A frozen urine specimen is required for an organic acids lab test, but the urine refuses to freeze after 4 days in a regular freezer turned as low as it will go. What would explain this, and how cold might it need to be to freeze?

  • $\begingroup$ Various not-so-legitimate sources say around -120 to -80 degrees celcius. I'd be surprised if there is a simple way to estimate the actual freezing temperature in this case. $\endgroup$ – Brian Feb 20 '14 at 13:58
  • $\begingroup$ That definitely sounds very far from the mark. A slight bit of time with a search engine afforded this source, which states that the freezing point of urine is normally between -1.3 and -2.2°C. I suspect either the freezer is not working properly, or the urine has been significantly preconcentrated, or the urine has been mixed with something else. $\endgroup$ – Nicolau Saker Neto Feb 20 '14 at 15:03
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    $\begingroup$ Add a tiny jot of ground glass, a boiling chip, or scratch the side of the container with a glass stir rod. You need nucleation centers. $\endgroup$ – Uncle Al Feb 20 '14 at 16:00
  • $\begingroup$ @UncleAl: unless its sterile filtered there will be a few cells already in there (e.g. exfoliated epithelial cells) so nucleation centers should not be a problem. $\endgroup$ – cbeleites unhappy with SX Feb 22 '14 at 19:19
  • $\begingroup$ I'd procude a control sample myself and put it in there to see what happens. Of course, tap water does freeze, right? $\endgroup$ – cbeleites unhappy with SX Feb 22 '14 at 19:21

Urine is an aqueous solution of urea, uric acid, sodium salts and whatnot. These solutions usually exhibit freezing-point depression.

As Brian has correctly pointed out in a comment, the effect isn't that strong and a quick and dirty look in the literature shows that a freezing point around -1.3 to -2.2 °C.

You might want to try a cooling bath with dry ice in isopropanol or acetone or other drastic methods to achieve shock-freezing.

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    $\begingroup$ Freezing point depression, as caused by salts such as NaCl, is usually on a much smaller scale. I tried calculating using concentrations from wikipedia, and the freezing point depression came out to around .5 K. Why do we see this 80-120 K freezing point depression? $\endgroup$ – Brian Feb 20 '14 at 14:38
  • $\begingroup$ some additions can reduce freezing points dramatically. $\endgroup$ – permeakra Feb 20 '14 at 15:36
  • $\begingroup$ According to another source, freezing point depression based on the urea content is -5.6 °C. $\endgroup$ – tanius Nov 22 '18 at 15:52

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