AbstractExperimentally obtained melting points acquired by the class were used to construct a melting point diagram. These values indicated an estimated eutectic point of 40% naphthalene at 30°C. Unknown D was determined to be succinic acid through the use of a mixed melting point technique.
IntroductionWhen conducting experiments in a laboratory setting it is important to know if the substances are fairly pure. A relatively easy way to determine the purity of a substance is by determining its melting point. Pure substances exhibit a sharp melting point range, with a 1-2 °C spread. However, impure substances exhibit melting point ranges that are much broader starting from 3°C onward. Comparing a known substances melting point to experimentally determined melting point ranges found in literature is often satisfactory in purity determination. Unknown substances, however, must be subjected to further testing. Using the mixed melting point technique, an unknown substance is mixed in equal proportion to known substances with a close melting point.
The compound having the same melting point as originally discovered identifies a pure substance. An impure substance is one that upon mixture exhibits a broadened and depressed melting point than originally discovered through experimentation. The use of melting points is a useful tool in both purity examination and unknown determination.
Experimental SectionThe experimental procedures can all be found in the PSU Organic Chemistry Fall 2008 laboratory manual. 1 The chemicals used in the experiment included 100% Naphthalene, C10H8, and 100% Biphenyl, C12H10. Various mixtures of the two substances were also used. A small sample of 100% Naphthalene was crushed into a fine powder and loaded into a capillary tube. This tube was first placed into a Mel-Temp apparatus where it was progressively heated. The melting point range was determined and another sample was tested using the oil tube technique.
Various mixtures of the substances were then subjected to a melting point test. Percentages of these mixtures included- 90%N/10%B, 70%N/30%B, 50%N/50%B, 30%N/70%B, and 10%N/90%B. The melting point range was determined for each of these compounds. However, these compounds were assigned individually so the melting point ranges were determined by students, then reported and organized into a table.
These temperature values were then averaged according to their range and plotted against mole percent of Naphthalene. Mole percent of Naphthalene was calculated through the following equation-Mole % Naphthalene(N)=_____Weight%N x (1mol/128 g)________(weight%N x(1mol/128 g)+(weight%Biphenyl x(1mole/158g)The second section of this experiment tested the melting point an unknown compound. A small sample of unknown D was taken and crushed into a fine powder and subsequently loaded into a capillary tube. The melting point range was determined using a Mel-Temp apparatus. This melting point range was compared to selected unknowns, with two being selected for further testing of the unknown. These two substances with closely related melting point ranges were then crushed into a fine powder. Equal amounts of unknown D were added to each mixture. When thoroughly mixed, the compounds were loaded into individual capillary tubes and tested using the Mel-Temp device.
Results and DiscussionUsing the data derived from the class, a melting point diagram was constructed (Graph 1). The classroom data was not large or consistent enough to determine with enough confidence each compounds true melting point. However, it can be determined that the eutectic point lies between 30-70% Naphthalene.
Graph 1With the given classroom melting range data, a eutectic point around 40% Naphthalene at 30 °C seems reasonable. On average, the classroom data was fairly consistent. There were, however, some melting point ranges that exceeded 20 °C. This margin could have resulted from the substance being heated too fast. Another source of differentiation could be from the two methods used to determine the melting point. The most consistent data centered around 100% Naphthalene. The classroom range was between 78-83 °C, with a reported range of 80.2°C.2Unknown D was determined to have a melting point range of 182-186 °C. The two pure substances with the closest melting points to that of unknown D were Sucrose (mp 185°C) and Succinic Acid (mp 188°C). The mixture of the unknown and sucrose melted at approximately 149°C. The mixture of the unknown and succinic acid melted at approximately 159°C. Although both pure compounds combined with the unknown produced a depressed melting point, the higher melting point with succinic acid is indicative of unknown D being that substance. An improperly mixed substance could have induced this large error.
ConclusionThe determination of melting points often aids in purity analysis and in the identification of substances. The narrow range of 100% naphthalene and the broadened and depressed range of the compounds mixed with biphenyl demonstrate the behavior of melting points. The experiment concluded that the eutectic point of naphthalene and biphenyl was around 40% naphthalene at 30°C. The experiment also used melting points to determine the identity of unknown D to be succinic acid.
References1PSU Organic Chemistry 337 Lab Manual, Fall 20082CRC Handbook, CRC Press: New York, 1999Fessenden; Fessenden; and Feist, Organic Laboratory Techniques, 3rd Ed, Brooks Cole:California, 2000