Melting-points of mixtures

Specific Directions. (A) Weigh out 1-0 g of benzoic acid on a triple-beam balance or on a Sanolite hand balance, and dissolve in 140 

100 ml of alcohol. Make a similar solution of naphthalene. Place the solutions in small flasks or bottles and cork them to prevent evaporation of the solvent. Prepare three dry test tubes and mark them 1, 2, 3. In tube 1 prepare a mixture of 75 per cent benzoic acid and 25 per cent naphthalene by adding with a pipette 3 ml of benzoic acid solution and 1 ml of naphthalene solution. In tube 2 prepare a mixture of 50 per cent naphthalene and 50 per cent benzoic acid by adding 2 ml of each solution. In tube 3 prepare a solution of 25 per cent benzoic acid solution and 75 per cent naphthalene by adding 3 ml naphthalene solution and 1 ml benzoic acid This amount is sufficient for fifteen students. 

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The melting points of mixtures and solutions depend on the nature and the relative amount of each component of the solution. They are, however, lower than those of the separate components. Solder, for example, an alloy of tin and lead, melts at 183°C, a much lower temperature than either of its components tin melts at 231 °C and lead, at 328°C. 

Melting point of mixture of isomers purification by acid-base separation. 

Newman [22] has estimated indirectly the melting point of nitrocellulose containing 12.6% N by the following method. He determined the melting points of mixtures of nitrocellulose and y-butyrolactone obtaining values from 39 toll2°C. By extrapolation he calculated the melting point of nitrocellulose to be 890°K, i.e. 617°C. 

Figure 15-4 Melting points of mixtures of methanol and hydrogen bromide...

Approximate melting points of mixtures of the trans- and m-0-nitro-a-phcnylcinnamic acids are as follows ... 

Information about the combinability of dyes is obtained from dye suppliers. Moreover, dye producers offer premixed disperse dyes whose components are selected in such a manner that the mixture is absorbed evenly at relatively low temperature and, consequently, makes fewer demands on the dispersion stability. At the same time, the depression of the melting point of mixtures should not be overlooked [119], When using mixed dyes, differences in the fastness of the components must be accepted. 

Advantage is taken of the depression of melting points of mixtures to prove whether two compounds having the same melting points are identical. If X and Y are identical, then a mixture of the two will have the same melting point but if X and Y are not identical, then a small amount of X in Y or of Y in X will cause the melting point to be lowered. 

The melting points of mixtures may be used to determine the percentage composition of mixtures whose components are known. For example, in Figure 22, assume that a mixture of A and X melts at 75°. The composition of this mixture will be either 80... 

It is manufactured from limestone (CaC03) and bauxite (mainly AI2O3). The melting point of mixtures of limestone and bauxite is in the range ca. 1500 to 1600°C and the sintering interval for rendering the batch liquid is limited to a very narrow temperature range. 

An interesting example of analysis based on formation of benzylthiouronium salts is the work of Muramoto and Hirao109. It is, in fact, quantitative analysis of binary mixtures of sulphonates, e.g. of o- and p-toluenesulphonates and other aromatic pairs. They measured the melting points of mixtures of the benzylthiouronium salts prepared from the samples and interpolated the values between those for the pure compounds. Muramoto110 extended it to further aromatic pairs. 

In order to decrease the stereochemical possibilities in the final product as a result of working with racemic modifications, the acid XLIX was resolved through the quinidine salt. Both forms of the acid were obtained and a similar series of reactions was carried out using the Zei orotatory /3-methyl-5-TO-nitrobenzoylaminovaleric acid. There was thus obtained optically active l-methyl-7-ketopyrrolizidine (XXXIII) which could not consist of a mixture of more than two forms. Like the product from the racemic acid, this was an oil. When treated with hydroxylamine, however, a single, pure optically active oxime was isolated. This proved to be identical in melting point, crystalline form as observed under the microscope, and specific rotation with retronecanone oxime, and the melting points of mixtures of the two compounds showed no depression. The picrates of the aminoketoximes obtained from natural and synthetic sources were also found to be identical and showed no depression of melting-decomposition point when mixed. Likewise, the picrolonates of the oximes were identical. 

A mixture of carbromal and bromvaletone has been encountered. The proportions of bromine or nitrogen are insufficiently different to be used to calculate the amounts of each present. A determination of the melting-points of mixtures of known proportions in 10 per cent differences after evaporation from ether solution will give a graph for comparison with the unknown mixture. A mixed melting-point of this and the calculated mixture will check the accuracy of the assessed figure. 

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