Purity mixed melting points

In addition to the use of a melting point determination as a criterion of purity, an equally valuable application is for the identification of oiganic compounds. If the melting point is known within one degree, the major proportion of possible substances is immediately eliminated from consideration. The study of the general chemical properties of the compound and a mixed melting point determination (Section 1,17) will largely establish the identity of the compound. 

A mixed melting point is the only absolutely fail-safe way of determining the purity of a sample. 

A mixed-melting-point experiment is an ideal test of a material s purity since 7"(meit) never drops unless the compound is impure. 

Figure 5.18 Adding a chemical to a host (mixing) causes its chemical potential /i to decrease, thereby explaining why a melting-point temperature is a good test of purity. The heavy solid lines represent the chemical potential of the pure material and the thin lines are those of the host containing impurities...

To eliminate nitrate anion, 1,3-D HN03 (198g) is mixed with water (910mL) and cooled to 3-5°C. Then, a cold NaOH solution (40%, 100g) is added. The solid formed is filtered and dried in air. Yield of 1,3-D 118 g (90% purity). Melting point 184-186°C [37]. 

With all crystalline substances, the crystal form is an intrinsic indication of purity. Impurities are often recognized macroscopically, but more generally their presence is revealed by microscopic observation of a different crystal habit or amorphous character. There are cases where the impurity crystallizes with the main product and is not directly detectable. Such mixed crystals, however, frequently have a different crystal form from that of the pure compound, and are distinguishable if a direct comparison is made with a pure sample. The control of purity of sulfonic acids, which is made difficult by the lack of characteristic melting points, is simplified somewhat by the fact that numerous salts can be prepared from them, and one or more of these will usually be found to have a recognizable characteristic crystal form. 

The enthalpies of solution were measured with a LKB 8700-1 precision calorimetry system. The experimental procedure and test of the instrument have been given before (6,7). EC (Fluka, purissimum) was distilled under reduced pressure and the middle fraction was stored over molecular sieves (4 A) for at least 48 hr. ACN (Merck, pro analysis) was dried over molecular sieves and used without further purification. The purity of both solvents (determined shortly before use), as deduced from GLC, was always better than 99.8%. The volume fraction of water, determined by K. Fischer titration (8) was always less than 3.10-4. The mixed solvents were prepared by weight as shortly as possible before the measurements. AH° of Bu4NBr in W-ACN mixtures have been measured at 25°C while those in W-EC are at 45°C, which is above the melting point of pure EC. 

Typically, polymer-grade l-LA with high chemical purity and optical purity (i.e., over 98-99% l-LA and less than 1-2% d-LA) is used for commercial PLA production. When l-LA is dehydrated at high temperature into L-lactide, some l-LA may be converted into d-LA. d-LA mixed with l-LA contributes to meso-lactide (the cyclic dimer of one d-LA and one l-LA) and heteropolymer PLA (with both d-LA and l-LA units). Heteropolymer PLA exhibits slower crystallization kinetics and lower melting points than homopolymer PLA (of pure l-LA units or pure d-LA units). 

Purification and Characterization. The product mixture remaining after removal of the methylene chloride is normally of sufficient purity for direct characterization. Weigh the solid product and calculate the percent yield. Determine the melting point of your material. 4-ferf-Butylcyclohexanol (mixed isomers) has a melting point of 62-70 °C. 

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