Reef Chemistry Question of the Day #38 Concentrated Seawater

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #38

You folks are doing very well on these, so we'll start the week with a tough one.

Suppose you make concentrated seawater, either to try to raise salinity in a reef aquarium through a water change with high salinity seawater, or in the early stages of drying out seawater to collect salts from it.

What are the first three salts to precipitate from the drying or concentrated seawater?

A. Calcium carbonate, magnesium carbonate, magnesium sulfate
B. Calcium carbonate, magnesium sulfate, magnesium chloride
C. Calcium carbonate, calcium sulfate, magnesium sulfate
D. Calcium carbonate, calcium sulfate, magnesium carbonate
E. Calcium carbonate, magnesium carbonate, calcium chloride

Good luck!


















.

 

[DRThompson]

Well, I'm sure Calcium carbonate ......Pretty confident on Magnesium chloride since it's manufactured from brine solutions(evaporation).....going with B

 

[Sam8fish]

Goin with C. The first salt to precipitate is calcite, then comes gypsum which later turns into anhydrite the. Next would be halite amoungst many more chlorides and sulfates which would include magnesium sulfate but not magnesium carbonate. Although it is often used in sea salt it is not the natural precipitate but is used as an anti caking agent.

 

[DFW]

C because the sulfates go first?

 

[Randy Holmes-Farley]

And the answer is.... D. Calcium carbonate, calcium sulfate, magnesium carbonate

According to "Chemical Oceanography" by Frank Millero, the order of salt precipitation from evaporating seawater is shown below, along with the percent water remaining in the solution. Note that many of the early precipitates continue to form more as the evaporation continues, such as the sodium chloride and the calcium sulfate.

1. calcium carbonate + magnesium carbonate 50% water remaining
2. Calcium sulfate (gypsum and then anhydrite) 28 --> 10 % water remaining
3. Sodium chloride 9% water remaining
4. Sodium calcium sulfate (glauberite) 7.6% water remaining
5. potassium magnesium calcium sulfate (polyhalite) 2.6% water remaining
6. Magnesium sulfate (epsomite then hexahydrite then kieserite) 1.4 --> 1.2 --> 1.0% water remaining
7. Potassium magnesium chloride (carnallite) 0.85% water remaining
8. Magnesium chloride (bischofite) 0.4% water remaining


You can see that some of these do not precipitate until there is almost no water left.

Happy Reefing.

 

[beaslbob]

Geee

you mean you can't just evaporate the sea water to get salt.

and then readd water to get sea water?

 

[Randy Holmes-Farley]

Geee

you mean you can't just evaporate the sea water to get salt.

and then readd water to get sea water?

I assume you are you being sarcastic?

Many of those would redissolve, but no, you cannot dry seawater and expect to redissolve it because some (like the calcium carbonate) won't redissolve.

 

[beaslbob]

I assume you are you being sarcastic?

Many of those would redissolve, but no, you cannot dry seawater and expect to redissolve it because some (like the calcium carbonate) won't redissolve.

Yeppers.

LOL

discussing saltwater with non saltwater aquariumists sometimes the idea of just using sea salt comes up.