Melting point of a binary system
with Cobra4
TEC
Related topics
Melt, melting point, melting point diagram, binary system, miscibility gap, mixed crystal, eutectic mixture,
Gibbs‘ phase law.
Principle
In plotting the cooling curves of binary mixtures one determines the temperatures of melting and solidification of specimens with differing fractions (molar fractions) of the two components. These results are
entered in a temperature versus concentration diagram.
Material
1 Cobra4 USB-Link
1 Cobra4 Sensor-Unit 2 × Temperature, NiCr-Ni
1 Thermocouple NiCr-Ni, sheathed
1 Retort stand, h = 750 mm
1 Right angle clamp
1 Universal clamp
(2)Test tube, 18 × 188 mm
1 Test tube rack, 12 holes, d = 22 mm, wood
1 Powder funnel, d = 65 mm
1 Microspoon
Set of Precision Balance Sartorius CPA 623S and
1
measure software, 230 V
2 Mortar with pestle, 70 ml
1 Weighing dishes, 80 × 50 × 14 mm
1 Teclu burner, natural gas
1 Safety gas tubing
2 Hose clips, d = 12...20 mm
1 Lighter for natural/liquified gas
1 Naphthalene, white, 250 g
1 Biphenyl, 100 g
1 Standard petrol, b.p. 65-95 °C, 1000 ml
1 Software Cobra4 - multi-user licence
12610-00
12641-00
13615-01
37694-00
37697-00
37715-00
37658-10
37686-00
34472-00
33393-00
49224-88
32603-00
45019-05
32171-05
39281-10
40995-00
38874-00
48299-25
31113-10
31311-70
14550-61
Additionally required
1 PC with USB-Interface, Windows XP or higher
Fig. 1:
Experimental setup.
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P3031361
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1
TEC
M
Melting
poiint of a bin
nary syste
em
w
with
Cobra
a4
Safetty instructions
When
n handling chemicals, you should wear suitable prote
ective glove
es, safety g
goggles, an
nd suitable
e
clothing. Please refer to the
e appendix for
f detailed safety instrructions.
Tasks
1.. Record th
he melting point
p
diagra
am of a mixtture of biphenyl and na
aphthalene..
2.. Determin
ne the composition of the eutectiic mixture and
a
its melting point ffrom the melting pointt
diagram.
up and proc
cedure
Set-u
Grind
d sufficient quantities
q
o biphenyl and
of
a naphthalene for th
he 11 differe
ent test mixxtures listed
d in Table 1
separrately to powder using a mortar and
a pestle. Make up th
he test mixtures, weigh
hing the sub
bstances to
o
an acccuracy of 1 mg, and pour them in
nto separate
e, appropria
ately marked
d test tubess.
Mola
ar fraction of
o naphthalene
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Mass of na
aphthalene in
i
g
0
0.3
338
0.6
688
1.0
051
1.4
426
1.8
816
2.2
220
2.6
639
3.0
075
3.5
528
4
Molar fraction
f
of bipheb
nyl
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
M
Mass of biph
henyl in g
4
3.66
62
3.312
2.94
49
2.57
74
2.18
84
1.78
80
1.36
61
0.92
25
0.47
72
0
Table 1: Molar fractions and weights of binary mixtures.
m
Comb
bine the Co
obra4 Senssor-Unit 2 × Temperatture with th
he Cobra4 USB-Link and
a connecct the tempe
erature prob
be to tempe
erature
S
t. Start the PC and con
nnect the Cobra4
C
input T1 of the Sensor-Unit
USB--Link with a USB sockket of the co
omputer. Call up the “measure“ program
p
in Windows. The
T sensorr is automattically recog
gnized
when
n the USB-L
Link is conn
nected to th
he compute
er. An ID number
(01) is allocated to the senssor, which iss indicated in the displlays of
the Cobra4
C
USB
B-Link.
Load the experiiment “Meltting point of
o a binary system” (E
Experiment > Open exxperiment). The measurement pa
arameters for
f this
experriment are loaded now.
Now double-clicck on the temperature
t
e channel to calibrate
e your
senso
or by enterring a temp
perature va
alue measu
ured with a thermome
eter under “Calibrate” (Fig.2). Affter having made thes
se settings,, press “Ap
pply” and th
hen “OK” to
o reach the
e field for the
t
recordin
ng of meassured value
es. Arrange
e the displa
ays as you
u want
them. Clamp a test
t
tube co
ontaining on
ne of the te
est mixtures
s at an
angle
e above th
he burner and carefu
ully heat it until the
e solid Fig. 2: Calibrationn.
2
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ghts reserved
P3031361
M
Melting
po
oint of a binary systtem
with Cobra4
T
TEC
substance melts and the
t resulting liquid is clear.
c
Do no
ot heat the liiquid up
to the boiling point! In
nsert the NiCr-Ni therm
mocouple in
n the melt and start
the measu
urement ( ). You can follow the course
c
of th
he cooling curve
c
by
observing the monito
or display. When the mixture ha
as nearly reached
r
room temp
perature, sttop the me
easurement ( ) and send
s
the re
esults to
measure (Fig. 3). Folllowing this, briefly rehe
eat the mix
xture to rem
move the
thermocou
uple. Clean the thermo
ocouple tho
oroughly wiith petrol. Use
U
the Fig. 3: Saving measurements
m
s.
same proccedure to record the cooling
c
currves for eac
ch of the other
o
10
mixtures. Fig. 4 show
ws the gra
aph as it iss presented
d by the programme
p
when the measurement is
stopped.
Fig. 4:
Cooling curve of a mixture of naphthalene and biphenyl.
Theory an
nd evaluatio
on
Mixtures of two substa
ances have
e lower meltting points than
t
those of
o the individual substa
ances. This effect
is greater, the more of
o the one substance th
hat is mixed
d in the othe
er substancce. When graphs are plotted
p
of the meltting points of
o various mixtures
m
aga
ainst the co
ontent of the
e one substtance in the
e other, desscending curves are obtained. The two curves in
ntersect at a lowest po
oint, the euttectic point. The quanttitative
o substance
es at this po
oint is the eutectic
e
mixxture. The m
melting poin
nt diagram shows
s
proportionss of the two
the depend
dence of th
he melting point on the composittion of the system
s
at cconstant prressure. Bip
phenyl
and naphthalene can be mixed in any ratio
o in their liq
quid phasess. Since the
ese substan
nces do not form
liquid crysttals, a sepa
aration into two
t
phasess occurs when the meltt cools down
n. Pure nap
phthalene crystallizes out firrst because
e it has the higher meltting point. As
A this occu
urs, the rela
ative concentration of bipheb
nyl in the remaining
r
m increasses and the melting poiint of the melt also deccreases. Fin
melt
nally, the re
esidual
melt reach
hes the com
mposition of the eutectic mixture and
a freezes as a fine m
mix of both types of cryystals.
To plot the melting point
p
diagra
am, the cooling curve
es of melts of severall different composition
c
ns are
measured.. Each of these curves (Fig. 4) hass a first critiical point. This
T
is the point at whic
ch the comp
ponent
with the highest melting point sta
arts to crystallize out. The curve remains ho
orizontal herre for some
e time,
as melting enthalpy iss released. You can exxactly determ
mine this te
emperature using the <survey>
<
function
in the uppe
er menu ba
ar. Subsequ
uently, a seccond critica
al point indiccates that th
he eutectic temperaturre has
been reach
hed. The co
ooling curve
e of the two
o pure comp
ponents and
d the eutectic mixture only has a single
critical poin
nt. This is in
ndicated byy a drop and
d then a rise
e in tempera
ature. Only the first critical points of the
cooling currves are of importance
e. The corre
esponding te
emperature
es are plotte
ed against the
t molar frraction
of one of the
t compon
nents of the
e mixtures (Fig.
(
5). The molar fraction is the
e ratio of the
e quantity of
o one
component to the tota
al amount in
i the syste
em (see als
so Table 1). On extrap
polating the
e two parts of the
curve in the melting point
p
diagram
m downwarrds, the com
mposition an
nd the meltiing point of the eutectic mixture can be
e read off at the point of
o intersection E.
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P3031361
PHY
YWE Systeme Gm
mbH & Co. KG © All
A rights reserved
3
TEC
M
Melting
poiint of a bin
nary syste
em
w
with
Cobra
a4
The melting
m
poin
nt curves divvide the dia
agram into fo
our areas:
Area I: Unsatura
ated solution
n of the two
o substances B and N
Area II: Supersaturated solu
ution of N in
nB
Area III: Supersa
aturated sollution of B in N
Area IV: Heterog
geneous mixxture of N, B and the eutectic.
e
When
n only the melt
m and the gas phasse are pressent, then th
he number of degreess of freedom
m is 2. The
e
tempe
erature and
d concentration can be
e varied with
hin wide lim
mits (area I in Fig. 5). O
On starting at
a a point in
n
area I and lowe
ering the te
emperature, deposition
n begins on
o reaching
g the meltin
ng point cu
urve. Three
e
phase
es are now present and the numb
ber of degre
ees of freedom is only 1; the melting point of the system
m
is dep
pendent on the concen
ntration of the
t melt. Po
oint E in the
e melting po
oint diagram
m is a quadruple point,,
becau
use it markss the intersection of fo
our phases (two solid, the
t liquid melt
m and the
e gas phase
e). The sys-tem iss non-varia
ant because
e, having tw
wo compone
ents and fou
ur phases, there are n
no degrees of freedom
m
remaining. The eutectic
e
mixxture therefo
ore behavess as though
h it were a single
s
comp
ponent syste
em.
Fig. 5:
Me
elting point dia
agram of different mixtures of naphthalen
ne and
bip
phenyl.
Data and resultts
Naph
hthalene and
d biphenyl form
f
a eutectic mixture
e which has the followin
ng composition:
0
0.44;
0.56
e
tem
mperature iss TE = 300.8
8 K.
The eutectic
4
PHYWE Systeme GmbH & Co. KG © All rig
ghts reserved
P3031361
Melting point of a binary system
with Cobra4
TEC
Appendix
Hazard symbol, signal word
Hazard statements
Precautionary statements
H225: Highly flammable
liquid and vapour.
H315: Causes skin irritation.
H304: May be fatal if swallowed and enters airways.
H336: May cause drowsiness or dizziness.
H411: Toxic to aquatic life
with long lasting effects.
P210: Keep away from
heat/sparks/open
flames/hot surfaces. –
No smoking.
P273: Avoid release to
the environment.
P302 + P352: IF ON
SKIN: Wash with plenty
of soap and water.
P301 + P310:: IF SWALLOWED: Immediately
call a POISON CENTER
or doctor/physician.
P331: Do NOT induce
vomiting.
P501: Dispose of contents/container to an approved waste disposal
plant.
FAM – Normalbenzine 65 – 95 °C
DANGER
Biphenyl
Warning
H319: Causes serious eye
irritation.
H315: Causes skin irritation.
H335: May cause respiratory irritation.
H410: Very toxic to aquatic life with long lasting effects.
Naphthalene
H302: Harmful if swallowed.
H350: May cause cancer.
H410: Very toxic to aquatic life with long lasting effects.
P273: Avoid release to
the environment.
P281: Use personal protective equipment as required.
P308: IF exposed or
concerned:
P313: Get medical advice/attention.
P501: Dispose of con-
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P3031361
PHYWE Systeme GmbH & Co. KG © All rights reserved
5
M
Melting
poiint of a bin
nary syste
em
w
with
Cobra
a4
TEC
tents/container to an appro
oved waste disposal
plant.
W
Warning
6
PHYWE Systeme GmbH & Co. KG © All rig
ghts reserved
P3031361