Reef Chemistry Question of the Day #98 RO Membrane Penetration

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #98

Which of the following chemicals is most likely to penetrate a reverse osmosis membrane (assuming no other filters are being used)?


A. Sodium ion (Na+)
B. Ammonia (NH3)
C. Magnesium ion (Mg++)
D. Chloride ion (Cl-)

Good luck!



















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[GHill762]

b. ammonia

 

[GHill762]

edit: sorry, double post.

 

[beaslbob]

Third guess
c. Magnesium

(Looks like the smallest)

 

[tonizzy22]

Answer is B all others are charged and will hold onto more water molecules making them harder to pass

 

[tgp4274]

I say D. Chloride ion (Cl-)

 

[Cory]

I'd say ammonia. But I hope that the demonization resin will chatch it in the end!

 

[mike007]

I say d.

 

[Randy Holmes-Farley]

And the answer is...B. Ammonia (NH₃)

RO membranes largely separate materials based on size, with smaller things getting through the small pores, and larger things staying behind.

Even though ammonia looks bigger (with its attached hydrogen atoms) than the simple ions such as sodium, magnesium, or chloride, and in fact, is bigger, the ions themselves have several very tightly held water molecules that do not easily come off. So the ions are carrying extra baggage that makes them even bigger when they are trying to squeeze through the pores of an RO membrane.

For this reason, small, uncharged molecules are the most likely to get through such a membrane. Other "leakers" include carbon dioxide (CO₂), hydrogen sulfide (H₂S), and silicic acid (Si(OH)₄).

This has more:

http://reefkeeping.com/issues/2005-05/rhf/index.htm

Happy Reefing!