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Q & A: saline slush refreezing

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Most recent answer: 09/01/2013
In the medical field where I work, we commonly make what we call "slush" from a 0.9% sodium chloride solution (approximately 9 grams sodium chloride in each 1 liter of water) for use in cardiac and organ donation procedures. We make this slush by adding bags of the saline solution to a cooler of ice and then adding isopropyl alcohol. Several hours later you have perfect margarita-style slush. However, any unused saline slush that is returned to room temp and then used to make slush again, as described above, makes rock hard slush in several hours. My qustion is: Why does refreezing saline result in a much denser ice/slush and, apparently, in less time?
- David (age 39)
Denver, CO

This is a very interesting and challenging question. I doubt that we can simply give the correct answer, but together we can probably find it with the help of a few experiments. Welcome to the messy world of experimental science.(But see below- after wandering around we probably came up with the answer.)

Obviously something has changed in your saline bags. There are two broad possibilities:

1. Something has leaked in or out.

2. Something inside is out of equilibrium, either before or after the initial freeze.

(1) is less likely, since you use double-bagged saline. Just to check before proceeding further to explore (2), I suggest

****Weigh a bag before and after the first freeze. Of course, make sure it's very dry etc. to avoid errors. Assuming it weighs the same, you could optionally double-check by opening it and measuring the [Na+] and/or [Cl-] and/or total conductivity (at some carefully controlled temperature), just to be sure that you have the same solution you started with. Actually, the latter types of measurements are also sensitive to certain changes of type (2).

(2): If things are out of equilibrium at room temperature, it's more likely after the freeze than after sitting around for a long time under stable conditions. What could be out of equilibrium? Maybe there could be small ice or NaCl crystals, slow to dissolve. Ice isn't so slow, so let's think about NaCl. (And on going further, I realized that quite aside from crystals you could simply have an inhomogeneous salt solution, see last section.)

How could we test for some NaCl crystals?

**** Try shining a little laser through a pre-freeze and post-freeze bag, each allowed to sit quietly for a while first. Do you see more little scattering events in the post-freeze bag? That could come from little NaCl crystals. (They'd also slightly reduce the salt concentration in solution, see above.)

What would cause NaCl crystals to form, and why would that have anything to do with the change in freezing? As water freezes, it excludes the salt, leaving a higher salt concentration in the remaining solution. That's one way to get a nice slush. If you cool water down to say -18°C. the salt will concentrate in the solution until its freezing point has dropped to 18°C, and then the ice/liquid ratio will stabilize. That shouldn't drive the formation of ice crystals, however.

What if you cool below -21°C? At that point no liquid is stable. In equilibrium, you have only ice and salt crystals.

****Key experiment: Stick a thermometer in your isopropanol-ice slurry. Is it below -21°C?

Let's say it is. You'll get a slush that will gradually solidify as the super-saturated salt solution gradually sheds salt crystals. That agrees with what you see, with another experimental implication. 

**** If you leave the bags in for a really long time on the first round, do they gradually freeze solid?

Let's say that's a yes. Then interrupting when you have a nice slush means that you've got some NaCl crystals in there. They may dissolve slowly at room temperature. [See laser test above.] 

****You could also try centrifuging some of the contents of the bags, and see if that affects the freezing behavior specifically of the post-freeze material by driving the small crystals toward the bottom of the centrifuge tubes.

Ok, so let's guess (maybe wrongly) that you've got some NaCl crystals left in the post-freeze bags. Why should they matter? For starters, you'll have a slightly reduced concentration of dissolved salt, so that could very slightly ease the freezing. You'll also have nucleation sites to help the new salt crystals grow as the ice forms, avoiding super-saturation of the solution. That could speed freezing significantly.

That suggests another possibility to me. When you form slush, you've separated water from denser salt solution. Maybe after melting you have relatively pure water on top and relatively salt water on the bottom. This will happen even if you never get it down to -21°C/ Unless you stir it up well, the purer water on top will start to freeze sooner and more solidly than you'd get in a homogeneous solution.

****Try shaking up the post-freeze bags after they've melted. Does that restore them to the initial behavior?

In fact, maybe that last idea and last test would be the best to try first. It seems most plausible.

If it's that simple (and now I suspect it is) there will be other symptoms:

**** You'll probably see some of those swirly effects that you get when you have inhomogeneous solutions, where the index of refraction isn't constant. (Thanks, Stuart Levy, for pointing this out.) You could sample a little of the solution from the top and bottom of the post-freezing bag, before shaking. The bottom part should be denser, more viscous, and have higher conductivity and ionic concentrations.

That should do for starters. We'll have more data next round.

Mike W.


(published on 09/01/2013)

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