Reef Chemistry Question of the Day #143 RO Membrane Rejection

Randy Holmes-Farley

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Reef Chemistry Question of the Day [HASHTAG]#143[/HASHTAG]

Which of the following chemicals would be most poorly rejected by an RO membrane?
In other words, which gets through most easily?

A. Sodium
B. Ammonia
C. Potassium
D. Chloride
E. Nitrate
F. Phosphate
G. Ferric iron
H. Ferrous iron

Good luck!

















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JimWelsh

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Ions are rejected by an RO membrane depending upon molecular/atomic weight and charge, where the greater the weight, the greater the rejection, and the greater the charge, the greater the rejection. Ammonia has both the lowest molecular weight of all the ions listed at 17.031, and also no charge. Answer is B. Ammonia.
 

arman

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Reef Chemistry Question of the Day [HASHTAG]#143[/HASHTAG]

Which of the following chemicals would be most poorly rejected by an RO membrane?
In other words, which gets through most easily?

A. Sodium
B. Ammonia
C. Potassium
D. Chloride
E. Nitrate
F. Phosphate
G. Ferric iron
H. Ferrous iron

Good luck!

















.
So whats the answer?
 

arman

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Reef Chemistry Question of the Day [HASHTAG]#143[/HASHTAG]

Which of the following chemicals would be most poorly rejected by an RO membrane?
In other words, which gets through most easily?

A. Sodium
B. Ammonia
C. Potassium
D. Chloride
E. Nitrate
F. Phosphate
G. Ferric iron
H. Ferrous iron

Good luck!

















.
So whats the answer?
 
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Randy Holmes-Farley

Randy Holmes-Farley

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And the answer is, and many have noted, ammonia.

While ammonia itself is larger than a sodium ion, all of the ions in the question have very tightly attached water molecules adhering to them. Typically 2 to 8, but for a complex ion such as carbonate there can be many more (~13 for carbonate). Those water molecules will only come off with substantial applied energy, so the ions carry them around and really act like much bigger entities than they really are when considering whether to go through an RO membrane.

Ammonia has no such strongly bound water molecules, so in effect it is the smallest entity on the list, and passes through an RO membrane fairly readily (like water does).

This has important implications because some water supplies that use chloramine to disinfect. When the chloramine hits the carbon block, it breaks down, releasing ammonia. The ammonia gets through the RO and sticks to the DI. If you let a DI begin to deplete, the ammonia can come out in high concentrations (higher than the input).

I had this happen once when I didn't notice the DI was spent, and when I made limewater from it, it stank of ammonia, which was released to the air at the high pH of limewater.

Happy Reefing!
 

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