Sciencemadness Discussion Board

Solubility of a mixture of isomers

Plastifabulous - 4-9-2009 at 08:08


I am trying to determine a value for the water solubility of a mixture of diasteromers of HBCD (hexabromocyclododecane). The technical product consists of 3 main isomers, with approximate ratio 10% alpha, 5% beta, 85% gamma. The water solubility of the individual isomers is known to be:

alpha: 49 ug/L (micrograms/L)

beta: 15 ug/L

gamma: 2 ug/L

The question is, what is the water solubility of the "technical product"?

This has become important during discussions of aquatic toxicity. Opinion seems to have split into 2 camps:

Industry - water solubility is 2ug/L "the major product"

Environmentalists - water solubility is 66ug/L "the sum of all solubilities"

Who is right?
Is it really possible to get 66ug/L into solution??
Can a value be approximated without further information on chemical activities, etc?

chemoleo - 4-9-2009 at 17:31

As it is used as a flame retardant, it is present in quantities much exceeding that of its solubilities in water per liter.

Therefore, if you take a gram of the technical product, and dissolve it in H2O, it will indeed dissolve 66 ug/L of all HBCD isomers together. As the alpha and beta isomers are more soluble, they will be leached out preferentially even though gamma is present in greater quantities.

i.e. per gram of industrial product it requires

alpha: 0.1/0.000049 = 2041 liters for its complete dissolution (2 cubic meters)
beta: 0.05/0.000015 = 3333 liters
gamma: 0.85/0.000002 = 425000 liters


I don't really see at all how the figure of 2ug/L can be the accepted one - it is only true for the major product (gamma isomer) but omits the higher solubilities of the alpha/beta products. The alpha and beta products are therefore much more a problem than the gamma product; i.e. in 3333 liters, all alpha and beta products are fully dissolved (per gram) while only a fraction of the gamma product is in solution.





For anyone who wants to know more:
http://www.empa.ch/plugin/template/empa/*/54785/---/l=2

Can't believe it's been found in the air on Greenland and in water bodies in norther Finnland - the stuff is literally everywhere!


[Edited on 5-9-2009 by chemoleo]

crazyboy - 4-9-2009 at 18:41

7.35μ/L

Multiply the solubility of each isomer by its percentage in the technical grade and add them all up.

watson.fawkes - 4-9-2009 at 19:42

Quote: Originally posted by chemoleo  
As it is used as a flame retardant, it is present in quantities much exceeding that of its solubilities in water per liter.

Therefore, if you take a gram of the technical product, and dissolve it in H2O, it will indeed dissolve 66 ug/L of all HBCD isomers together.
Not to disagree, but I want to point out a tacit assumption made here, namely that the solubilities of the various isomers are independent of each other. This is certainly a reasonable assumption; it's just that it may not be true in this case. Plausibility, to my eye, comes from the idea that the various isomers co-crystallize readily, and at arbitrary ratios. But I don't have data one way or the other.

In the worst case, though, the total solubility would be that of highest solubility fraction, the alpha isomer at 49 μ/L. As chemoleo mentioned, this would also be by preferential leaching. In either case, the industry claim of low total solubility is dead wrong.

chemoleo - 5-9-2009 at 15:21

Yes you are right there, watson - but like you say it would be at least 49 ug/L.
The industry ought to test the solubility of total material from the technical mixture - it is baffling that they haven't done so.

It is kind of - well, frightening- an aliphatic bromo-compound that is cyclised - ensuring its long stability - that ends up all over the world, with unknown biological effects - and the industry clearly misleads people by showing such a low solubility coefficient.


PS crazyboy - please explain why this makes sense? :o

crazyboy - 5-9-2009 at 18:34

Quote: Originally posted by chemoleo  

PS crazyboy - please explain why this makes sense? :o


Whoops sorry that's wrong now that I think about it I think it would be 66μg/l unless there is some cross solubility interference.

unionised - 6-9-2009 at 04:45

There are two answers because they are the answers to two different questions.
If I take a lot of this stuff and shake it with a little water the solution will end up saturated with all 3 components (assuming there's no interaction) so the solubility will be 66µg/l

On the other hand, if I want to dissolve a gram of the stuff I need (to a fair aproximation) enough to dissolve all the major product so I need 85% of 500000 litres of water plus more water to dissolve the other isomers as Chemoleo calculated.

From an environmental point of view I want to know how toxic the 3 isomers are before I can say what difference it makes.

Plastifabulous - 8-9-2009 at 06:45

Hi everyone, thanks for your answers!

Just to give my opinion, I assumed it would be something like 30-60 µg/l, as there would have to be some interference between the isomers.

And yes, it is worrying that it's being found everywhere. I'm pretty confident that use will be restricted very soon. The question is whether it is completely banned and how much time will be given to phase it out.