Reef Chemistry Question of the Day #191 Settling of Organic Dosing Compounds

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #191

This is a follow up question to question 189: Evaporation of Organic Dosing Compounds

Aquarists use a variety of organic compounds for dosing to drive bacterial growth and reduce nutrients. Sometimes these solutions may sit around a long time before or during dosing.

When a solution of the following chemicals is mixed at equal concentrations and then allowed to sit for a very long time at room temperature, which chemical will have the highest concentration at the top of the fluid in the dosing reservoir?

A. Methanol (one component of NOPOX)
B. Ethanol (as in vodka and NOPOX)
C. Acetic Acid (as in vinegar and NOPOX)
D. Isopropanol (one component of NOPOX)
E. Water
F. All will be at the same concentration

Good luck!

 

[Randy Holmes-Farley]

No takers yet?

 

[Rybren]

Hmm.

I would think that the least dense one would rise to the top of the solution and then the second least dense one and so on. However, since each compound would evaporate at a different rate, the final concentrations will be different than at the start point. Therefore, I have no idea which one would have the highest concentration.

But, if the end state occurred on the second Tuesday of a week and there was a full moon, then the answer would be 'C' (or the longest answer)

HTH

 

[Randy Holmes-Farley]

Ah, I should clarify that the reservoir is closed, so no evaporation.

 

[Drauka99]

"A" has been good to me before, "f" is tempting though since they should all be fully dissolved but I guess with no agitation they might stratify

 

[Bob O]

I'm with Rybren on this one - less dense will rise to the top

 

[KingBlingTX]

I'm going with F. Since they are all solution, concentration gradients will force diffusion to counter any localized high/low concentration areas.

 

[Sabellafella]

Water? (Let me edit and explain my answer, everything would evaporate quicker then water if left for a 'very long time')

 

[beaslbob]

f.

1) they all attach with the water.
2) the fish keep things mixed up

 

[Cory]

Ill guess water.

Can we have a chemistry question of the day about detritus?

 

[Randy Holmes-Farley]

And the answer is...F. All will be at the same concentration

Any water solution that contains dissolved small molecules or ions will stay mixed forever. Normal random movement of the molecules will keep it mixed and is not impacted enough by gravity to cause any layering of the solution. This applies to seawater as well: the ions stay completely mixed forever.

If a solution starts layered, it may take some time to become mixed when there is no flow, and this is a tool scientists sometimes use. For example, in density gradient centrifugation, a centrifuge tube is layered with higher density at the bottom using a concentration gradient of something like sucrose or potassium bromide. Higher density at the bottom, then add the test material and the top and centrifuge like crazy until the metail of interest mocvves to the density point that matches its density. The layered potassium bromide or sucrose stays separated during this time, but if you wait a long time it will become uniform.

http://www.sigmaaldrich.com/technical-documents/articles/biofiles/centrifugation-separations.html

 

[Drauka99]

Well... F was telling me it was the answer but I let my thinking get in the way.

 

[beaslbob]

dern

Ruined my reputation.

got one correct

 

[Rybren]

Random chance? There's still hope for you

 

[beaslbob]