Cooling Liquids for Rotary Evaporator Condensers

By Carlton Hoyt, 11 April 2016

If you have a dry ice condenser you're obviously using dry ice, but coiled condensers, which cool the vapor inside the condenser using a liquid passed through an internal cooling coil, are much more popular. That begs the question: what liquid should I use as a coolant?

If your vapor temperature is high enough, you can simply use water. You can use a recirculating chiller to cool the water or you can simply connect the condenser to a tap. Don't forget the Rule of 20 - you want the coolant to be at least 20°C cooler than your vapor temperature.

If you need your coolant to be near or below 0°C you cannot use water. What then?

Ethanol or methanol can be used for low-temperature applications. They are non-viscous, non-corrosive, and have extremely low boiling points.

Ethylene glycol can also be used and is more versatile than alcohols or water, as it has both a low freezing temperature and a high boiling point. Furthermore, its natural freezing temperature of -12°C can be depressed to -45°C by creating a 60% ethylene glycol and 40% water solution. The downside is that ethylene glycol is moderately toxic, so like anything you would find in a lab, don't be an idiot and try to drink it.

A glycerol-water solution may also be used, and while it is more viscous it is non-toxic. A 65% glycerol / 35% water solution has a freezing point of about -37°C.

A number of commercially available synthetic liquids formulated specifically for use in circulators are also available (mostly from companies who manufacture circulators and chillers). These generally have very favorable properties, such as low toxicity, low viscosity, high stability, high conductivity, low corrosion, and a wide working temperature range. They tend to be far more expensive, however. It's a "you get what you pay for" situation.

Whatever you do, do not use automotive antifreeze. There are substances in automotive antifreeze which could damage the seals of recirculating chillers, and most automotive antifreeze uses propylene glycol which can oxidize under heat and is vulnerable to biological fouling.

Customers who have questions about cooling their rotary evaporator condensers are always welcome to contact us!