What happens when you touch liquid nitrogen

REFRIGERATED BATHS / COLD TRAPS

theme

properties

In addition to water and air, other cooling media are frequently used

  • Ice,
  • Ice / salt mixtures,
  • Solvent / dry ice mixtures
  • liquid nitrogen

Solid cooling media

For the production of cooling baths with ice / salt mixtures, salts with a low health risk are used, the dissolving process of which is endothermic (sodium, potassium, magnesium or calcium chloride.) And therefore withdraw the energy required for this when dissolving their surroundings. Provided that there is always enough salt in the mixture with the ice, such cooling baths are well suited for constant setting of minus temperatures.

The lowest attainable temperature depends on the one hand on the salt itself and on the other hand on its mixing ratio with the ice. With a mixture of 3 parts of ice + 1 part of sodium chloride you achieve a low temperature of - 21 ° C, with a mixture of 3 parts of ice + 1 part of magnesium chloride - 33 ° C and with a mixture of 0.8 parts of ground ice + 1 part Calcium chloride hexahydrate - 55 ° C.

Even colder cooling mixtures are often required for vacuum distillation. Solid carbon dioxide, also known as dry ice, is used for this. Because of the harmlessness of its vapors, dry ice is a popular coolant.

Liquid cooling media

Carbon dioxide sublimes at -78.5 ° C. If dry ice is mixed with organic solvents, it removes the energy required for sublimation and thereby cools them down. Acetones, methanol or isopropanol are often used as the transfer media.

If the low temperature is still not sufficient, liquid nitrogen is used. Nitrogen boils at -196 ° C and thereby removes the energy required for it from its surroundings.

General safety information

To be on the safe side, you should not wear metal finger rings during this work, as they are good heat conductors and maximize the risk of cold burns locally.

The properties of the receptacles for cooling baths must be tailored to the cryogenic conditions. The material from which they are made must not become brittle at low temperatures, but should rather have an insulating effect on the outside - it may even have to be chemically inert. If plastic bowls are sufficient for ice baths at the frost line, Dewar flasks are indispensable at lower temperatures. These are internally mirrored, evacuated hollow glass bodies (risk of implosion!). For your safety, you should only use vessels with a protective jacket and avoid any mechanical stress, especially when immersing the cold trap (in particular: pay attention to the immersion depth!)

Safety instructions: Solid cooling media

Even if the salts used to make the ice / salt mixtures are largely harmless from a toxicological point of view, you should never touch them with your hands. Use a larger, sturdy spatula for this.

Even in cooling baths with ice / salt mixtures, temperatures as low as - 55 ° C can arise. Of course, you have to protect your hands with insulating gloves even at such low temperatures!

Safety instructions: Liquid cooling media

When adding dry ice, the solvent foams up. The entry must therefore be made carefully to avoid overmolding. If you have a choice of the solvents mentioned, you should give preference to isopropanol because of its high viscosity and relatively low toxicity. Don't forget the fire hazard posed by these three organic liquids if you deal openly with them!

Because even small amounts of water can neutralize the insulating Leydenfrost effect, you may only fill dry Dewar vessels with liquid nitrogen. Otherwise there is a risk of implosion! First add only a little liquid nitrogen to the Dewar and cool it down by gently swirling it around.

Caution: If you stand for a long time, oxygen (bp. –183 ° C) can condense in from the air and accumulate in the otherwise colorless liquid nitrogen. You can recognize this by a blue color. Because liquid oxygen has a strong oxidizing effect, you must no longer use this mixture. Destroy it by evaporation.

Of course, oxygen also condenses when cold traps are left open in liquid nitrogen after use! So never forget to pull the cold traps out of the cryogenic medium after use and to remove any condensed substances.