Temperature vs Volume Distillate Plot Interpretation

One method you can use to visualize how well a distillation worked is to record the volume of distillate and the temperature that it is collected. With this data, you can plot a temperature vs volume distillation curve. On this graph, the volume of distillate is plotted on the x-axis and temperature on the y-axis. A flat or horizontal stretch on the curve represents a large number of drops or volume being collected with little change in temperature. This signifies a compound being distilled; in other words, a compound’s boiling point.

A vertical area on the curve represents a large change in temperature with no drops being collected. This signifies the transition between the boiling points of two compounds.

One thing that you should notice is that sometimes, right before the transition between the two boiling points, the temperature actually decreases. This occurs because the lower boiling point compound has been totally removed from the distillation flask and it sometimes takes a few minutes for the higher boiling point compound’s vapor to reach the thermometer bulb. During this period where no vapor is hitting the thermometer, the temperature actually starts to decrease. Once the vapor of the higher boiling point compound hits the thermometer, the temperature will shoot up to that boiling point. This decrease in temperature before the boiling point transition is useful because it gives you an indication of when to change distillation collection flasks.

If you understand the concepts discussed above, then you should understand that an ideal distillation curve for a two-component mixture should look like the plot shown below. It should have two flat stretches representing the boiling points of the two compounds in the mixture and one vertical stretch that represents the transition between the two boiling points. When you make a temperature vs volume distillation curve, what you are looking for is how closely the plot resembles this ideal separation. If you see very defined boiling points (flat stretches) and a steep vertical transition between the boiling points, you know that your distillation was successful.

Figure 1: Chart relating temperature and volume of distillate collected.