# Spectrophotmetric Analysis

Topics: Phase, Thermodynamics, Melting point Pages: 7 (2025 words) Published: December 13, 2012
Exercise 1
Binary Solid-Liquid Diagram

CHEM 112.1 – 2L
Group 2

I. Data and Results
Table 1.1 Break and arrest temperature of diphenylamine-naphthalene mixture. Table 1.1. Mole fraction of naphthalene, χB, and corresponding arrest and break temperature at each run. Run number| Mole Fraction,XB| Ave. break temperature, ˚C| Ave. arrest temperature, ˚C| 1| 1| 72| 35.4375|

2| 0.868388| 72| 35.4375|
3| 0.725218| 62| 35.4375|
4| 0.568895| 49| 35.4375|
5| 0.397522| 70| 35.4375|
6| 0| 45| 35.4375|
7| 0.208813| 48.5| 35.4375|
8| 0.305919| 46| 35.4375|

I. Discussion
Whenever a mixture is cooled or heated, it undergoes changes in composition. This change is depicted by a phase diagram. It determines whether two or three substances are mutually miscible, whether the equilibrium can exist over a wide range of conditions, or whether the system must be brought to definite temperature, pressure, and composition before equilibrium is established. Interpretation of the phase diagram is usually done with the use of Gibb’s phase rule. It describes the relationship among the number of degrees of freedom (F), the number of components (C), and the number of phases (P) at equilibrium:

F = C – P + 2eq. 1
The degrees of freedom refer to the number of intensive variables such as temperature, pressure and composition that can be changed without disturbing the number of phases in equilibrium. It cannot have a negative value. All intensive variables are fixed whenever F is equal to zero (invariant). When F=1 (univariant), one of the variables can be varied. When F=2 (bivariant), two of the variables can be varied. Experimentally, phase diagrams are often determined at a fixed atmospheric pressure since it is one of the easiest variable to make constant, thus reduces equation 1 to:

F = C – P + 1eq. 2
In the experiment, a binary system was considered. Given that the number of components is two, the above equation is further reduced to:

F= 3– P(1 – 3)
In a binary solid-liquid system, when a single liquid melt formed from two immiscible solids is cooled sufficiently, a solid is formed. The temperature at which the solid is first formed is the freezing point of the solution that is dependent on the composition. This exercise demonstrates the cooling curves of diphenylamine and naphthalene mixture of varying compositions. From the cooling curves, phase diagram of naphthalene-diphenylamine mixture, which is a binary solid system, will be constructed. Thermal analysis, which is technique used for phase determination, continuously monitor physical and chemical changes of a sample as the temperature of the sample is increased or decreased. Initially, the required amount of solid naphthalene was transferred to the flask. This was heated in a water bath until the temperature reached 90˚C to ensure that all naphthalene present, having melting point of 80.2 has melted. The following runs were prepared by adding the required amount of the component to the previous mixture in order to minimize the quantities of reagents used. The mixtures were also heated up to 90˚C for runs 1-5. The temperature allows the components in the mixture to be dissolved since their boiling points are quite low relatively. For runs 6-8, the mixture was heated up to 62˚C. This is because diphenylamineamine is the dominating component in the mixture. Diphenylamine has melting point of 52˚C. The tube was then wiped and inserted in the Dewer flask fully packed with crushed ice. A Dewer flask is a vessel designed to provide good thermal insulation since the vessel does not allow the heat to easily escape.

Fig. 1.2 Illustration of a Dewer flask.
In the experiment, a series of mixtures of known composition was prepared and heated above their melting point. The rate of cooling for each sample was determined and then the phase diagram was constructed. In the cooling curve of pure substances, the...