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Question about thermometer placement for distillation

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hixidom

Rising Star
Hi folks.

I have a vacuum distillation set that I bought online for pretty cheap. Schematically, it is equivalent to this. I have only tried using it once: A couple months ago, I tried distilling a beer for kicks (and as a test run). I didn't have a water pump or vacuum pump at that time, but that shouldn't be relevant to my question...

I noticed that, due to the placement of the thermometer, I cannot measure the temperature of the liquid being heated until after it reaches (and potentially passes) the boiling point. Since it takes the vapor a while to reach the thermometer and it takes the thermometer a while to come to equilibrium with the vapor that reaches the thermometer, it seems to me that there is a delay of at least a minute between the thermometer reading and the actual temperature of the liquid being heated. This means that, since my hotplate doesn't have a temperature reading, I will always overshoot the desired temperature.

Am I doing something wrong here, or is this just an innate flaw in this type of distillation setup? Would it be better to have the thermometer closer to the surface of the liquid (if not submerged in it), or to have it in the vat of oil that the round-bottom flask is submerged in so that it measures the hotplate temperature more directly?

Thank you for any insight you can provide.
 
The function of the thermometer in distillation is to measure the temperature of the vapor that is about to be condensed. You see, the boiling point of a liquid and the condensation point of its vapor are at essentially the same temperature. You want to measure the condensation temperature, because that is the boiling point of the fraction that is condensed and collected at that time.

Also consider that often in distillations some sort of separation column is employed. In such cases, the temperature of the still pot gives no good indication of the condensation temperature of the vapor at the still head (and condenser).

The delay due to thermal inertia of the thermometer is a source of inaccuracy, in a way. But look at it this way: a thermometer that is colder than the surrounding vapor will act as a condenser and will collect heat from the vapor than condenses on it. If the distillation does not proceed too fast, vapor will mostly condense on the thermometer and not in the condenser, until the thermometer reaches equilibrium temperature, allowing the vapor to move into the condenser. So having the bulb of the thermometer just below the entry of the condenser is ideal.

Before attempting distillations, it never hurts to read some useful information.
 
I place the mercury bulb of my thermometer centered at the bottom of the sidearm (that leads to the condenser). Wait for liquid to condense onto the thermometer and drop back into the flask, that is your starting temperature.

Bear in mind when using a vacuum the temperature changes, you can try purifying ethanol from vodka as a neat practice. Hesring the flask in hot oil and/or insulating everything up to the thermometer with aluminum foil to make an air bath may also help if your hot plate isn't putting out enough heat.
 
I always put my thermometer at the top of my column (reflux condenser on top of a vigreux column). That way your measuring the temperature of the vapour about to be condensed. It's especially important to read the temp at the top when using a fractionating column because the vapours at the top are cooler than this in the boiler.

Without a column, you can measure the vapour temp in the boiler or at the recovery bend as they will be much the same.

EDIT: forgot to mention; you can't really "overshoot" the heating if you've got an appreciable amount of liquid in your boiler because the latent heat will keep the solution at it's boiling point, so more heat will just result in faster boiling.

If you want a cheap test of your still, mix denatured alcohol with water and distill (do not drink the product though). You can make different strength solutions to observe the difference in boiling points of different alcohol concentrations. If you do this, thoroughly wash your still before using it to make a product intended for consumption.
 
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