Conducting Reactions Below Room Temperature

Cooling Bath Setup

For a general set-up, prepare the cooling bath of choice as described below and immerse the reaction flask into the bath (Figure 1). Don't fill the bath vessel all the way, but leave enough room to allow for immersion of the reaction flask.

Note: if the reaction is moisture sensitive, be very careful when adding reagents to the flask or any other part of the apparatus (e.g. a dropping funnel). If an opening is generated while the glassware is immersed in the cooling bath, then room temperature air quickly flows inside and carries moisture in.

Figure 1. Example for a cooling bath set-up in a three-neck round-bottom flask with dropping funnel, thermometer under inert atmosphere.

1. Making an Ice-Water Bath

1. For an ice-water bath with an additive, weigh and add the appropriate amount of ice to the bath vessel of choice. A Dewar is helpful but not necessary. The vessel can be plastic, rubber (e.g. a bucket), or glass (e.g. a large crystallization dish). For this experiment, add 500 g of ice to a 1 L vessel.
2. Weigh the appropriate amount of additive consulting an ice-bath (Table 1), and add the additive to the ice. For this experiment, weigh and add 165 g (=5x 33 g) of NaCl to the bath vessel.
3. Add a small amount of deionized water to the bath vessel and stir thoroughly using a stir rod. Add just enough water until all the ice is covered.
4. Check with a thermometer to ensure that the desired temperature has been reached. Adjust the amount of additive if necessary. The bath will not maintain its temperature for very long and adjustments need to be made in frequent intervals of about every 20–30 min. For this it may be necessary to pipette off some of the water in the bath and add more ice and additive.

Substance	g/100 g H2O	Final Temperature (°C)
Na2CO3	20	-2.0
NH4NO3	106	-4.0
NaC2H3O2	85	-4.7
NH4Cl	30	-5.1
NaNO3	75	-5.3
Na2S2O3 ● 5H2O	110	-8.0
CaCl2 ● 6H2O	41	-9.0
KCl	30	-10.9
KI	140	-11.7
NH4NO3	60	-13.6
NH4Cl	25	-15.4
NH4NO3	45	-16.8
NH4SCN	133	-18.0
NaCl	33	-21.3
CaCl2 ● 6H2O	81	-21.5
H2SO4 (66.2%)	23	-25
NaBr	66	-28
H2SO4 (66.2%)	40	-30
C2H5OH (4°)	105	-30
MgCl2	85	-34
H2SO4 (66.2%)	91	-37
CaCl2 ● 6H2O	123	-40.3
CaCl2 ● 6H2O	143	-55

Table 1. Salt/ice cooling mixtures that are obtained by mixing the salts with water or ice at the specified temperatures and in the specified amounts.¹

2. Making a Dry-Ice Bath

1. Put on cryogenic protection gloves and safety goggles. Always practice this when handling dry ice and never touch it with bare hands as it can rapidly burn skin and cause frostbites.
2. For a bath vessel with a volume of about 1 L, take about 1/3 of a block of dry ice (usually available in ~2 lb blocks) and break it up into a few smaller pieces.
3. Add the pieces of dry-ice to the bath vessel.
4. Slowly add the organic solvent (e.g. acetone) to the dry ice while stirring with a glass rod. There is a vigorous fizzing as a result of CO₂ gas development.
5. Continue to slowly add solvent and stir until a homogenous slurry forms and most of the dry ice dissolves. This is to ensure that the heat transfer to the reaction flask is as uniform as possible.
6. Insert a cold-temperature thermometer into the bath to ensure that the desired temperature is reached.
7. Check on the dry-ice-bath in regular intervals and add more chunks of dry ice when a rise in the bath temperature is noticed. The interval time depends on the degree of insulation, but is usually around every 45–60 min.

3. Making a Liquid Nitrogen Bath

1. Put on cryogenic protection gloves and safety goggles. Always practice this when handling liquid nitrogen as it can rapidly burn skin tissues and eye fluids, causing frostbites or permanent eye damage.
2. For a bath without additives, add the appropriate amount of N₂ to a Dewar to obtain a temperature of -196 °C. Move to step 3.3 if this is the desired temperature.
3. For a bath with additives, add an organic solvent of choice (consult Table 3 to find the right solvent for the right temperature) to the Dewar first, then slowly add liquid N₂ to the solvent.
4. Insert a cold-temperature thermometer into the bath to ensure that the desired temperature has been reached. In contrast to the other baths, the liquid N₂ bath inside of a Dewar maintains its temperature for hours at a time.
5. Check on the bath in appropriate intervals (a few hours) to see if more N₂ is needed.