Estimation Technique: Bond contribution methods In the absence of any other info, QSAR methods give good approximation. Hine and Mookerjee 1975 bond contribution method 292 compounds Nirmalakhadan and Speece, 1988 connectivity indexes same data set as H&M but excludes amines, ethers, aldehydes & ketones good to within a factor of 1.8 for most compounds Meylan and Howard 1991 bigger data set (345 compounds) also good to within 1.8 Pitfalls How good are the calibration data? Measured or estimated from VP/soly? Human error? How big is the data set?

29. table 6.4

30. Examples:

31. Example: PCBs by M&H method Calibration set includes 12 halogenated benzenes: mean error = 21% and 3 PCBs error = 47% (is this good enough?) Validation set includes some PCBs and chlorobenzenes, they are predicted OK. Best to start with a known compound: 4-CBP logKh = -0.63 2-CBP log Kh = -0.09 subtract Car-H = -0.1543 add one Car-Cl = +0.0241 result = -0.76 (err = 7%) -0.22 (err = 78%) measured: 4,4� CBP = -0.79; 2,5 CBP = -0.47 Cl in the 2 position has a large effect on Kh. These estimation methods cannot account for that.

32. Other properties can be used to predict HLC works best when compounds are closely structurally related.

33. PAHs

34. PCBs- chlorine number

35. Problem 6.3

36. homework