Fractional distillation of binary solvent mixture
By - 05/25/2006
Dr. A. Rajasekaran
Objective
To separate the binary mixture by simple distillation using fractionating column
Prinicple/Background of the Experiment
Distillation is a purification technique in which compounds with different boiling points can be separated by controlled heating. Vapors from a sufficiently heated sample can be recondensed and collected, purer than the initial mixture.The liquid which has not vaporized is called the residue, and the liquid which is collected in the receiver is called the distillate.
Since not all chemicals distill the same way, there are several distillation techniques can be preferred depending on the nature of constituents to be purified or to be separated. These include simple distillation, fractional distillation, steam distillation and vacuum distillation.
A simple distillation (figure 2) is for purifying liquids of one component (separating nonvolatile liquid impurity or to purify a liquid from solid contaminants), multiple liquids where the differences in boiling points is very large (a low boiling liquid from a high boiling liquid)(b.p difference around 50-70°C). Simple distillations are not effective in removing multiple solvents from one another with a high degree of success.
In fractional distillation (figure 3), a fractionating column is inserted between the distillation flask and the distillation head. The fractionating column provides a large surface area in which the mixture can be continuously vaporized and condensed.
The principle of a fractionating column is that, as the vapours ascend the column from the boiling mixture below, the high boiling components are condensed and returned to the flask, the ascending column of vapour being thus steadily “scrubbed” by the descending column of liquid condensate. The ascending column of the vapour becomes therefore steadily richer in the lowest boiling component, and the descending column of condensate steadily richer in the highest boiling component.
Figure 1 represents the typical curve for simple and fractional distillation. In an ideal fractional distillation, two distinct fractions are obtained. The first corresponds to the component with the lower boiling point and the second to the high-boiling point component. What characterizes a good fractional distillation is the sudden increase in temperature between both fractions, or in other words, a very small volume distilled at temperatures other than the boiling points of the pure liquids. In simple distillation, a much more gradual increase in temperature is observed, reflecting the impure nature of the distillate
Figure 1. Simple and fractional distillation curves
Steam distillation is used for separating mixtures of chemicals such as oils, resins, hydrocarbons, etc. which are insoluble in water and may decompose at their B.P.
Vacuum distillation is used for separating liquids boiling above 200◦C
Fractional Distillation Apparatus
The set-up for fractional distillation is shown in Figure 3. It consists of a round-bottomed distillation flask where the liquid is placed, a fractionalting column, a distillation head that connects the distillation flask to the condenser; and a distillation adaptor that connects the condenser to the receiving flask. The condenser is a tube surrounded by a water jacket to cool and condense vapors. The distillation head holds a thermometer to allow the temperature of the vapors to be monitored during the distillation.
Figure 2 Simple distillation set-up
Procedure:
Assemble the apparatus as shown in Figure 3, use graduated cylinder to collect the distillate. Place the liquid (25 mL of cyclohexane (b.p.80.7°C) and 25 mL of toluene(b.p.111°C) or 25 mL of benzene (b.p.80°C)or 25 mL of of toluene) to be distilled into the distillation flaskPlace a boiling chip in the distillation flask to ensure smooth bubbling and prevent bumping of the liquid up into the distillation head. Heat the liquids and distil slowly, so that the total distillation occupies about 1 hour and 30 minutes. As the liquid boils, vapors rise into fractionating column and to the distillation head and condensed liquid will be seen dripping from the thermometer bulb. Eventually the vapors enter the side arm of the distillation head and continue into the condenser. Once in the condenser the vapors are cooled due to the water circulating in the outer jacket; the vapors condense back to a liquid that runs down the condenser and is collected in the receiving flask. Increase the heating rate near the midpoint of the distillation otherwise the head temperature will drop.
Take temperature readings at the first drop and at each 2 ml increment. Continue until you have distilled 48 ml(collect the fractions having boiling points (a) 80—85°, (b) 85-107° (c) 107-111°, these fractions should have the volumes about 23, 2, 23 mL respectively). Construct a table in your notebook as given below, to record the temperature at the distillation "head" as a function of volume distilled.
Volume distilled (mL) 2 4 6 8 10 12 14 16 18 20
Temperature without column
Temperature with column
Make a plot of Temperature (ºC) Vs. volume of distillate Collected (ml) and determine the b.p of each of these solvents.
Note:
1.The atmospheric pressure is always less than standard atmospheric pressure, and therefore the observed boiling points will always be lower than reported b.p in literature.
2.Never distill a flask to dryness because explosive peroxides tend to form and are concentrated enough to blow a flask to pieces.
3.Use always a boiling stone to prevent bumping, that is a sudden gushing of liquid that rushes upwards and may splash out of the flask in simple distillations and hinder fractional distillations. A boiling stone is a small piece of marble gravel.
Author Contact Information
Dr. A. Rajasekaran,
Additional Professor, School of Pharmacy, Faculty of Medicine and Health Sciences,Asian Institute of Medicine Science and Technology, Sungai Petani, Kedah, Malaysia
Phone: 006-04-4422884 Fax: 006-04-4422881 email:rsekaran2001in@yahoo.co.in