Reef Chemistry Question of the Day #284: Freezing Seawater

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #284

The freezing point of 35 ppt seawater is described as being -2 °C (or 28.4 °F).

Can the ocean freeze?

Sea water has been defined as a weak solution of almost everything. Ocean water is indeed a complex solution of mineral salts and of decayed biologic matter that results from the teeming life in the seas

oceanservice.noaa.gov

Suppose you take a sample from your aquarium with a salinity of 35 ppt and send it off through the mail for analysis on a very cold day.

At what temperature would all of the water in the sample first become ice as it cooled from room temperature?

A. 0 °C
B. -1 °C
C. -2 °C
D. -2.5 °C
E. less than -2.5 °C

Good luck!

Previous Reef Chemistry Question of the Day:

Reef Chemistry Question of the Day #283: Effect of Salinity on Magnesium Ion

Reef Chemistry Question of the Day #283 Suppose that a reef aquarium has a specific gravity of 1.027 and you decide to lower it to a specific gravity of 1.026. You accomplish this lowering by removing some salt water and replacing it with pure RO/DI water (0 ppm TDS) at the same temperature...

www.reef2reef.com

 

[KStatefan]

E. Less than -2.5° C

 

[Dburr1014]

D

 

[Randy Holmes-Farley]

Bump. I'll give the answer tomorrow, but for eager beavers, there's a fascinating paper from 1954 that gives a blow by blow description of what happens to seawater as it is cooled, based on experiments they ran.

Start reading at the Discussion section on page 181.

https://images.peabody.yale.edu/publications/jmr/jmr13-02-02.pdf

 

[Randy Holmes-Farley]

And the answer is...

At what temperature would all of the water in the sample first become ice as it cooled from room temperature?

A. 0 °C
B. -1 °C
C. -2 °C
D. -2.5 °C
E. less than -2.5 °C

It is actually way, way less and unless you live in far northern Alaska, Canada, or Siberia, you almost certainly never get cold enough.

It takes a temp below -54 °C before seawater totally solidifies.

As the top first begins to freeze at -2 °C, the salts are getting more and more concentrated below the ice. Different chemicals precipitate at different temperature as more and more ice forms, and eventually the last thing to precipitate is mostly calcium chloride.

The article linked above gives a nice blow by blow description of the process.

Happy Reefing!

 

[Fish Styx]

E. Because as it freezes it leaves behind the salt, thereby increasing the salinity of the remaining water. That then requires a lower temperature to freeze.

 

[907_Reefer]

It is actually way, way less and unless you live in far northern Alaska, Canada, or Siberia, you almost certainly never get cold enough.

It takes a temp below -54 °C before seawater totally solidifies.

Well, even in mid/southern AK it gets cold enough to ruin your shipment of 35ppt solution..! This batch came in weather around 0 to +10F (-17 to -12C), froze partially, expanded to leak, the remainder registered as 1.019 after thawing..

 

[taricha]

Wait, so freezing saltwater - the ice crystals are essentially pure h2o and the major ions get concentrated in the increasingly salty liquid water below?
Do low-concentration analytes of interest like phosphate, nitrate, ammonia etc also get concentrated with the salts in the brine or do they stay evenly distributed between the freshwater ice and salty brine because of their low concentration - meaning you could freeze like 50-80% of the saltwater, remove and melt the ice crystals and then run freshwater tests without having to worry about all the salts?

 

[Randy Holmes-Farley]

Wait, so freezing saltwater - the ice crystals are essentially pure h2o and the major ions get concentrated in the increasingly salty liquid water below?
Do low-concentration analytes of interest like phosphate, nitrate, ammonia etc also get concentrated with the salts in the brine or do they stay evenly distributed between the freshwater ice and salty brine because of their low concentration - meaning you could freeze like 50-80% of the saltwater, remove and melt the ice crystals and then run freshwater tests without having to worry about all the salts?

If the freezing is slow enough, I think it will be pure water. Freezing faster risks ions getting trapped “by mistake” before they get out of the way of the growing ice crystal front.

 

[Rick Mathew]

If the freezing is slow enough, I think it will be pure water. Freezing faster risks ions getting trapped “by mistake” before they get out of the way of the growing ice crystal front.

That water is AMAZING stuff...

 

[Randy Holmes-Farley]

That water is AMAZING stuff...

Purification by crystallization is a very useful process for many materials. Synthesis chemists do it all the time, though other, higher tech, processes such as chromatography can work better for many materials. Lots of pharmaceuticals, for example, can be purified by crystallization.

Crystallization is usually in a solvent rather than the liquid phase of the crystallizing material, but water acts nicely as it’s own solvent.

 

[HomebroodExotics]

Why does it start freezing from the top? How does the starting point get decided? Very interesting stuff.

 

[biom]

Why does it start freezing from the top? How does the starting point get decided? Very interesting stuff.

Because water in its solid form (ice) weights less than water in its liquid form so ice floats on the top of the water. This is one of the unique characteristics of the water that make life possible on Earth.

 

[taricha]

right, even if you cool it from the bottom, the ice crystals will go straight to the top.

BTW a quick Google scholar for desalination by freezing shows that it's a real thing, but never achieved at usefully large scale.
The difficulties are how to wash the concentrated salt off your clean ice, and... freezing water takes a LOT of energy removal.

 

[907_Reefer]

Somewhat related, for fun, this is how we separate water from gasoline up north. Have a machine that has been sitting outside? Empty fuel into a bucket, freeze it (by leaving outside), water separates into ice, scrape off, put fuel back into machine.

 

[Rick Mathew]

Somewhat related, for fun, this is how we separate water from gasoline up north. Have a machine that has been sitting outside? Empty fuel into a bucket, freeze it (by leaving outside), water separates into ice, scrape off, put fuel back into machine.

we do the same thing here in Florida but we use a different procedure...Picture below illustrates

Doesn't make the wife happy but it works