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`The Techniques
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`3. Open the stopcock at B (Figure 6-7).
`4. Turn on the aspirator to the maximum extent.
`5. Tighten the screw clamp at A until the tubing is nearly closed.
`6. Slowly tighten the screw clamp at C until the tubing is closed completely. Watch the
`bubbling action of the ebulliator to see that it is not too vigorous or too slow. Adjust A
`until a fine steady stream of bubbles is formed with C closed.
`7. Record the pressure obtained, after waiting a few minutes to allow any residual
`solvent to be removed. Readjust A if necessary. If the pressure is not satisfactory,
`check all connections to see whether they are tight. Do not proceed until you have a
`good vacuum.
`
`Beginning Distillation
`
`8. Raise the heat source (see Technique 1) into position with wooden blocks , or other
`means , and begin to heat.
`9. Increase the temperature of the heat source. Eventually a reflux ring will contact the
`thermometer bulb, and distillation will begin. Record the temperature range and the
`pressure range during the distillation. The distillate should be collected at the rate of
`about 1 drop per second. The Claisen head and distilling head may have to be wrapped
`with glass wool or aluminum foil (shiny side in) for insulation during the distillation if
`it is slow. The boiling point should be relatively constant so long as the pressure is
`constant. A rapid increase in pressure may be due to increased use of the aspirators in
`the laboratory or to rapid decomposition of the material being distilled. Decomposition
`will produce a dense white fog in the distilling flask. If this happens, reduce the
`temperature of the heat source, or remove it, and stand back until the system cools.
`Investigate the cause.
`
`Changing Receiving Flasks
`
`10. To change receiving flasks during distillation when a new component begins to
`distill (higher boiling point at the same pressure), open the clamp at C slowly, and
`immediately lower the heat source. (Watch the ebulliator for excessive backup. It may
`be necessary to open clamp A.) The wooden blocks under the receiver are removed, or
`the clamp is released, and the flask is replaced with a clean, preweighed receiver.
`11. Reclose the clamp at C and allow several minutes for the system to reestablish the
`reduced pressure. Bubbling will commence after the liquid is drawn back out of the
`ebulliator. This liquid may have been forced into the ebulliator when the vacuum was
`interrupted.
`12. Raise the heating source back into position under the distilling flask and continue
`with the distillation. When the temperature falls at the thermometer, this usually indi(cid:173)
`cates that distillation is complete. If a significant amount of liquid remains , however,
`the bubbling may have stopped, the pressure may have risen, the heating source may
`not be hot enough, or perhaps insulation of the distillation head is required. Adjust
`accordingly.
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`fractional distillation is required, while in Figure 7-6C a simple distillation provides an
`adequate separation.
`In the extreme case of a negligible vapor pressure for B, the vapor will be pure
`A. For example, the distillation of an aqueous salt solution will behave like this:
`Ptotal = P~20NHp + P~alt Nsalt
`P~alt = 0
`Ptotal = P~ oNH o
`A solution whose mole fraction of water is 0.7 will not boil at 100 °C, since Ptotal =
`(760)(0. 7) = 532 mmHg and is less than atmospheric pressure. Eventually, the solu(cid:173)
`tion can be heated to the boiling point. At 110 °C, the solution boils because Ptatal =
`(1085)(0.7) = 760 mmHg. The vapor is pure water, and its observed boiling point is
`100 °C. (The vapor pressure of pure water at 110 oc is 1085 mmHg.)
`
`2
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`2
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`7.4 COLUMN EFFICIENCY
`
`A measure of column efficiency is given by theoretical plates. A column would have 1
`theoretical plate if the first distillate (condensed vapor) had the composition located at
`L2 (20% A) when starting with a liquid with composition L 1 (5% A) as shown in
`Figure 7-4. This would correspond to a simple distillation, or one vaporization(cid:173)
`condensation cycle. A column would have 2 theoretical plates if the distillate (vapor)
`had the composition L3 (50% A) starting with a liquid with composition L1 (5% A).
`The 2-theoretical-plate column essentially carries out " two simple distillations," cor(cid:173)
`responding to lines L1 V 1 L2 and L2 V 2L3 in Figure 7-4. Thus , five theoretical plates
`are necessary to separate nearly pure A from B (lines L1V1L2 , L2V2L3 , L3V3L4 ,
`L4V4L5 , and L5V5 in the example shown in Figure 7-4). In effect, a 5-plate column
`corresponds to "five simple distillations." In practice, as shown in Figure 7-5, the
`first theoretical plate corresponds to the initial vaporization from the distilling flask,
`and the remaining 4 plates are obtained from the condensation-vaporization cycles in
`the column.
`Most columns do not allow distillation in discrete steps, as indicated in Figure
`7-4. Instead, the process is continuous, allowing vapors to be continuously in contact
`with liquid of changing composition as it passes through the column. In principle,
`almost any material can be used to pack the column so long as it can be wetted by the
`liquid.
`
`The relation between number of theoretical plates needed to separate an ideal
`two-component mixture and the difference between the boiling points of the compo(cid:173)
`nents is given in Table 7-2. The values have been calculated assuming an average
`boiling point of 150 oc for each mixture and for a column operating at equilibrium.
`Since columns are seldom operated at equilibrium, more theoretical plates than listed
`may be necessary for a complete separation. For example, 3 or 4 plates rather than the
`listed 2 plates may be necessary to separate a mixture with a boiling -point difference of
`
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`Technique 7: Fractional Distillation, Azeotropes
`
`575
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`of the foil are crimped so that the glass wool is not visible. The column is then wrapped
`with this insulation. A piece of cloth may also be used as insulation.
`It is also important to distill the mixture as slowly as possible. Much of the
`liquid should be allowed to return through the column to establish good vapor-liquid
`equilibrium. The values given in Table 7-3 depend on establishing good equilibrium.
`The distillation must be conducted fast enough to maintain a constant takeoff (rate
`at which material collects in the receiver) so that the temperature at the thermometer
`bulb remains constant. In other words, a distillation conducted too slowly will be
`unsatisfactory.
`
`7.6 FRACTIONAL DISTILLATION: METHODS
`
`For a fractional distillation, the apparatus shown in Figure 7-2 and a column similar to
`one of those shown in Figure 7-7 are used. The distilling flask, condenser, and vacuum
`adapter should be clamped so that the column is perpendicular to the desk top. No
`cooling water is used in the fractionating column. The receiving flask will have to be
`supported by wire gauze on an iron ring attached to a ring stand. If an oil bath or a
`heating mantle is used, wooden blocks should be placed under the heat source so that it
`can be removed easily.
`The steps given in Technique 6, Section 6.4, p. 556 for simple distillation are
`also used in fractional distillation. Besides those steps, care must often be taken to
`insulate the column with glass wool and aluminum foil, or a cloth towel if the material
`to be distilled has a high boiling point, and to distill as slowly as possible (Section 7 .5).
`For the best possible separation, the temperature of the material in the distilling
`flask should be raised slowly so that the liquid-vapor can move up the column and be
`equilibrated. If the contents of the flask are heated too quickly, the column fills with
`liquid (flooding). Flooding decreases the efficiency of the separation. If there is flood(cid:173)
`ing, the heat source must be lowered so the liquid can return to the distilling flask.
`The temperature at the thermometer bulb should remain constant as the pure
`low-boiling component is removed (Figure 7-3). When most of this component is
`distilled, the distillation rate decreases. At this point, the distilling flask is heated to a
`higher temperature, and an intermediate fraction is collected until the temperature of
`the vapor at the thermometer bulb stabilizes at the higher value. The higher-boiling
`component is collected in another container. Ideally, results should be as shown in
`Figure 7-3.
`
`· ' pi
`
`II:
`
`"
`i! I
`f
`!
`i
`i
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`t
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`7. 7 NON IDEAL SOLUTIONS: AZEOTROPE$
`
`Many mixtures of compounds, because of intermolecular attractions or repulsions, do
`not show ideal behavior. Because of this nonideal behavior, Raoult's law is not fol(cid:173)
`lowed. There are two types of vapor-liquid composition diagrams that result from this
`nonideal behavior: the minimum-boiling-point and the maximum-boiling-point dia(cid:173)
`grams. The minimum or maximum points in such diagrams correspond to a constant-
`
`l
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`56
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`576
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`The Techniques
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`boiling mixture called an azeotrope. An azeotrope has a fixed composition, which
`cannot be altered by normal distillation (simple or fractional), and a fixed boiling point.
`Hence, an azeotrope acts as if it were a pure compound.
`
`A. Minimum-Boiling-Point Diagrams
`
`The most common two-component mixture that gives a minimum-boiling-point azeo(cid:173)
`trope is the ethanol-water system shown in Figure 7-8. A minimum-boiling-point
`azeotrope results from a slight incompatibility of the substances, which leads to higher(cid:173)
`than-expected combined vapor pressures from the solution. The higher combined vapor
`pressures bring about a lower boiling point for the mixture than that of either of the two
`components. One notes that the azeotrope V3 in Figure 7-8 has a composition of about
`96% ethanol-4% water and a boiling point of 78.1 °C. This boiling point is not much
`lower than the boiling point of pure ethanol (78. 3 °C) . However, this small difference
`means that one can obtain only 96% ethanol-4% water in a simple or fractional distilla(cid:173)
`tion of an aqueous solution of ethanol. Even with the best fractionating column, one
`cannot obtain 100% ethanol! The remaining 4% of water can be removed by adding
`benzene and removing a different azeotrope, the benzene-water-ethanol azeotrope (bp
`65 °C). Once the water is removed, the excess benzene is removed as an ethanol(cid:173)
`benzene azeotrope (bp 68 °C). The resulting material will be free of water and is called
`"absolute" ethanol.
`The behavior on fractional distillation of an ethanol-water mixture of composi(cid:173)
`tion X (Figure 7- 8) can be described as follows. The mixture is heated (line XL1) until
`it is observed to boil (L1). The vapor (V 1) will be richer in the lower-boiling compo(cid:173)
`nent, ethanol, than the original mixture. The condensate (L2) is vaporized to give V 2 .
`The process continues, following the lines to the right, until the azeotrope (V 3) is
`
`1 000
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`r
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`temperature
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`X
`95.6 100%
`%Ethanol--?
`CH 3CH 20H
`FIGURE 7-8. Ethanol-water minimum-boiling-point phase diagram
`
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`592
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`The Techniques
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`or subtraction is necessary, since the reference pressure (created by the initial evacua(cid:173)
`tion when filling) is zero. Hence, the height difference !:l.h gives the pressure in the
`system directly.
`A barometer must be at least 760 rnrn tall; an open-end manometer is often 1 rn
`tall. A closed-end manometer is often less than 20 ern long and is thus very convenient
`for laboratory use. These manometers are conveniently used with an aspirator that only
`rarely creates pressures lower than 10 to 20 rnrnHg. Since the open-end and the closed(cid:173)
`end manometers cannot be read more finely than ± 1 rnrnHg, they are not suitable for
`very low pressures. They should not be used for high-vacuum applications. Other types
`of manometers must be used with high vacuum. The manometer common for high(cid:173)
`vacuum systems is the McLeod gauge, which will not be discussed here.
`
`9.4 CONNECTING AND USING A MANOMETER
`
`The most common use of a closed-end manometer is to monitor pressure during a
`reduced-pressure distillation, as discussed in Technique 6, Sections 6.5, 6.6, and 6.7,
`pp. 558-563. The manometer is placed in a vacuum-distillation system as shown in
`Figure 9-6. Generally an aspirator is the source of vacuum. Both the manometer and
`the distillation apparatus should be protected by a trap from possible backups in the
`water line. An alternative trap arrangement is shown in Technique 6, Figure 6-7, p.
`559. The trap assembly shown in Figure 9-6 is more easily constructed. You may
`notice in the figure that the trap has a device for opening the system to the atmosphere.
`This is especially important in using a manometer, since one must always make pres(cid:173)
`sure changes slowly. If this is not done, there is danger of spraying mercury throughout
`the system, breaking the manometer, or spurting mercury into the room. In the closed(cid:173)
`end manometer, if the system is opened suddenly, the mercury will rush to the closed
`end of the U tube with such speed and force that the end will often be broken out of the
`manometer. Air should be admitted slowly by opening the valve (Figure 9-6) on the
`trap cautiously. Similarly, when the vacuum is being started, the valve should be
`closed slowly so that mercury will not suddenly be sucked out of the gauge. Rapid
`opening of an open-end manometer system inevitably results in spraying a fountain of
`mercury through the open end.
`When one is conducting a reduced-pressure distillation, if the pressure is lower
`than desired, it is possible to adjust it by means of a bleed valve. One removes the
`screw clamp on the valve shown in Figure 9-6 and connects the base of a Tirrill-style
`Bunsen burner to the water trap, as shown in the alternative arrangement. The needle
`valve in the burner can be used to adjust precisely the amount of air that is bleeding into
`the system and hence the pressure. When the valve is opened fully, atmospheric pres(cid:173)
`sure is admitted; when it is fully closed, maximum vacuum is achieved.
`The boiling point of a liquid is a function of the applied pressure. The boiling
`point is often reported at atmospheric pressure or some other pressure that cannot be
`exactly reproduced by using the aspirator. By the nomograph chart in Technique 6
`(Figure 6-2, p. 552), it is possible to determine the boiling point of a liquid at any
`given pressure if the value is known for at least one other pressure.
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`594
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`The Techniques
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`All chromatography works on much the same principle as solvent extraction
`(Technique 5) . Basically, the methods depend on differential solubilities (or ad(cid:173)
`sorptivities) of the substances to be separated relative to the two phases between which
`they are to be partitioned. In this section, column chromatography, a solid-liquid
`method, is considered. Thin-layer chromatography is examined in Technique 11; gas
`chromatography, a gas-liquid method, is discussed in Technique 12.
`
`10.1 ADSORBEN S
`
`Column chromatography is a technique based on both adsorptivity and solubility. It is
`a solid-liquid phase-partitioning technique. The solid may be almost any material that
`does not dissolve in the associated liquid phase; those solids most commonly used are
`silica gel, Si02 · xH20, also called silicic acid, and alumina, Al20 3 · xH20. These
`compounds are used in their powdered or finely ground (usually 200- to 400-mesh)
`forms.
`
`Most alumina used for chromatography is prepared from the impure ore baux(cid:173)
`ite, Ah03 · xH20 + Fe20 3. The bauxite is dissolved in hot sodium hydroxide and
`filtered to remove the insoluble iron oxides; the alumina in the ore forms the soluble
`amphoteric hydroxide Al(OH)4-. The hydroxide is precipitated by C02 (which reduces
`the pH) as Al(OHh. When heated, the Al(OH)3 loses water to form pure alumina,
`Ah03.
`
`hot NaOH Al(OH)4- (aq) + Fe20 3 (insoluble)
`Bauxite (crude)
`Al(OH)4- (aq) + C02 ~ Al(OH)3 + HC03-
`
`2Al(0Hh ~ Ah03(s) + 3H20
`Alumina prepared in this way is called basic alumina, because it still contains some
`hydroxides. Basic alumina cannot be used for chromatography of compounds that are
`base-sensitive. Therefore, it is washed with acid to neutralize the base, giving acid(cid:173)
`washed alumina. This material is unsatisfactory unless it has been washed with
`enough water to remove all the acid; on being so washed, it becomes the best chroma(cid:173)
`tographic material, called neutral alumina. If a compound is acid-sensitive, either
`basic or neutral alumina must be used. One should be careful to ascertain what type of
`alumina is being used for chromatography. Silica gel is not available in any form other
`than that suitable for chromatography.
`
`10.2
`
`INTERACTIONS
`
`If powdered or finely ground alumina (or silica gel) is added to a solution containing an
`organic compound, some of the organic compound will adsorb onto or stick to the fine
`particles of alumina. Many kinds of intermolecular forces cause organic molecules to
`bind to alumina. These forces vary in strength according to their type. Nonpolar com(cid:173)
`pounds bind to the alumina using only Vander Waals forces. These are weak forces,
`
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