If such ideal conditions are not possible as is usually the case”then simple distillation can be applied as long as he liquid in question is composed of compounds that differ in volatility such that their boiling points differ by at least 40 to 50 degrees Celsius. Because the very essence of simple distillation is relies upon the idea that more volatile compounds have lower boiling points and thus when heated to this boiling point will occupy most if not all of the vapor above the liquid pot residue.
Because the difference in boiling point for the compounds in a mixed liquid must at least differ by 40 to 50 degrees Celsius in order for purification through Simple Distillation, this procedure should not result in a high amount of impurities in he distillate or pot residue since the difference between both boiling points is great enough that most of the lower boiling point liquid should vaporize without vaporization of the higher boiling point liquid.
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The experimental set up for the simple distillation procedure is the standard procedure which invokes the use of a a heat source, a magnetic stirrer, a receiving flask for the distillate to be collected in, a condenser with an accompanying inflow of cold water, a stilled, a thermometer, a rubber adapter, an adapter, and check clips. The check clips are seed to stabilize the glass joints while the condenser cools the vaporized gas to liquid.
Because the stilled is where the vapor collects, and the thermometer attached to the top of the stilled must record the temperature of the vapor” and thus boiling point of the distillate”the bottom of the mercury bulb of the thermometer must be directly adjacent to the bottom of the opening of the arm of the stilled. When distilling a stir bar must also be used in order to disturb the heat evenly throughout the solution.
The apparatus must be loosely covered in cotton and aluminum foil, such that the apparatus is transformed into an isolated system from the environment that does not receive nor release the added heat into the surroundings. This ensures that all of the added heat and temperature change results from boiling point properties of the compounds in the solution.
Fractional distillation, like simple distillation is also a separation technique that takes advantage of the differing boiling points of two compounds in a liquid. This technique however, differs from simple distillation in the sense that it can be applied to differences in boiling points of two compounds more sensitive than 40 to 50 degrees Celsius, i. E. 0 to 30 degree Celsius of a difference.
This implies that while the lower boiling point liquid occupies most if not all of the vapor at its respective boiling point, in fractional distillation this vapor is composed of the vaporized lower boiling point compound as well as a significant amount of the higher boiling point liquid. In fact, if simple distillation were to be used to separate a binary mixture when fractional distillation was in fact the appropriate technique to be applied, such a distillation would yield an impure distillate.
This character would be self-evident in the temperature against volume rape of such an inappropriately applied simple distillation as the temperature would steadily increase and eventually level off only once, indicating that the distillate collected was no more special than the condensed liquid that could be collected from simply heating a compound and then cooling it although the distillate obtained would be more concentrated in lower boiling point liquid.
One method in which this characteristic of simple distillation could be applied to purify a binary mixture with compounds that do have sensitively differing boiling points is that the simple distillations could be applied in series. To carry this out, he initial mixture would be broken up into smaller fractions and each fraction would be distilled according to simple distillation procedures until a pure drop of lower boiling point liquid could be collected”since this pure concentrated compound boils before the other less volatile compound.
This obviously is not practical as it yields a very small volume of distillate; however the theory which supports such a procedure is the same theory which the procedure of fractional distillation is built upon. The only difference between the apparatus set-up used for simple distillation and that which is used for fractional distillation is hat fractional distillation makes use of a fractional distillation column which is in between the stilled and the flask containing the pot residue.
Some examples of fractional distillation columns are Figurer columns and Hempen columns. Figurer columns are marked by indentations while the Hempen column is often packed with material such as glass beads or stainless steel sponge as well as glass tubing sections. The purpose of such a column is a bit muddled at first however when placed in the context of the theory of the series of simple distillations it can be understood that this column simply concatenates the rise of simple distillations into one process.
The reasoning is a hybrid of both Dalton and Royalty Law in that each compound will exhibit a characteristic partial pressure in the vapor at each level of the column with accompanying mole fractions. Therefore at each level of the column there are differing mole fractions of each compound in the vapor with an increasing amount of mole fraction of the more volatile compound at higher levels of the column. Such a gradient is obtained by maintaining the bottom of the distilling column hotter than the top.
As mentioned previously, this has the effect of producing a series f simple distillations within the column due to the fact that the vapor that condenses near the top of the column is repaired when it is near the bottom, hotter portion of the column. Such liquid is repaired and recombined with vapor that is concurrently rising from the still pot, this combined vapor becomes increasingly concentrated in lower boiling point liquid while the temperature of the stillest rises, approaching the boiling point of the pure lower boiling point liquid.
Because the column provides in essence, a length of simple distillations, the length can also affect the degree to which the binary mixture is purified. The fractional distillation column is designed as such that each level corresponds to an ideal simple distillation in the series of simple distillations which the fractional distillation experiment is modeled after. Because conditions are never as ideal as desired, a column can be characterized by the degree to which its distilling behavior effectively models the ideal series of simple distillations meant to be performed within it.
Therefore the efficiency of columns is often described in terms of theoretical plates”in this case the term plates simply refer to the level f the column and its respective theoretical simple distillation. Similarly, HEAT” or height equivalent to a theoretical plate”merely describes the length of the column in terms of theoretical plates contained where length per theoretical plate is the unit describing such a length.
The efficiency of fractional distillation columns can therefore be altered by using column packing material whose surface area of contact with the vapors are directly proportional to the amount of series of simple distillations which can be executed. Other factors affecting the efficiency are the length of the column”which relates to the HEAT as rebelliously stated”the maintenance of the temperature gradient that is used to reappoint the returning condensate, and the difference between boiling points of the liquids.
Applying the aforementioned concepts of Simple and Fractional Distillation to Figure 1 in the Appendix, it can clearly be discerned that there are two distinct plateaus at two different temperatures which correspond to the boiling points of each of the compounds in the binary mixture. The first plateau is that of the lower boiling point, more volatile compound and occurs near 52 degrees Celsius with the second plateau of the higher boiling point, less volatile impound occurring near 89 degrees Celsius; in between these two plateaus is a steady increase in temperature of the temperature.
Because Figure 1 from the Appendix varies temperature with respect to volume, Figure 1 indicates that while the temperature was remaining the constant during the plateaus an increasing volume of distillate was actually being collected in the receiving flask (falcon tubes). Through similar reasoning, it can also be concluded that the rapid increase of temperature in between the plateaus corresponds to only a slight increase of distillate collected in between the plateaus.
The distillate collected during the first plateau, during the rapid increase in temperature in between both plateaus, and the last plateau are Fraction A, B and C, respectively. After reviewing Figure 1, it was hypothesized that the first plateau corresponded to a compound with a boiling point from 52 to 54 degrees Celsius and the second plateau corresponded to a compound with a boiling point from 84 to 89 degrees Celsius. When referring to the boiling points of the possible compounds it was determined that the first and second plateaus likely corresponding to acetone” boiling point of 56. Egress Celsius– and Heptanes”boiling point of 98. 4 degrees Celsius. While the actual boiling point of Heptanes is 8 degrees Celsius higher than the experimentally hypothesized boiling point, it was the closest boiling point that matched that of the second plateau in Figure 1. The discrepancy between the actual and experimental boiling point was most likely due to the fact that the heating applied was not enough or human error”of which will be described shortly. For Fraction A, approximately 14 ml was obtained, for Fraction B approximately 6 ml was obtained, and for Fraction C approximately 7 ml was obtained.
These results immediately raise concern as Fraction B should ideally be a very small amount of mixed compound since the amount of liquid obtained is inversely proportional to the degree of efficiency obtained through the particular fractional distillation. This error resulted mostly because of the amount of liquid the receiving falcon tubes could hold. Fraction B as described by the graph was actually never separated. When separating the first fraction of liquid, the falcon tube filled too quickly, thus requiring another falcon tube to continue collecting Fraction A.
Out of confusion and lack of preparation at a critical point in the experiment, Fraction B as described in Figure 1 was actually collected in the Falcon tube. This impurity therefore is most likely the source of the discrepancy between the actual boiling point of heptanes and the experimental temperature of the second plateau in Figure 1. Therefore although the subsequent chromatography results are referred to as Fractions A, B and C, such reference is unfortunately of no relation to the theoretical identities of Fraction A, B, and C as defined in ideal fractional distillation experiments.
The chromatography exults, Figures 3-5 depict the ratios of compound obtained in the Fractions A, B, and C respectively while Figures 6-8 correspond to a 1:1 mixture of Fraction A with Acetone, a 1:1 mixture of Fraction C with Heptanes, and the unknown mixture before distillation, respectively. In each of the chromatography results of the fractions the Area Report is used to determine the ratio of the compound in each of the fractions, this area report is merely describes the area under each of the peaks as a percentage of the total area under all of the peaks, where each peak is characteristic of a compound in the binary mixture.
As GO relates retention time to the volatility of the compound, compounds that elute at greater retention times correspond to the compound that is less volatile or of higher boiling point and vice versa. Therefore Figure 3 that depicts Fraction A, or the distillate of lower boiling point it shows that the ratio of lower boiling point compound to higher boiling point is 1. 829 : 1 or about 2 : 1. Similarly Fraction B in Figure 4 shows a ratio of lower boiling point compound to higher boiling compound of about 1. 4:1, and Fraction C in Figure 5 shows such a ratio to be about 1. 33 : 1.
The experiment therefore did have some success as well as failure. When referring to the pre- fractional distillation GO results (Figure 8), a ratio of about 6. 5 : 10 is obtained for lower boiling point liquid to higher boiling point liquid. Therefore the GO results in Figures 3-4 show a significant increase in the concentration of lower boiling point liquid indicating that the lower boiling point compound was separated to a greater degree. Despite this however, Figure 5 shows that there is still a significant amount of lower boiling point liquid in the distillate of higher boiling point liquid.
Therefore even though these figures do show an increase in lower boiling point distillate as the experiment progressed, the ideal results would yield Fraction A to be most if not all lower boiling point liquid, Fraction B to have a greater amount of higher boiling point liquid than lower boiling point liquid, and Fraction C to be most if not all higher boiling point liquid. In order to determine whether the unknowns were those as hypothesized previously in the analysis of Figure 1, two assays were prepared: one assay of a 1:1 mixture of Fraction A solution and Acetone and one assay of a 1:1 mixture of Fraction C mixture and
Heptanes. While Figure 6 does show some absorbency at the characteristic higher boiling point peak, this was dismissed as due to error resulting from impurities since the ratio of lower boiling point liquid to higher boiling point liquid increased to 4. 1 : 1. Similarly, Figure 7 shows a very slight absorbency at the characteristic lower boiling point peak. This peak was also dismissed as error resulting from impurities since the ratio of lower boiling point liquid to higher boiling point liquid decreased to about 1 : 43.
Therefore the identities of the lower boiling point and higher boiling point compounds in the unknown 30 ml unary mixture Acetone and Heptanes respectively and thus correct as previously hypothesized. Conclusion: This experiment was a success in the sense that solutions of greater concentration of lower boiling point Acetone and higher boiling point Heptanes were separated and their identities as determined by the fractional distillation temperature against volume graph were correctly determined and confirmed with GO chromatography.