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

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


1
Chapter 6Air-Organic Solvent and Air-Water
Partitioningin other wordsHenrys Law
  • equilibrium partitioning between air and water

2
KH PoL/Csatw
Kow Csato/Csatw
Air
Koa Csato/PoL
A gas is a gas is a gas T, P
Koa
KH
Octanol
PoL
Water
NOM, biological lipids, other solvents T,
chemical composition
Fresh, salt, ground, pore T, salinity, cosolvents
Kow
Pure Phase (l) or (s)
Csato
Csatw
Ideal behavior
3
Ranges of KH
4
Partitioning between air and any solvent
(recall that in an ideal solution, g 1) If g is
constant, even close to solubility, then
units of pressure over mole fraction (no one uses)
units of pressure over molar conc Pa-m3/mol or
Pa-L/mol
dimensionless units or Lwater/Lair
5
VP/solubility
if activity coefficients do not change, even as
the chemical approaches saturation, then Henrys
law may be estimated as the compounds vapor
pressure divided by its aqueous solubility
this is, I think, a useful concept that has been
lost in the new edition of the text. If a
compound has both a low VP and a low solubility,
it can be difficult to judge what its HLC will be.
6
VP ranges over 1012
HLC ranges over 107
solubility ranges over 1012
7
Factors influencing HLC
  • Temperature
  • Salinity
  • Cosolvents

8
Temperature dependance of HLC
units for Kaw in this equation must be
pressure-L/mol (and must match R)
Note you can use any units for Kaw in this
equation except dimless
If you want to use dimless units, use this form
of the equation
air
DvapH
DH Henry
DH Henry DH vaporization minus the excess
enthalpy of solubilization When solvent is
similar to solute, DHE may be negligible
water
DHE
Pure liquid
9
?If you cant find HE, then just use DHvap
10
Effect of salinity and cosolvents on HLC
Salinity will increase HLC by decreasing the
solubility (increasing the activity coefficient)
of the solute in water. Account for salinity
effects via Setschenow constant
Cosolvents will decrease HLC by increasing the
solubility (decreasing the activity coefficient)
of the solute in water. Account for cosolvent
effects via
sic is the cosolvent term, which depends on the
identity of both the cosolvent and solute fv is
the volume fraction of cosolvent
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
refractive index describes polarity
molar volume describes vdW forces
additional polarizability term
H-bonding
Note that this is a generic equation for
estimating the partition of a compound between
air and any solvent. It is similar to the
equation we used to estimate vapor pressure and
solubility, but is slightly less complicated
13
Table 6.2
14
For water
That darn cavity term is back!
15
Measurement of Henrys 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

where G volume of gas Vw volume of water
18
Henrys Law Constants of Polychlorinated Biphenyl
Congeners andTheir Variation with
TemperatureHolly A. Bamford, Dianne L. Poster,
and Joel E. Baker ,
paper
19
HLC of all 209 PCB congeners
HLC (and VP, too) are lowest for congeners with
few ortho chlorines
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.

21
lt Higher --Temperature-- Lower gt
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
  • Hollys 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
  • Hollys 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 HM 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?

28
KH from fragment constants structure-property
relationships structure-property relationships
used to predict many things ? specific
structural units increase or decrease and
compound's KH by about the same amount. KH
estimation method
where f are factors for structural units, and F
are correction factors for affects such as
polyhalogenation, etc. Note factors for
fragments attached to aliphatic carbons (C-H) are
not the same as those attached to aromatic
carbons (Car-H) Example C-Cl -0.30 Car-Cl
0.14
29
table 6.4
aliphatic alcohols
Benzene biphenyl
30
Examples
hexane log Kiaw (n-hexane) 14(C-H) 5(C-C)
0.75 0.75 is the correction factor for a linear
or branched alkane log Kiaw (n-hexane)
140.1197 5-0.1163 0.75 1.84 experimental
value is 1.81 benzene logKiaw (benzene)
6(Car-H) 6(Car-Car) logKiaw (benzene)
6(0.1543) 6(-0.2638) -0.66 experimental value
is 0.68
31
Example PCBs by MH 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
1,1,1-TCA Cair 0.9 mg/m3 Cwater 2.5 mg/m3 Is
this compound volatilizing from, or absorbing
into, the arctic ocean at 0C and at 10C? Salinity
0.35o
36
homework
Problems 6.5 and 6.1
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