Henry's law

# Henry's law - Chapter 6: Air-Organic Solvent and Air-Water...

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Chapter 6: Air-Organic Solvent and Air-Water Partitioning in other words Henry’s Law equilibrium partitioning between air and water

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Air Water Octanol A gas is a gas is a gas T, P Fresh, salt, ground, pore T, salinity, cosolvents NOM, biological lipids, other solvents T, chemical composition Pure Phase (l) or (s) Ideal behavior P o L C sat w C sat o K H = P o L /C sat w K oa K H K ow = C sat o /C sat w K ow K oa = C sat o /P o L
Ranges of K H

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Partitioning between air and any solvent iL il il i il p x p f * γ = = (recall that in an ideal solution, γ = 1) If γ is constant, even close to solubility, then: il iH iL il il i il x l K p x p f = = = ) ( * ' iL il il i iH p x p l K * ) ( ' = = units of pressure over mole fraction (no one uses) il i iH C p l K = ) ( units of pressure over molar conc Pa-m 3 /mol or Pa-L/mol RT l K C C l K iH il ia ial ) ( ) ( = = “dimensionless” units or L water /L air
VP/solubility if activity coefficients do not change, even as the chemical approaches saturation, then Henry’s law may be estimated as the compounds vapor pressure divided by its aqueous solubility il sat Li iH C p l K * ) ( 2245 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.

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VP ranges over 10 12 solubility ranges over 10 12 HLC ranges over 10 7
Factors influencing HLC Temperature Salinity Cosolvents

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Temperature dependance of HLC cst RT H l K i al iH + - = ) ( ln ' E il i vap i al H H H - = H “Henry” = H vaporization minus the excess enthalpy of solubilization When solvent is similar to solute, H E may be negligible water Pure liquid air H E vap H H “Henry” - = 2 1 ) 1 ( ) 2 ( 1 1 ln T T R H K K aw T H T H Note: you can use any units for K aw in this equation except dim’less units for K aw in this equation must be pressure-L/mol (and must match R) - + = 2 1 1 2 1 1 ln T T R RT H K K av aw awT awT If you want to use dim’less units, use this form of the equation
If you can’t find H E , then just use H vap

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## This document was uploaded on 10/25/2011.

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Henry's law - Chapter 6: Air-Organic Solvent and Air-Water...

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