Solid-Liquid Phase Diagram of Naphthalene-Diphenylamine Two-Component System...
Solid-Liquid Phase Diagram of Naphthalene-Diphenylamine Two-Component System Michael Go, Nate Manaloto, Ronald Reyes* *Faculty, Ateneo de Manila University; email:
[email protected]
Abstract. The experiment aims to be able to construct a phase diagram of Naphthalene-Diphenylamine two-component system at constant atmospheric pressure for thermal analysis. From the phase diagram, the eutectic composition (XE) and eutectic temperature (TE) are determined and are compared with the literature value. Commercial Naphthalene is used to undergo this test, and see if it is of the same caliber as the standard Naphthalene. Keywords. Naphthalene-Biphenylamine. Eutectic point. Newton’s Law of Cooling. Thermal Arrest. Thermal Break.
Introduction. In investigating the heterogeneous
Figure 1. A Phase Diagram for a two component system in
equilibrium between solid and liquid phases of a two-
which the solids are partially miscible and the liquids are
component system, a phase diagram is constructed. In
complete miscible.
constructing phase diagrams, cooling curves forms
The binary solid-liquid diagram in Figure 1 shows the
the basis for “thermal analysis”. From the phase
stability of different phases as a function of
diagram, the eutectic composition (XE) and eutectic
temperature and composition. This example (Figure
temperature (TE) are determined.
1) shows a case where the solid components are partially miscible, α+β. α(s) represents a solid state mixture predominantly composed of substance A, with B present as an impurity, and β(s) represents the opposite case where A is an impurity. When a substance is dissolved in a liquid and the freezing point of the liquid is lowered, this is called freezing pint depression, a colligative property that depends on the number of solute particles present in the solvent. The shape of the phase boundaries between the (α + liquid region) + (β + liquid region), the liquidus curves, describes the freezing point depression for this mixture. The equation of the liquidus curves can be derived from the Clausius-Claperyon equation
under the assumption that the solution behaves
Naphthalene followed by successive additions of
ideally:
Diphenylamine
to
the
previous
run;
and
a
Diphenylamine-rich series similarly prepared with the
T(XA) = Tf.A. + ln(XA)RTFA2/dHA = TA-((1-XA) + (1-XA)2/2 + …) RTf.A2/dHA
Naphthalene-rich series*. Heat the mixture in water Tf.A is the freezing point of compound A, and is also shown in Figure 2. dHA is the heat of fusion for compound A and XA is the mole fraction of compound A. An analogous equation can be written for compound B. The two liquidus curves intersect at the eutectic point, C.
bath until completely melted, and then, remove the water bath and measure the temperature periodically (e.g. every 15 seconds for the first 5 minutes, 30 seconds for the next five minutes, and every minute for the latter parts until the eutectic temperature is reached) until the system is essentially solid.
In the absence of a phase change, the rate of change in temperature follows Newton’s Law of cooling. The Newton’s Law of cooling predicts that there is an
Table 1. Approximate range of composition for the two-component Naphthalene-Diphenylamine mixture Approximate Range of Composition
exponential approach to the ambient temperature. A
Pure A -
Pure B -
solid is formed because the rate of cooling is changed
Naphthalene: 10 g.
Diphenylamine: 10 g.
as part of the heat exchanged with the surroundings
Run
B (g)
Wt. % A
Run
A (g)
Wt. % B
that contributes to the phase transition. During the
1A
0.0
100
1B
0.0
100
2A
1.5
87
2B
1.5
87
3A
2.0
74
3B
2.0
74
4A
2.5
63
4B
2.5
63
5A
3.0
51
5B
3.0
51
freezing point of a pure substance, when the temperature remains constant, this is called thermal arrest. In a two-component system, as the temperature is lowered, one component begins to freeze while the other component still remains in the liquid state. In this freezing process, the liquid’s concentration mixture changes as more and more solid forms, and
Results.
this consequently changes the freezing point. For this reason, the rate of cooling is not constant, but is different from the rate of cooling of the original liquid. This change in the rate of cooling is known as thermal break. When the liquid reaches a certain ratio of the two components, a thermal arrest is observed. This temperature and concentration point is also known as the eutectic point. Experimental.
The
binary
Figure 2. The constructed phase diagram with the freezing points plotted, temperature (y-axis) and mole fraction of Naphthalene (x-axis). The best fitted curve is used and the eutectic point and temperature were determined. Eutectic composition (XE) = 0.76381045 Eutectic temperature (TE) = 21.8121115
system
will
be
Discussion: In the constructed phase diagram, a few
naphthalene-diphenylamine. Make two series of runs:
changes were made. The best fitted curve was used in
a Naphthalene-rich series beginning with pure
order to get the eutectic point. The experimental value
of the eutectic point is 0.764 mole fraction
We would like to acknowledge the Chemistry
composition of Naphthalene and 21.81⁰C for the
Department of the Ateneo de Manila University for
temperature. Comparing it with the values taken by
supporting our project and providing us the
other studies, it was found that the eutectic point is
equipments necessary for the completion of the
around 0.36-0.38 mole fraction of naphthalene, and
project.
the temperature roughly 31⁰C. The high deviation of the results from the previous one can be seen in the
References.
experimental procedure, and the result itself. As can
University of Colorado. “Binary Solid-Liquid Phase
be seen in the results, the points were not successive
Diagram.”
and not complete. As such, there was a huge break
fraction of Naphthalene interval. As such, the eutectic point can only be roughly estimated by extending the best fitted curves. Having more points can lead to a more visualize-able and more accurate results. The recommendation for the experiment is that it can add more solutes for each runs that can extend the points further nearer the eutectic point, or if not change the five runs where the solute are added 1.5 g. each run instead of adding in a 0.5g increment. The major source of error in the experiment is temperature reading. Since the temperature is read not automatically by a machine, but manually by the experimenter. Furthermore, when the system is not stirred, the whole solution is not in equilibrium which leads to a deviation in temperature reading that eventually leads to supercooling of the system. Although this will eventually be eliminated, however, this could have been prevented if the system was stirred consistently. The recommendation is that an automatic stirrer can be used rather than a manual one. Acknowledgement.
Williams Diagram.”
Accessed
College.
on
“Binary
Accessed
on
October
6,
Solid-Liquid October
6,
2010.
Phase 2010.
Figure 1. A Phase Diagram for a two component system in which the solids are partially miscible and the liquids are complete miscible.
Figure 2. The constructed phase diagram with the freezing points plotted, temperature (y-axis) and mole fraction of Naphthalene (x-axis). The best fitted curve is used and the eutectic point and temperature were determined.
Figure 3. A run of Diphenylamine with 3 grams of Naphthalene as impurity, the graph illustrates the thermal break and the thermal arrest.
