Purity and purifications of solids using melting points.
CHE 337, Section 001
Department of Chemistry
Portland State University, Portland, OR
Melting points of Naphthalene/Biphenyl mixtures differing in their percent compositions were observed and plotted on a graph that indicated eutectic point of the mixture to be at 50 mole percent Naphthalene. Using melting point technique to identify mixture Unknown H was determined to be 3-ethoxy-4-hydroxybenzaldehyde. Introduction
Melting point is a technique used by chemists to identify unknown substance. Compounds depending on their chemical structure have a specific melting point. Pure substance has a sharp melting point with the maximum range of 5 C. Mixtures on the other hand have wider range. Based on these concepts unknown mixtures/substances can be determined by combining them with the substance of the known melting point. Experimental Section
Materials and Instruments:
- Melt Temp device used to electrically heat up substance to a known temperature. - Glass capillary to hold substance in a Melt Temp while heating. - Naphthalene
- Unknown H
Naphthalene and Biphenyl melting point determination.
Using procedure for melting point determination with Mel-Temp on p.43-45 in FFF melting points of pure Naphthalene, pure Biphenyl and mixtures of the two were observed and recorded. Mixtures of Naphthalene and Biphenyl according to their mass percent used in experiment: 10% Naphthalene, 90% Biphenyl
30% Naphthalene, 40% Biphenyl
50% Naphthalene, 50% Biphenyl
70% Naphthalene, 30% Biphenyl
90% Naphthalene, 10% Biphenyl
Values of melting points obtained in the observation were recorded in Table 1 and plotted into Graph 1. Identification of an Unknown using Mixture Melting Points.
MP of unknown H was determined using procedure indicated on p. 43-45 in FFF. Two known samples with the closest to unknown H mps were determined. Two mixtures were prepared, each combining unknown with the known sample in 50/50 proportion. MP of each mixture was observed and recorded. Mixture that had sharp melting point was mixture that contained identical parts. Results and discussion
Melting points for mixtures containing different percent of Naphthalene to Biphenyl were observed, recorded, averaged and graphed in Table 1 and Graph 1. Average was taken from several results obtained by organic chemistry lab.
Table 1. Average melting point specific to % Naphthalene in Naphthalene-Biphenyl mixture
% Naphthalene inNaphthalene-Biphenyl mixture (%)
| Corresponding Melting point (°C )
Mole Percent of Naphthalene
Graph 1. Melting Point Diagram for Naphthalene and Biphenyl.
Observed melting points of pure Naphthalene and Biphenyl (in Table 1) are consistent with CRC Handbook1, that indicates melting temperatures for these substances at 80.2°C and 71.0 °C, respectively. Based on the graph above eutectic point lies at 50% mole percent Naphthalene. Class results were averaged, therefore one inaccurate result would effect the average point, this could be a potential source of error in the experiment. However, class average for MP of pure substances came close to values in CRC Handbook.
Another Source of error is limited data points that were observed. Best fitted line illustrated in Table 1 can contain a source of error due to the insufficient number of data points.
Using Melting point technique unknown H, was determined to have MP in the range from 73.3 to 75.4.Based on its melting point it was mixed with 3-Ethoxy-4-hydroxybenzaldehyde and pure Biphenyl. Results are reported in Table 2.
| Melting Point ( °C )
| 76.1 - 78.7
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