Dear Andrea,

Different forms of Henry's law are discussed in the technical literature. 

The table depicted in the following link lists some of Henry's law and constant. The table reveals that a certain change in the solvent (water) has a significant effect on Henry's constant as expected when the values of TDS and TOC are relatively high in water. 

It should also be noted the Henry's law is a limiting law that only applies for dilute solutions. The range of concentrations in which it applies becomes narrower the more the system diverges from ideal behavior. Roughly speaking, that is the more chemically different the solute is from the solvent.

It also only applies for solutions where the solvent does not react chemically with the gas being dissolved. A common example of a gas that does react with the solvent is carbon dioxide (CO2), which rapidly forms hydrated carbon dioxide and then carbonic acid (H2CO3) with water.

https://chemengineering.wikispaces.com/Henry%27s+Law

Hoping this will contribute to the discussion on this topic,

Rafik

Dear Rafik,

I appreciate your quick response to my question. The table that you attached was informative for the different forms of Henry's Law Constants for inorganic gases in water and will be useful to me.  My question is more directed at organic compounds partitioning between air and water and the effect of the mineral and organic content of the water on the partitioning.  Are there additional links you can provide for this topic?

Andrea

Temperature, TDS, TSS are the important factor for it.

Dear Mahadev,

I agree that Temperature, TDS, TSS are the important factor affecting Henry's law Constants. Temperature effects can be predicted by the van't Hoff equation.  Do you know of quantitative or semi-quantitative equations that demonstrate the relationship between TDS and TSS and Henry's Law Constants?

Andrea

Dear Andrea, 

The second edition of "Environmental Organic Chemistry" by Schwarzenbach, Gschwend, and Imboden has a very extensive discussion of your question in Part II : Equilibrium partitioning between gaseous, liquid, and solid phases, that extends over nine chapters.

Given the variety of phenomenon that are involved it is very difficult to accurately predict such equilibria as the gas-liquid equilibrium of Henry's Law as a function of the aggregate water quality parameters, even for a single organic contaminant. One approach is to estimate Henry's Law constant as the ratio of the vapour pressure of the pure substance and the aqueous solubility, and then caculate a correction to the aqueous solubility based on various processes.  For example,  I recall the text discusses at length the partition of organic contaminants between bulk water and humic substances, and the estimation of this behaviour from octanol-water equilibria data. The same can be done with ion-exchange on mineral particulates, inorganic complexation, sorption to particles and so on... Applying what can be a litany of nested assumptions and correction factors, a new value for the partition coefficient in question can be obtained. I remember doing this for Dr. Joe Ryan's graduate course exams on the subject at CU Boulder (Dr. Ryan was a student of Gschwend I believe). However, the problem is that if the uncertainties are considered for each step, the propagation of error may result in no significant difference from the conditions in pure water. I remember discussing this with Dr. Ryan, that this approach could not possibly be very precise and was somewhat of an academic exercise. 

However,  in the absence of much good quality data and mindfulness of the potential complexity of the relevant phenomena, I would use the approach of the above text to try to obtain a "order of magnitude" result that might suggest which phenomena may be most significant, a kind of reality check. 

Sorry for the long message, but I hope there was something useful there. 

Dear Andrea,

This is a good question. For rather non-polar organic chemicals I suggest to distinguish two different phenomena that take place in the water phase:

1) Salting out effect. There will be a slight effect of the ionic strength on the activity coefficient that controls the partitioning behavior of the dissolved solute. Basically the activity coefficient increases with increasing ionic strength resulting in an increasing Henry's law constant. This can be calculated using the Setschenow law and constant. This effect is generally rather limited in freshwater, whereas the salinity of marine waters often has an effect in the order of 30-50%.

2) Binding to third phases within the water. I understand the Henry's Law constant as the partition ratio that describes the partitioning between the dissolved forms of the chemical in air (without particles and aerosols) and Water (without binding third phases). This Henry's law constant should then not be affected by the binding to third phases. To the contrary, the distribution coefficient between air and water (now including binding to third phases) will decrease due to binding to third phases in the Water.

A relatively new approach to quantify both effects is the analytical determination of the (enhanced) capacity of the aqueous solution relative to pure water. This can be precisely determined by equilibrating the aqueous solution and pure water with a polymer that is loaded with the chemical(s) of interest. At equilibrium, the total concentration is then measured in the aqueous solution and water and the enhanced capacity (E) is determined as E=Ctotal,solution/Cwater. The enhanced capacity can subsequently be used as correction factor for partition ratios and distribution coefficients. This "analytical passive dosing" approach is presented in the following two publications, and the first publication also includes experimentally determined Setschenow constants for some PAHs:

https://www.researchgate.net/publication/51859155_Measuring_Binding_and_Speciation_of_Hydrophobic_Organic_Chemicals_at_Controlled_Freely_Dissolved_Concentrations_and_without_Phase_Separation

https://www.researchgate.net/publication/230828502_Partitioning_of_fluoranthene_between_free_and_bound_forms_in_stormwater_runoff_and_other_urban_discharges_using_passive_dosing

Best regards

Philipp

Article Measuring Binding and Speciation of Hydrophobic Organic Chem...

Article Partitioning of fluoranthene between free and bound forms in...

Beware of using only the Van't Hoff equation for estimating temperature dependency of Henry's Law Constants.  The HLC temperature dependence curve is compound-specific, so you would need to see studies of the particular compound(s) of interest.  See Staudinger's paper in Chemosphere for a compilation of these relationships.

Also take into account organic films at the water surface . . .  let us know your specific problem to get more relevant comments.