Sizing up the importance of flask size in rotary evaporation

I’ve got some big issues to discuss with you in this post. You probably guessed it already, we will talk about the size of your evaporating flask. As we’ve already seen, the parameters of your laboratory evaporation accessories have an impact on the process. Flask size is one of the factors you should certainly consider when optimizing your reactions. Read on to see which flask size fits your application best.

Whoever said that size does not matter has never tried making strawberry jam. Every year, my wife and I order a big basket of “bad” strawberries from a farmer at our local market. We use these “ugly strawberries to make jam. This year, we bought a batch of tiny strawberries and they were bursting with flavor and sweetness. But oh my goodness, to wash and cut these things up was an ordeal, I tell you. My wife and I were almost fighting for the biggest ones at the end, all in a selfish attempt to avoid handling tiny little berries.

So yes, in life, as in laboratory equipment, size matters. And sure, sometimes bigger is better, but it is not always sweeter.

Let me give you rotary evaporation accessories as an example. I just discussed the importance of glass thickness of your flask in a previous post. But, it is not just how thick, but also how big the flask is that can influence laboratory evaporation.

Generally, the larger the evaporating flask, the greater the evaporation rate.

This is because a larger flask allows for an increased velocity at the perimeter of the flask. With higher speed, the solvent inside the flask becomes more agitated and the active surface area of the solvent is enlarged. This all results in a higher evaporation output.

Larger glass wall surfaces also allow for better heat energy transfers from the heating bath, through the evaporation flask to the solvent. The picture below demonstrates this phenomenon:

evaporation rate, flask size, labroatory evaporation, rotary evaporation

Bigger flasks are also more likely to prevent foaming over, hence minimizing the negative effects of boiling delays and bumping. Lastly, larger flasks offer the advantage of being easier to handle.

But don’t be too quick to disregard smaller flasks.

Smaller strawberries are easier to eat in one bite and they are often sweeter than larger strawberries. Similarly, smaller flasks can offer benefits over larger flasks.

For example, smaller flasks are more suitable for quantitative analysis, such as cases where you want to further assess the residue after the evaporation. You can also handle the rotary evaporator in the vertical axis more flexibly when using smaller flasks. Even more, larger evaporating flasks have thicker glass walls, which in turn lower the efficiency of your evaporation, as discussed in the last evaporation blog post.

The theory makes sense, but I wanted to find out how much influence the flask size really has on the evaporation process. So I set up an experiment to test this.

I turned on my Rotavapor® and started a distillation using different sized evaporating flasks, ranging from 250 mL to 4 L. For the flasks up to 1 L, I used 200 mL solvent, for flasks larger than 1L, I used 750 mL solvent. The other parameters are included in the table below:

Heating bath temperature60°C
Cooling temperature10°C
Pressure556 mbar
Flask size250 mL - 4 L
Content200 mL, 750 mL
Immersion Depthfill level

To evaluate the results, I set the evaporation outputs of the two 1 L flasks to 100%. Then I divided the outcomes into two groups, according to the amount of the solvent in relation to the 1 L flasks. If I set the evaporation output of the 1 L evaporating flask at 100%, the 2 L flask achieved 147% and the 4 L flask 209% output. In contrast, the 250 mL flask obtained only 38% of the output compared to the 1 L flask. The results clearly demonstrate that the evaporation rate increases with larger flask size.

flask size, evaporation rate, evaporation performance, rotary evaporation, laboratory evaporation

As expected, larger surface area is correlated with larger flask volume and increased evaporation output.

From this, I would recommend using as large of a flask as possible as long as it is still suitable for your application. This means, if you want to collect the residue, you should use a smaller flask to avoid sample loss and to make sample transfer easier and more efficient.

Another tip I’d give you is to select a flask that can accommodate at least twice the starting sample volume. Your set-up should ideally look like the picture below with optimal filling quantity of 1/3 to ½ of the evaporating flask’s volume:

flask size, flask volume, rotary evaporation, laboratory evaporation

If you’ve heard a variation of the saying that it does not matter how big the tool is, it matters what you do with it, I can assure you, it is also true for rotary evaporation. Flask size perfection is important, but there are a lot of other factors you can optimize to make sure the process satisfies your needs. Read about using the delta 20 rule, pressure optimization and finding the optimal condenser size in the latest blog posts and keep tuning in for more tips and tricks.

Till next time,

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