Reef Chemistry Question of the Day #30 Chemical Layering

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #30

Let's have a hard question today!

This question relates to the "de-mixing" of chemicals in seawater.

Assume you have a well-mixed reef aquarium with lots of turbulence at 80 degrees F. Assume it has 0.05 ppm phosphate, 1 ppm nitrate, 430 ppm calcium, and 1300 ppm magnesium. The molecular weights of these ions are 96, 62, 40, and 24 g/mole, respectively. The salinity is 35 ppt (sg = 1.0264).

You withdraw a liter of the water and fill a tall, thin bucket. You let that sit for six months with no stirring or mixing at all. It remains at 80 degrees F.

Then you come back, and carefully remove samples from the top and the bottom and test them.
Assume there is no biological action or abiotic precipitation taking place in the bucket.

What are you most likely to find?

1. The top is depleted in all of these ions relative to the bottom.
2. The bottom is depleted in all of these ions relative to the top.
3. The top is depleted in phosphate, nitrate and and calcium, but the reverse is true for the lightest of these ions, magnesium.
4. The top is depleted in phosphate and nitrate, but the reverse is true for the two lightest of these ions, magnesium and calcium.
5. Only the heaviest of these ions, phosphate, is depleted at the top, and the reverse is true for all of the others.
6. The top and bottom are equal in concentration for all of these ions.
7. There is no way to know in advance.

Hint: You may not need every piece of information supplied in the question.

Good luck!

















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[leptang]

Ill take a stab at it... #1 because of gravity?

 

[Velodog2]

Thermodynamics says it is 6.

 

[birdsnest]

I'm going with 6 as well

 

[Randy Holmes-Farley]

Anyone else?

 

[redfishbluefish]

I've got to believe with all the crap Randy included that this isn't right, but in the olden days a solution is where all solutes are evenly distributed. So 6 would be the correct answer.

 

[DFW]

1, because all the water evaporated and left the residue on bottom.

 

[Randy Holmes-Farley]

And the answer is....6. The top and bottom are equal in concentration for all of these ions. Good job!

The only way that ions in seawater can have different concentrations in different locations is if they are added or removed nonuniformly. Simple random diffusion will eventually ensure they are the same everywhere, except the monolayer or so that is near an interface. Most inorganic ions are repelled from the air/water interface, and some organics are attracted there, but everywhere in the bulk water will be the same.

For example, no matter where you take a sample, phosphate should be pretty much the same everywhere in a reef aquarium, unless something unusual is happening nearby. So if you measure just downstream of a GFO reactor it will be lower, and that lower stream of phosphate may take a little bit of time to mix in. Likewise, phosphate may be elevated within a layer of sediment of it is being locally produced by degradation of organics and is not readily mixing with the bulk water of the aquarium.

 

[Randy Holmes-Farley]

I've got to believe with all the crap Randy included that this isn't right, but in the olden days a solution is where all solutes are evenly distributed. So 6 would be the correct answer.

Just being devious.

 

[Randy Holmes-Farley]

1, because all the water evaporated and left the residue on bottom.

:lol:

That would be true too.

 

[DFW]

:lol:

That would be true too.

Yeah! I got one right! I knew that all of those high school chemistry classes would pay off one day!:loco:

 

[Randy Holmes-Farley]

Yeah! I got one right! I knew that all of those high school chemistry classes would pay off one day!:loco:

:lol:

Glad they were so useful.

 

[yarddoctor123]

I've wondered if precipitated compounds become insoluble in salt water or if their state is in flux with varying temp and salinity. Randy, specifically the fine milky stuff that comes up when I vacuum sand bed. I treat that water with kalk and it turns clear with the thicker sludge on the bottom. Phosphate reading of the top water is zero. What do I have left in the sludge? And will that go back up into solution in lower salinity and higher temp?

 

[Randy Holmes-Farley]

I've wondered if precipitated compounds become insoluble in salt water or if their state is in flux with varying temp and salinity. Randy, specifically the fine milky stuff that comes up when I vacuum sand bed. I treat that water with kalk and it turns clear with the thicker sludge on the bottom. Phosphate reading of the top water is zero. What do I have left in the sludge? And will that go back up into solution in lower salinity and higher temp?

I'm not sure I fully understand the question.

All precipitated compounds are in some sort of flux, although in some cases, such as globs of organic matter, that "flux" may be very slow.

For things like calcium carbonate, it is actually quite fast. Calcium and carbonate ions are coming onto and off of the surface constantly and rapidly. Magnesium and other ions are also reversibly attaching to the calcium carbonate surface, including organics, phosphate, strontium, etc.

On this specific case, you are adding kalkwasser and get a precipitate? If it is clear limewater, that is likely mostly magnesium hydroxide, which should redissolve under aquarium conditions. Some calcium carbonate may also form, especially if stirring is not adequate to rapidly mix in the kalkwasser, and that may not redissolve completely and will sit there in the sand, undergoing the reversible surface reactions mentioned above.

 

[mainereefer]

to me this dosnt make sense... salt and water dont mix and will separate, the reason salinity is different at the top of your tank compared to 6 inches down same with ocean water. if the salinity drops at the top of the water, wouldnt everything else follow?

 

[Randy Holmes-Farley]

to me this dosnt make sense... salt and water dont mix and will separate, the reason salinity is different at the top of your tank compared to 6 inches down same with ocean water. if the salinity drops at the top of the water, wouldnt everything else follow?

But salinity isn't different six inches down unless there is something dramatic happening, such as fresh water being added on top.

Already mixed seawater will never separate into layers of different salinity.

And assuming nothing is adding more fresh water or salt water in an ongoing way, even a not well mixed salt solution will eventually become homogeneous due to random diffusion and convection.

Solids salts (like calcium carbonate sand) certainly settle out due to gravity, but that's a different issue.

 

[yarddoctor123]

You did answer my question. After reading "What Your Grandmother Never Told you About Lime", I started using it as a "purifier" or flocculant. I vacuum my sand bed and pull up a lot of that milky sludge. I use 1plastic baby spoon of Mrs Wages powder/5g of "dirty" tank water. I stir, don't shake, the cocktail, and let it sit overnight. The resulting clear liquid I draw off and use for certain cultures, and the remaining sludge I use for a phyto culture. The phyto responds very well and very quickly to the addition of that sludge. I was just curious if the precipitated phosphates would be released into the water column in a low phosphate condition (hungry phyto culture) thermodynamically or if the precipitates remained insoluble and unavailable in salinity of .03 - .04 and 60 - 85 degree temperatures.
So you gave me good news. I hadn't taken Mg into consideration, either. Can I assume that the sludge is replenishing whatever is lacking very quickly in my very dynamic cultures regardless of temperature and salinity?

 

[mainereefer]

Solids salts (like calcium carbonate sand) certainly settle out due to gravity, but that's a different issue.

wasnt that part of the question? with calcium settling wouldnt mag follow? or do they bind when a coral uses them?

 

[Randy Holmes-Farley]

The question didn't have any precipitation taking place:

"Assume there is no biological action or abiotic precipitation taking place in the bucket."

It just started and ended with clear seawater in a bucket.

 

[yarddoctor123]

wasnt that part of the question? with calcium settling wouldnt mag follow? or do they bind when a coral uses them?

Sorry, Randy 's original question that involved NO abiotic precipitation, spurred my own question about precipitates. I didn't mean to cause any confusion[emoji2] .