Make liquid chlorine with a simple freezing mixture

With the help of a cooling mixture of snow and table salt, some gases can be liquefied, such as sulfur dioxide. For other gases, such as chlorine, however, the temperature that can be achieved with this mixture is not low enough, and dry ice is preferred instead. A lesser-known alternative is a mixture of snow and calcium chloride hexahydrate, which can be easily produced from dehumidifier granules. This mixture can achieve temperatures just below -50 °C, which is more than sufficient for liquefying chlorine.


Production of calcium chloride hexahydrate:

In a large beaker, 303 g (16.83 mol) of water is added to 620 g (4.22 mol) of calcium chloride dihydrate (dehumidifier granules), and the salt is dissolved while stirring and heating.



Place the cloudy solution in an ice bath and allow it to cool, stirring occasionally with a glass rod, until a thick paste of calcium chloride hexahydrate forms.



The mass is placed in a Buchner funnel in several portions and sucked off.



In my case, the filtrate had a volume of approx. 100 ml and the crystals weighed approx. 750 g. They are stored in an airtight container because they are hygroscopic.


Production and liquefaction of chlorine:

Place 17.5 g of coarsely powdered trichloroisocyanuric acid (TCCA) in a suction flask (250 ml), add a stirring bar, and place the flask on a magnetic stirrer. Connect a dropping funnel with 54 ml of 15% hydrochloric acid. Connect the gas generator to a glass tube (I used a test tube without a bottom) filled with granules of anhydrous calcium chloride using a lab tubing. Connect another lab tubing to the other end of the prepared drying tube and insert a glass capillary into the end of the lab tubing.

The cooling mixture is made from dry snow (below 0 degrees Celsius) and ice-cold calcium chloride hexahydrate in a ratio of 1:1.43. To do this, both components are weighed beforehand and placed in two containers. Snow and salt are then added alternately in large portions with a spoon through a powder funnel into the insulated container (Dewar vessel)



from a small thermos flask (see photos above), stirring with a wooden stick. The container should be filled to 1 to 2 cm below the rim. For the approximately 350 ml thermos flask, I needed an estimated 250 g of calcium chloride hexahydrate and 175 g of snow. Use a thermometer to check whether the temperature is low enough (at least -35 °C, preferably below -40 °C). If not, stir the mixture a little longer.

Insert the gas inlet tube with the glass capillary into the ampoules, which are made of thick-walled glass tubes, fused at one end and narrowed at the other, and start the chlorine development in the gas generator by slowly adding hydrochloric acid to the TCCA (1 drop every 3 seconds). Break up the foam that forms in the gas generator by stirring magnetically and accelerate the gas generation by heating gently.



After adding the first drops of hydrochloric acid, place the ampoules in the cold bath so that the chlorine gradually condenses, which takes 5 to 30 minutes depending on the quantity.



The filled ampoules are transferred to a small beaker containing a fresh cold mixture and sealed with a strong burner. I made the ampoules from borosilicate glass, which is tough, so in this case you have to fix them in place at the bottom (using a test tube holder) while sealing them. Once the seal has cooled down, remove them from the cold bath and allow them to warm up slowly to 0 °C. There should be no smell of chlorine, otherwise the ampoules are leaking. To test the pressure resistance, I heated my ampoules for 10 minutes to 65-70 °C (thick ampoule) or 70-80 °C (thin ampoule). Once tested in this way, they can be stored at room temperature without hesitation. The pressure inside is then approximately 7 bar.

After using about 3/4 of the hydrochloric acid, I obtained approximately 1.1 and 2.6 ml of liquid chlorine in two ampoules, which corresponds to a total weight of 5.5 g at a density of 1.5 g/cm³. This is a yield of 43%.



Because I was unable to melt the large ampoule for a long time, approximately 2.5 to 3 ml of the already liquefied chlorine evaporated. Under normal circumstances, a yield of approximately 75% would therefore be realistic.

Note: An alternative to a Dewar vessel would be a Styrofoam box, for example. However, a standard beaker would probably heat up too quickly from the outside. The cooling mixture maintained a temperature between -45 and -40 °C for over an hour, but did not reach the literature value of below -50 °C because the conditions were not ideal. After the experiment, I noticed that there were still large salt crystals at the bottom of the insulated container. It would be best to obtain more soluble, i.e., smaller crystals when producing calcium chloride hexahydrate by using a little more water and stirring constantly. In addition, the salt should ideally be dried over sulfuric acid.


Source: Hermann Hammerl (1878) Über die Kältemischung aus Chlorcalcium und Schnee. Sitzungsberichte der Kaiserlichen Wiener Akademie der Wissenschaften, 78, 59-79.


Explanation:

When calcium chloride hexahydrate is mixed with ice, an endothermic reaction takes place. The driving force behind the reaction is the increase in entropy caused by the calcium and chloride ions dissolving and the melting snow. In contrast to the refrigerant mixture of table salt and ice, this method achieves temperatures as low as -55 °C (instead of -21 °C) because calcium chloride lowers the freezing point of water more than sodium chloride.




Question:

Do you agree with this explanation? If yes, why does the calcium chloride lead to a stronger freezing-point depression than NaCl? I would like to completely and deeply understand, why calcium chloride (hexahydrate) lowers the temperature so much more than NaCl.

Thanks. Since the temperature never rose to more than -40 °C, sealing the ampoules wasn't that difficult. The loss of chlorine from the thick ampoule was just a stupid mistake.

The solubility indeed seems to correlate somewhat with the melting point depression. But there are salts with lower solubility than NaCl which lower the melting point of water more (e.g. magnesium chloride, see below).



and ammonium nitrate has a higher solubility than calcium chloride, but you can't get such low temperature using it.



So this property doesn't seem to explain the effect of CaCl2 vs. NaCl.

Thanks for the explanation! So it only depends on the concentration of ions in solution, right? I showed some graphs above which made me think that the melting point depression depends on the identity of the ions. But if the concentration of ions (instead of mass) is used, the following graphs should result, right?


(I'm not sure any longer if the -55 °C for CaCl2 are true. Newest sources claim 49.95 °C eutectic)

There are some differences between the graphs in reality, even when the molar mass is taken into account.

The eutectic temperature of AlCl3 + H2O is said to be -52 °C.

Combining 2 or 3 salts (all consisting of different ions) could even generate lower temps, I think.

Quote: Originally posted by Texium

Freezing point depression is a colligative property. It is dependent on the total concentration of solute particles in solution, regardless of their identity. CaCl2 dissociates into three ions when it dissolves, so it is 1.5 times as potent per mole of salt as NaCl or NH4NO3. If you made solutions of equal concentration of those three salts, the CaCl2 would have the lowest freezing point, and the NaCl and NH4NO3 would have equal freezing points. So solubility is important of course, since higher concentration = lower freezing point, but high solubility combined with more ions in solution is especially good. Endothermic dissolution is important so that your solution gets colder as the salt dissolves, but it’s independent from what the magnitude of the freezing point depression is. I’d be curious to see how cold you can get a mixture with AlCl3•6H2O since it splits into 4 ions, though it doesn’t have quite as high solubility. Also, very nice post overall! Thank you for sharing your write-up.