Unknowns melting-point determination

Determine the melting point of pure cinnamic acid (133°) and pure urea (133°). Intimately mix approximately equal weights (ca. 01 g.) of the two finely-powdered compounds and determine the melting point a considerable depression of melting point will be observed. Obtain an unknown substance from the demonstrator and, by means of a mixed melting point determination, discover whether it is identical with urea or cinnamic acid. 

Infrared, NMR, and mass spectra are usually reliable as fingerprints. Mixed melting point determinations with solid unknowns or with solid derivatives of liquid unknowns rarely give misleading results. Identity should... 

If the melting point of the sample is unknown, you can often save time by preparing two samples for melting-point determination. With one sample, you can rapidly determine a crude melting-point value. Then repeat the experiment more carefully using the second sample. For the second determination, you already have an approximate idea of what the melting-point temperature should be, and a proper rate of heating can be chosen. 

Determination of melting points (a-naphthylamine, a-naphthol, benzoic acid, succinic acid and p-nitrobenzoic acid). Use the apparatus shown in Fig. II, 10, 2, a. Construction of calibration curve for thermometer. Determination of m.p. of unknown compound. 

Compare melting point graphs to determine the molecular mass of the unknown substance. 

Observing and Inferring Using the graphs, determine the melting point for each of the following pure BHT BHT + PDB BHT + unknown. 

The nuclear chemists at the Lawrence Berkeley Laboratory worked with extremely small samples of lawrencium with short half-lives, which made it difficult to determine the new elements chemical and physical properties. Most of its isotopes spontaneously fission as they give off alpha particles (helium nuclei). Lawrencium s melting point is about 1,627°C, but its boiling point and density are unknown. 

Dubnium s (Unp) most stable isotope, Db-268, is unstable with a half-life of 16 houts. It can change into lawtencium-254 by alpha decay ot into tuthetfotdium-268 by electton cap-tute. Both of these teactions occut thtough a series of decay processes and spontaneous fission (SF). Since so few atoms of unnilpentium (dubnium) are produced, and they have such a short half-life, its melting point, boiling point, and density cannot be determined. In addition, its valence and oxidation state are also unknown. 

Once the unknown is determined to be a ketone or an aldehyde. the melting point of the imine derivative is determined. The melting point and other physical characteristics (i.e.. solubility of unknown and boiling point or melting point of the unknown) are used to determine the unknown s identity. The information is compared to tables in books which contain the melting points of derivatives and other physical data for organic compounds. 

In any attempt to determine the structure of an unknown biological compound, researchers must deal with two fundamental problems (1) If you don t know what it is, how do you know if it is pure (2) If you don t know what it is, how do you know that your extraction and purification conditions have not changed its structure Morgan addressed problem 1 through several methods. One method is described in his paper as observing constant analytical values after fractional solubility tests (p. 312). In this case, analytical values are measurements of chemical composition, melting point, and so forth. 

Using this set of equations, it is determined that for P phases there are PN + P equations with PN + P unknowns (Wesdorp et al. 2005). This set of equations may be solved to determine the number of phases, the composition of each phase and the melting temperature. To solve this series of equations, several experimental values are required. The melting point, heats of fusion for the pure compounds, the activity coefficients for the liquid phase, the activity coefficient for the solid phase and a method to solve a complex set of nonlinear equations are reported in Wesdorp et al. (2005). 

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