The Purity and Purification of Solids- Melting Points of Napthalene and Biphenyl

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 substance’s 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...