Why you should give the glass thickness of your evaporation flask a second thought

Balance, balance, balance. It is a recurring theme on the blog (and in life), but there is not much I can do about it except for keep trying to tell you the two sides to every story. In this post, I go on to examine how the thickness of your evaporation flask affects safety and efficiency differently. I offer you some experimental data and suggest the optimal glass thickness to finish your evaporation faster and without glass breakage.

My daughter invited us for brunch the other day and we had a lovely time. She was inspired to try something new and served us milkshakes for the first time. Well, we couldn’t exactly taste her creation, because she had added so much banana and so little milk that the milkshakes were too thick for us to drink. Her husband laughed and declared he could very quickly solve the problem, re-poured the milkshakes in the blender and added half a bottle of milk in there. Well, now we could easily drink the milkshakes, but the taste was rather…thin and diluted.

We had a good laugh, tried again by adding more bananas and strawberries and got the perfect consistency and perfect taste.
But finding a good balance between thick and thin is not just important for making milkshakes, it is also important for achieving high distillation efficiency.

What am I talking about? The thickness of your evaporation flask of course.

Finding an optimal glass thickness is both a safety and efficiency issue.

For one, the glass must be resistant to breakages and withstand conditions, such as high temperatures as well as rapid temperature changes at low pressure. Here, one would prefer thicker glass.
On the other hand, the evaporation efficiency highly depends on the amount of heat energy that reaches the solvent inside the evaporation flask. No need to state the obvious, but a thicker glass barrier would hinder heat transfer, so the thinner the better.

I scratched my head trying to find a way out of this paradox, before deciding to settle this as a scientist would: with experiments. Precisely, I wanted to assess the influence of the glass thickness of the evaporation flask on the evaporation rate during solvent distillation.

To do this, I used acetone as a solvent in five 1 L evaporating flasks with different glass wall thickness, ranging from 1.6 to 2.7 mm. I also tried the experiment with water as a solvent with three different flasks, ranging from 1.5 to 2.6 mm in thickness.

I set up my parameters on a Rotavapor® as follows:

Heating bath temperature60°C
Cooling temperature10°C
Pressure556 mbar (acetone)
72 mbar (water)
Flask size1 L
Content500 mL
Immersion Depthfill level
Rotation speed280 rpm

I guess it’s safe to say, the results were as expected. The thinner the glass walls of the evaporation flasks, the higher the evaporation output. This effect was extremely pronounced in the case of acetone. There, an evaporating flask with a thickness of 1.6 mm achieved 54% higher evaporation output than a flask with 2.7 mm glass wall thickness. You can see a graph with my results below:

evaporation, flask, glass thickness of flask, evaporation flask, evaporation rate

But as I previously mentioned, there is a fine line between glass that is too thin and glass that is too thick. Thinner glass is more prone to breaking, thicker glass slows down the evaporation output.

In general, the evaporation flask should be as thin as possible, but still withstand very low pressures, high temperatures and rapid temperature changes.

So what thickness should you chose?

Well, experimental data, customer feedback and testing all support standardized 1 L evaporating flasks with a glass wall thickness of around 1.8 mm as the condition that provides the optimal ratio between high heat transfer efficiency and high safety.

If you have special requirements, make sure your glassware and rotary evaporator supplier can handle glass modifications to obtain new designs for your individual needs.

I’m glad you stuck through thick and thin with me in this post. If you’ve been sticking through thick and thin with me with the blog as well, you might remember I’ve already written about balance and golden means, but in chromatography. Go ahead, refresh your memory by checking out posts on the golden mean in flash chromatography and balance and the retention factor in chromatography.

Meanwhile, I think I will refresh myself with a perfectly not-too-thick, not-too-thin milkshake. My daughter’s experiment has inspired me to experiment myself not just in the lab, but in the kitchen.

Till next time,

The Signature of Bart Denoulet at Bart's Blog