In this lab tutorial, we discuss simple distillation, including its distinction from fractional distillation, its underlying physical chemistry, and its basic setup.
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What is Simple Distillation?
Distillation is a technique used by chemists to separate components of a liquid mixture with different boiling points. To do so, the liquid mixture is heated to only boil one component, which separates from the mixture as a gas. This gas then passes through a cold tube, condensing it back into a liquid and flowing into a separate vessel.
Simple distillation involves performing this procedure once to separate two liquids with very different boiling points. If instead, you want to separate two similarly volatile liquids, you would instead want to use fractional distillation to get a pure separation. If you would like to learn more about fractional distillation, check out this article.
Physical Chemistry of Simple Distillation
Simple distillation works to effectively separate liquids due to the unique properties of liquids. In particular, all liquids involve an equilibrium between the condensed liquid phase and a layer of vapor above the liquid. As temperature increases, more liquid molecules have enough energy to liberate from the liquid as vapor. This increases the pressure exerted by the vapor on the liquid, called vapor pressure.
When the liquid heats so much that it reaches beyond its boiling point, all the liquid converts to vapor at equilibrium. Thus, at a given temperature, liquids with lower boiling points have higher vapor pressures than those with higher boiling points, as more volatile liquids are closer to becoming completely gas.
When two liquids form a homogenous mixture, any increase in temperature will release vapors from both liquids. However as mentioned before, the more volatile component releases more vapor than the other. According to Raoult’s Law, the exact proportion of a component in the vapor mixture depends on its vapor pressure and its mole fraction in the liquid mixture:
PA = XAPA°
XA = Mole fraction of A
PA = Partial pressure of component A
PA° = Vapor pressure of A
Phase Diagrams in Distillation
Using Raoult’s Law, chemists develop phase diagrams for binary mixtures involving three areas: where both components are liquid, where both are gases, and where a mixture of gas and liquid exists at equilibrium.
The liquid-and-gas phase has an elliptical shape with two corners at either end of the diagram. The two corners correspond to the boiling temperatures of both components. The x-axis corresponds to the mole fraction of one of the components.
In distillation, we start at a given mole fraction of component A and increase the temperature, moving upward in the diagram. Once we reach the edge of the liquid-and-gas phase, the first bubble of gas forms. If we draw a horizontal line from that edge, we find another point that lies on the other edge. This point corresponds to the mole fraction of A in that first bubble.
Notice that the boiling point of component A is lower than that of B. Thus, A has much higher volatility, so it makes sense to have such a high proportion in the gas. In your distillation apparatus, you will basically collect close to pure A in your receiving flask.
However, as mentioned before, simple distillation is most effective when the boiling points of the two components are significantly different. A minimum difference of 25°C between boiling points often serves as the standard for simple distillation. Any closer between the boiling points requires fractional distillation. This involves multiple rounds of distillation since the vapors off of that first round will have significant quantities of both components.
Simple Distillation Setup
To perform a simple distillation, you will need to set up the following apparatus:
The apparatus involves the following important components:
- A heat source, which raises the mixture to the appropriate temperature.
- A round-bottom boiling flask, which contains your liquid mixture or “analyte”.
- A sand bath, which ensures even heating of your boiling flask.
- A Vigruex column, which features internal “finger” structures that serve to collect vapors into liquid drops. These “fingers” primarily collect the trace vapors of the less volatile liquid(s), since they more easily condense. These drops then fall back into the analyte while the more volatile gas(es) pass into the condenser.
- A thermometer, which allows close monitoring of the vapor temperature.
- A condenser column, which features an external cold water jacket that cools the vapor, condensing it to a liquid. Importantly, this water jacket is completely separate from the vapor mixture, which flows through an internal tube.
- A receiver joint, which delivers the condensed liquid to the receiving flask. It features an inlet that you can use to apply a vacuum, which is useful in some separations.
- A receiving flask, into which the condensed liquid or “distillate” flows from the condenser.
Important: To ensure effective cooling of the distillate in the condenser, you should connect water tubes to the condenser inlets such that the water flows uphill. Put differently, water should enter the condenser from the lower inlet and exit from the upper inlet. When water instead flows downhill, a bubble of air forms at the top of the condenser, which limits the efficiency of the condenser.
Once you have set up your apparatus and checked all of your connections between glassware, you can apply heat and start distilling. The distillation is finished when one component is completely separated from the analyte. You can monitor this by observing the thermometer.
When immersed in vapors during distillation, the thermometer should read at or close to the boiling point of the component being separated. If the thermometer significantly drops in temperature after some time, that indicates the analyte has stopped producing vapors. This means that the component has finished distilling. Additionally, if the temperature suddenly increases, that indicates a different component with a higher boiling point has begun distilling. Under simple distillation, this means that the lower boiling point component has already finished distilling.