T. E. (Thomas Edward) Thorpe.

A dictionary of applied chemistry online

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in the barometer tube, and, on the other hand,
the boiling-points of the liquids under various



The behaviour of two liquids, A and B, when
brought into contact with each other, must be
affected (a) by the attraction of the like mole-
cules ; those of A for each other, and those of
B for each other ; and (6) by the mutual attrac-
tion of the molecules of A and B. If the attrac-
tion of the unlike molecules is relatively very
slight, it may be expected that the liquids will
be non-miscible, or nearly so, as in the case of
benzene and water. If the attraction of the



266



DISTILLATION.



unlike molecules is greater, but still relatively
small, miscibility within limits may be expected,
as, for example, with aniline and water. In such
cases there is frequently slight expansion and
slight absorption of heat on admixture. Com-:
paring together various pairs of liquids, as the
mutual attraction of the unlike molecules
increases relatively to that of the like molecules,
the following changes may be expected : (o)
increasing and finally infinite miscibility ; (6)
slight expansion, diminishing to zero, and
followed by increasing contraction ; (c) diminish-
ing absorption of heat, changing to increasing
heat evolution. These changes do not, in many
cases, run strictly 'pari passu, and among
liquids which are miscible in all proportions,
it is not unusual to find a small amount of
contraction attended by a slight absorption of
heat, as, for example, when a little water is
added to normal propyl alcohol, but when
certain compounds, such as chlorobeuzene and
bromobenzene, which . are chemically closely
related, are mixed together, there is no appreci.
able change of volume or temperature. For
such substances it is probable that'the different
molecular attractions, A for A, B for B, and
A for B, are very nearly equal, and that the
relation suggested by GaUtzine (Wied. Ann. 41,
770; and by D. Berthelot, Compt. rend. 126,
1703), namely, that a^^=f/a^.a^, holds good,
where a^^ represents the attraction of the
unlike molecules, and a^ and Og the respective
attractions of the like molecules.

It would appear, then, that there are two
very simple cases : I. that in which the
liquids are non-miscible ; 11. that of two
infinitely miscible and closely related liquids
which show no heat or volume change on
admixture.

I. It was found by Regnault that when two
non-misoible liquids are placed together over the
mercury iu a barometer tube, the observed
vapour pressure is equal to the sum of the
vapour pressures of the two liquids when heated
separately to the same temperature. It is only
necessary that both substances should be
present in sufScient quantity, and that the
tube should be shaken or sufiicient time allowed
for the evaporation of the heavier liquid. The
liquids evaporate independently of each other,
and Dalton's law of partial pressures is therefore
applicable to the distUlation of non-miscible
liquids. Each vapour behaves to the other as
an indifferent gas, and the boiling-point of
each liquid depends on the partial pressure of
its own vapour. The temperature is necessarily
the same for both liquids, and the total pressure,
if the distillation is carried out in the ordinary
manner, is equal to that of the atmosphere.
The boiling-point is, therefore, that temperature
at which the sum of the vapour pressures of the
components is equal to the atmospheric pressure.
For example, at 90-23° the vapour pressure of
water is 530-1 mm. ; and that of chlorobeuzene
is 210-1 mm. ; total, 740-2 mm. ; and it was
found that when chlorobenzene and water were
distilled together under a barometric pressure
of 740 '2 mm., the temperature varied only
between 90-25° and 90-35°, until there was
scarcely any chlorobenzene left in the residue,
when it rose rapidly to 10((°. The boiling-



point of a pair of non-miscible liquids is neces-
sarily lower than that of the more volatile
component, and may be far lower than that of
the less volatile.

The process of distillation with steam is
very frequently employed in the case of sub-
stances of high boiling-point, which are insoluble
or only slightly soluMe in water, such as aniline
or nitrobenzene. (For the application of steam
distillation to the preparation of ethereal oils,
V. von Rechenberg, Theorie der Gewinnung
und Trennung der atherischen Oele durch
Destination, Schimmel & Co. 1910.)

The composition of the vapour — and there-
fore of the distillate — ^from two non-miscible
liquids, like the vapour pressure and boiling-
point, is independent of the relative amounts of
the components, provided that both are present
in sufScient quantity and that evaporation can
take place freely. Calling the vapour densities
D^ and D^, and the vapour pressures at t°
p^ and Jig, there will be, in a litre of the mixed
vapour, 1 litre of A at t° and p^ mm., and 1 htre
of B at t° and p^ mm. The weights of vapour
will therefore be :
0-0899 xD^x273X2)^ 0-0899 xD^x 273x^)2

(273+<)x760 ^^ (273+<)x760

respectively, and the relative weights wiU be

DaX^'a (Naumann, Ber. 10, 1421, 1819, 2014,



DbXJ'b

2099 ; Brown, Chem. Soc. Trans. 35, 547).

The vapour density of chlorobenzene is 56-2,
and that of water is 9; and at 90-23° the
relative weights of vapour will be

m'^_ 56-2x210-l -
™'b 9x530-1
and the percentage weight of chlorobenzene
wiU be 71-2. In the actual experiment to which
reference has been made, the percentage of
chlorobenzene in the distillate was found to
be 71-4. Both boiling-point and vapour
composition agree weU with the calculated
values.

II. The vapour pressure of a mixture of
two infinitely miscible liquids which are chemi-
cally closely related to each other, or, probably,
for which 0^3= V^^-^b' ^ gi'^^^n by the formula
lOOp = M^^-f (100 — MJ^B. where M is the
molecular percentage of A, and p, p^, and p^
are the vapour pressures of the mixture and
of A and B respectively, at the same tempera-
ture, f. In other words, the relation between
vapour pressure and molecular composition is
represented by a straight line (Van der Waals,
Proc. Roy. Acad. Amsterdam, 3, 170 ; Young,
Chem. Soc. Trans. 81, 768 ; Young and Fortey,
ibid. 83, 45 ; ZawidsM, Zeitach. physikal. Chem.
35, 129). The relation has been found to hold
accurately for chlorobenzene and bromobenzene,
and with very slight error for other pairs of
closely related liquids.

In order to calculate the boiling-points of all
mixtures of two such liquids under a given
pressure, p, the vapour pressures of each must
be known at aU temperatures betwe^n their
respective boiling-points under that pressure.
The percentage molecular composition of
mixtures which would exert the vapour pressure



DISTILLATION.



207



p must then be oaloulated at a series of tempera-
tures between these limits by means of the for-
mula M = 100 ^°~^ Lastly, the values of M

must be plotted against the temperatures, when
the curve drawn through the points will give
the required relation between boiling-point and
moleoulaT composition. In the cases examined,
the agreement between the observed and cal-
culated results is quite satisfactory.

The relation between the composition of a
liquid mixture and that of its vapour (distillate)
has been the subject of many experimental
investigations (Brown, Chem. Soo. Trans. 35,
547; 39, 304; Lehfeldt, PhU. Mag. (v.) 46,
42 ; Zawidski, Zeitsoh. physikal. Chem. 35, 129 ;
Carveth, J. Phys. Chem. 3, 193 ; Winkelmann,
Wied. Ann. 39, 1 ; Linebarger, J. Amer. Chem.
Soo. 17, 615 ; Gahl, Zeitsch. physikal. Chem.
33, 179 ; Eosaneff, Lamb, and Breithut, ibid.
66, 349 ; RosanoS and Easley, J. Amer. Chem.
Soc. 31, 953) ; for a description of which (up
to the year 1903, v. Young's IVaotional Distilla-
tion, 71-113). Unfortunately, however, it is
only in a very few cases that the mixtures
investigated consisted of closely related liquids.
In the discussion of their results, Lehfeldt and
Zawidski made use of formulse which they
derived from a general equation proposed in-
dependently by Duhem and by Margulfes, and it
is to be noted that both formulse, in the simplest
oases, can be reduced to one originally given by

Brown, z^=c-—, where m\ and «j'b are

the relative weights of the two substances in
the vapour, mx and m^ the relative weights in
the liquid, and c is a constant. From Zawidski's

formula it would follow that c = — So far as

experimental evidence is available, it would
appear that Brown's formula is applicable to
mixtures of liquids for which a^-^^is/a^.a^,

and that for closely related liquids c =^.

When the components of a liquid mixture
are not chemically closely related, the relation
between vapour pressure and molecular composi-
tion is not, as a rule, represented by a straight
line, but by a curve, and, in all probability, the
form of the curve depends on the relation between
<^A.B and /v/oa-»b. as shown in Kg. 7. That is to
say, when

oab< v'oa-ob, lOOp >mija+(100-m)pb,
and vice versa.

The greater the difference between Oa3 and
Voa-^b the greater will be the curvature ; and it
will be seen that, for any given values of p^ and
Pb> if the deviation from straightness exceeds
a certain limit, there must be a point of maximum
or minimum pressure on the curve. It is
obvious, also, that the smaller the difference
between Pa, and Pb, the smaller wiU be the
deviation required to give a maximum or
minimum pressure. If aA.B is so much smaller
than Va-^b t^iat the two liquids are only
partially miscible, the maximum pressure will
not be represented by a single point, but by a
horizontal line forming part of the curve.

The influence pf chemical relationship is



well seen by the behaviour of the monhydric
aliphatic alcohols towards water. These alcohols
may be regarded as derivatives of water, formed
by the replace-
ment of a hydro- JPb
gen atom by the -"^
group CnRim+i;
the smaller the ^
alkyl group, the a
closer is the re- m
lationship of the ^
alcohol to water, o.
At the ordinary
temperature me- p^^
thyl alcohol is
miscible with
water in all pro-
portions, as also
are ethyl and propyl alcohols ; normal butyl
and j«obutyl alcohol are only partially mis-
cible with water ; and the solubility of the
higher alcohols in water diminishes with rise of
molecular weight, cetyl alcohol, for example,
being practically insoluble. The volume and
heat changes on admixture with water similarly
indicate that the attraction of the unlike mole-
cules diminishes as the molecular weight of the
alcohol increases. Lastly, the deviation of the
vapour pressure curves from straightness
increases (Konowalow, Wied. Ann. 14, 34), as
may be seen from Fig. 8.




MOUCULM FERCENTAGE OF B

FiQ. 7.




O 20 40 60 80 100

Molecular percentage of alcohol
Fig. 8.

(The temperatures are such that the vapour
pressure of each pure alcohol is 400 mm.)

The boiling-points of liquids which are not
closely related cannot, as a rule, be calculated

by means of the formula M = 100 ^°~^ . If,

as is usually the case, the vapour pressures of
the mixtures are higher than are given by the



2fi8



DISTIIXATION.



formula lOOj) = M^A+pOO -M):Pe. the ob-
served boiling-points will be lower than the
calculated ; if the vapour pressures are lower,
the boiling-points will be higher. Moreover,
when there is a point of maximum pressure on
the curve which shows the relation between
vapour pressure and molecular composition,
there must be a point of minimum temperature
on the boiling-point — molecular composition
curve, the composition of the mixture which
exerts a maximum vapour pressure, p, at t"
being the same as that of the mixture which
has the miniumm boiling-point, t", under the
pressure p. So, also, two substances capable
of forming a mixture of minimum vapour
pressure can also form a mixture of maximum
boiling-point.

Such mixtures of either minimum or maximum
boiling-point, when distilled, boil at a constant
temperature without change of composition,
like pure substances, and they have frequently
been mistaken for chemical compounds. The
composition, however, depends on the pressure,
which would not be the case with a definite
compound ; and, moreover, mixtures of minimum
boiling-point are formed owing to the relatively
small attraction of the unlike molecules for
each other. A very striking example of the
influence of pressure on the composition of the
mixture of constant boiling-point has recently
been observed by Wade in the case of ethyl
alcohol and water. Undei normal pressure the
mixture of minimum boiling-point contains 4-43
p.c. of water. At higher pressures the percentage
of water is slightly higher ; at lower pressures it
is smaller, and it is remarkable that the diminu-
tion in the percentage of water becomes more
and more rapid as the pressure falls, so that
below 80 mm. no mixture of minimum boiling-
point is formed at all.

In the case of three liquids, if each of the
three possible pairs is able to form a mixture of
minimum boilmg-point, it may happen that a
particular mixture of all three liquids wiU boU
constantly at a lower temperature than will any
of the pairs or single liquids. Thus benzene and
water form such ternary mixtures of minimum
boiling-point with ethyl, «-propyl, isopropyl,
and tertiary butyl alcohols (Young, Chem. Soc.
Trans. 81, 707 ; Fortey and Young, ibid. 81, 739).

A table of mixtures of constant boiling-point
is given in Young's Fractional Distillation,
67-69. Other mixtures have since been dis-
covered by Wade (Chem. Soc. Trans. 85, 938 ;
87, 1656 ; V. also 95, 1842).

The composition of the vapour from a
mixture of liquids which are not closely related,
cannot, as a rule, be calculated from the vapour
pressures of the components and the composi-
tion of the liquid, but for purposes of interpola-
tion the formulae of Lehfeldt or Zawidski may be
found useful {l.c.).

Fiaotional distillation. It has been stated

that Brown's formula, —^=c-^, is applicable

to mixtures of all closely related liquids 6o far
investigated, and that, in all probability,

c =~. The value of c is necessarily such that

the vapour is richer than the liquid in the more
volatile component. The first portion of



distillate obtained by condensation of the
vapour will, therefore, be richer, and the
residual liquid wiU be slightly poorer in that
component. Continuing the distillation, the
vapour evolved from the residual liquid will
again be richer than the liquid in contact with
it in the more volatile component, though
poorer than the first portion of vapour. If
successive portions of vapour be condensed, the
percentage of the more volatile component in
the distillates or fractions will steadily diminish,
untU, if the constant c differs greatly from unity,
the residue will consist of the less volatile liquid
in a pure or very nearly pure state. As the
distillation proceeds, the temperature will,
therefore, rise until, if c has a hjgh value, the
boiling-point of the less volatile component is
reached.

If the fractions obtained in the first distilla-
tion were redistilled, each of them could be
separated into smaller fractions, the first richer
and the last poorer in the more volatile com-
ponent, and by a process of systematic fractional
distillation it should, theoretically, be possible to
effect a separation of both components.

A mathematical and experimental investiga-
tion (BarreU, Thomas, and Young, PhU. Mag.
(v.) 37, 8) of the behaviour on distillation of
mixed liquids which follow Brown's law, showed
that with two components it is the liquid of
higher boiling-point which is the easier to
separate ; and that if there are three or more
components boiling at fairly equal intervals of
temperature, the substances of intermediate
boiling-point are the most diflScult to separate,
and the least volatile substance is the easiest.

The metho(J employed for the fractional
distillation of a mixture of two liquids can best
be explained by taking a concrete example. A
mixture of 100 grams of benzene (b.p. 80-2°)
and 100 grams of toluene (b.p. 110-6°) was
slowly distilled, and the distillate was collected
in a convenient number of fractions, the receivers
being changed when the boiling-point reached
certain demiite previously arranged tempera-
tures, the actual readings being corrected for
barometric pressure and for errors in the
thermometer scale. The results of the first
four fractionations, and also of the ninth, are
given in Table I., next page.

In the first distillation the temperature rose
almost at once to 86° (oorr.), and the first
pdrtion of distillate was therefore coUeoted in
receiver 4. On the other hand, the temperature
reached 110-6° before the whole of the liquid
had come over ; the residue, therefore, con-
sisted of pure toluene.

The distillate in receiver 4 was now placed
in a smaller flask and distilled, fractions being
ooUeoted in receivers 2, 3, and 4. When the
temperature reached 89-2°, the flame was
removed, and the contents of receiver 5 were
added to the residue in the still. The distilla-
tion was recommenced, and fractions were
collected in receivers 3, 4, and 5, the flame being
removed when the temperature reached 92-2°.
The distillation was continued in » similar
manner until, after addition of the contents of
receiver 12, the temperature rose to 110-6°,
when the residue of pure toluene was added to
that from the first fractionation. The third
and fourth fractionations were carried out like



DISTILLATION.

Table I.



289



Number of
receiver


Final
temperature
(corrected) =(


Weight o( fraction = AW


t


AW


I.


n.


m.


IV.


IX.


Pure benzene


o










8S-2


10-20 i


1


81-2








12-95


31-55


80-3


45-00 '


2


83-2





3-80


24-80


23-90


80-6


17-55


3


86-2





33-85


22-75


16-20


81-4


7-20


4


89-2


9-75


22-30


13-50


9-65


83-5


4-50


5


92-2


51-80


19-65


11-80


8-00


88-0


3-30 ,


6


95-4


28-8S


13-60


9-15


5-80


95-4


2-75


7


98-6


21-20


12-95


7-30


6-35


102-8


2-60


8


101-6


12-80


9-05


6-75


4-65


107-3


2-85


9


104-6


11-45


8-90


6-30


3-85


109-4


3-16


10


107-6


14-15


10-80


7-95


5-86


110-2


3-46 '


11


109-6


13-45


9-60


8-95


7-40


110-5


7-00


12


110-6


24-90


30-75


33-05


30-50


110-6


5-75


Pure toluene


(110-6)


10-90


22-95


31-35


42-10


(110-6)


72-10



the second. In the subsequent fractionations
the temperature ranges of the middle fractions
were gradually increased and those of the lowest
and highest fractions diminished. It was not
until the ninth fractionation that pure benzene
began to be collected. Eventually 81-4 grams
of pure benzene and 88-8 grams of pure toluene
were recovered.

FuU details of the systematic fractional
distillation of mixtures of two liquids and three
liquids, and of more complex mixtures, are given
in 'Fractional Distillation,' 114-143.

If the components of a mixture are not
closely related, the composition of the vapour
cannot, as a rule, be calculated by Brown's
formula, but even in such cases the percentage
of the more volatile component in the vapour
will be greater than in the liquid, and the
separation by fractional distillation is theoreti-
cally possible, unless the deviation of the
vapour pressure-composition curve from straight-
ness is so great that a mixture of maximum or
minimum vapour pressure is formed.

For mixtures to which Brown's law is
applicable, it may be stated quite generally
that the greiater the difference between the
boiling-points of the compounds, the more
readily can a separation he effected by fractional
distillation, but for other mixtures the form of
the curve representing the relation between
boiling-point and molecular composition, if
known, must be taken into account. If the




O lOO

Molecular percentage ofA

Fio. 9.

curve for two components were of the form a
(Fig. 9), the substance of higher boiling-point



could be separated very easily, but the more
volatUe component only with great difficulty, if
at all. On the other hand, if the curve were
of the form 5, nearly horizontal at the higher
temperature, the more volatile component
would be comparatively easy, and the less
volatile difficult to separate. The first case is
frequently met with, the second seldom. With
curves of the form c and d (Fig. 10) it would not.



v

c



u
a.

n

<

a.
Ill

UJ



o q r loo

Molecular percentage of A
Fig. 10.

under any conditions, be possible to separate
both components by distillation of any given
mixture, but it would be possible to separate
the mixrture of. minimum or maximum boiling-
point from that component which was in excess.
In a case represented by curve c, where the
minimum temperature corresponds to the
molecular composition q, the distillation might
proceed in either of three ways : (1) If the liquid
mixture had the composition q, it would distil
unchanged in composition at the constant
minimum temperature ; (2) if the molecular
percentage of A were less than q, the liquid
would tend to separate into two components,
the more volatile of which would be the mixture



270



BISTILLATION.



of minimum boiling-point, and the less volatile
the substance B ; (3) if the molecular percentage
of A were greater than q, the components
separable by distillation would be, first, the
mixture of minimum boUing-point, and, second,
the! liquid A. A well-known case of this kind
is that of normal propyl alcohol and water ;
the boiling-points under normal pressure are :
water, 100° ; w-propyl alcohol, 97 •2° ; mixture
of minimum b.p. 87 •7°. This mixture contains
43-2 molecules p.o. of the alcohol ; it was
described for many years as a hydrate of
propyl alcohol. If the curve had the form d, the
mixture would distil like a pure liquid if it had
the composition r, or it would tend to separate
into two components, the first being either
A or B, according as the moleotilar percentage
of A in the original mixture was greater or less




O 20 40 60 80 lOO

Molecular percentage of Water

Fig. 11.

than r, and the second the mixture of maximum
boiling-point. Formic acid and water behave
in this manner, as shown by Roscoe (Chem. Soc.
Trans. 13, 146; 15, 270). Boiling-points: water,
100° ; formic acid, 99-9° ; mixture of maximum
b.p., 107-1°; molecular percentage of acid
in mixture, 56-7°.

If, as with ethyl alcohol and water (Fig. 11),
the mixture of constant boUing-point distils at
nearly the same temperature as one of the two
pure liquids, it is impossible to separate that
liquid in a pure state by distUlation. In this
case, as the curve is exceedingly flat at the lower
temperature (Noyes and War-
fel, J. Amer. Chem. Soc. 23,
463), it is practically impossible
to separate the mixture of
minimum boUing-point (con-
taining 4-43 p.c. by weight of
water) in a piire state even
when water is in excess, although
it is quite easy to separate pure
water from dilute spirit.

The correct interpretation of
the results of the fractional dis-
tillation of a complex mixture
may be rendered very difficult
by the following causes : (1) the
presence of two components
boiling at nearly the same
temperature ; (2) the presence
of one or more components in
relatively very small quantity ;
(3) the formation of mixtures of
constant boUing-point. It is
only by plotting the total
weights of distillate against
the temperatures, or by dividing the weight
of each fraction. Aw, by its temperature range
and tabulating the ratios tw/Ut, that the



results are likely to be understood, and in any
case, the existence of a mixture of constant
boiling-point may not be discovered, and it may
be difficult to decide whether a distillate
collected at a practically constant temperature
consists of a single pure substance or of two
liquids boiling at nearly the same temperature. ,
AH the above difficulties are met with in the
fractional distillation of petroleum, which
contains homologous and isomeric paraffins,
polymethylenes and their alkyl derivatives
(naphthenes) and aromatic hydrocarbons, Rus-
sian petroleum being relatively rich, and Ameri-
can petroleum poor in naphthenes and aromatic
hydrocarbons. When American petroleum was
carefully distilled, it appeared at first as though
the distillate coming over between the ordinary



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