[awolokay]

I'm from Canada and in the frozen north we can get down to -40 °C or lower. According to various literature sources I've been able to find, the freezing point of 36% HCl is about -30 °C so freezing is a concern. But it's not WHETHER it will freeze at -40 °C, but HOW it will freeze - specifically, does this solution expand on freezing, like water? It's one thing to land up with a pipe blocked by frozen solution - quite another thing to have a pipe rupture!

I've contacted various HCl vendors and Googled without any luck. Having not found anything, it is my suspicion that the solution does not expand, but I'd like something more than a guess.

The only other point of reference I have, is freezing of caustic soda solutions. With the much higher freezing point, this is something that happens far more frequently and yet I have not seen or heard of rupture problems in such lines.

One final option would be to carry out a test. If so, does anyone have any suggestions as to a lab that could do this? I haven't been able to find one.

Thanks
 

[curiouscat]

Couldn't you test by, say, cooling a small graduated vial (careful about shattering!) with 35% acid in a cooling bath? -40 C shouldn't be very hard to reach using say CaCl2 + Ice or Octane + Dry Ice.

[awolokay]

I've certainly considered the cooling test, but in a plastic pipe rather than a glass vial. However, it's not very "friendly" working with 36% HCl and I haven't been able to find a lab willing to do the test for me.

[Arkcon]

We've discussed this recently:  http://www.chemicalforums.com/index.php?topic=61111.msg218105#msg218105

[awolokay]

Arkcon:
Many thanks - I had searched for posts on this topic, but didn't come up with the one you give.
I had also seen the phase diagram before, but I guess I had not clued into what it could tell me.
If I'm correct, taking a 36% solution, what it says is this will cool (assuming uniform cooling throughout the liquid) until the temperature hits -25 °C. At this point, crystals of the tri-hydrate will start to form. Since these have an HCl content of approx. 40%, the remaining liquid will have less HCl, and based on the curve a lower freezing point.
If cooling continues, tri-hydrate will continue to crystallize (forming an HCl slushie - lovely - but at least something that will not split pipes or totes) until -73 °C or thereabouts is reached. At this point the whole mass will freeze.
Since this is well below the temperature of concern for me, I don't care whether this freezing results in expansion or not - although I'd be interested to know purely as an academic point.

[curiouscat]

I don't think the phase diagram will tell you anything about expansion or the lack of it.