Melting point criterion

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. 

The properties of fillers which induence a given end use are many. The overall value of a filler is a complex function of intrinsic material characteristics, eg, tme density, melting point, crystal habit, and chemical composition and of process-dependent factors, eg, particle-si2e distribution, surface chemistry, purity, and bulk density. Fillers impart performance or economic value to the compositions of which they are part. These values, often called functional properties, vary according to the nature of the appHcation. A quantification of the functional properties per unit cost in many cases provides a vaUd criterion for filler comparison and selection. The following are summaries of key filler properties and values. 

At the onset of constitutional supercooling, the melting-point gradient exceeds the temperature gradient. Equating these gradients leads to the criterion for constitutional supercooling ... 

Many oils possess the property of becoming solid at temperatures slightly below the ordinary, and a determination of the soliditying- or melting-points becomes an important criterion of purity in these cases. The melting-point is not usually the same as the solidifying-point, on account of the peculiar properties of bodies, included under the term superfusion, etc. In addition, the temperature recorded differs some-... 

Elemental analysis can be used to establish purity, although a melting point >320°C is generally a sufficient criterion of product purity. The complex does not exhibit a readily interpretable NMR spectrum because of the paramagnetic nature of the complex. 

The melting point of the />-toluenesulfonylanthranilic acid is not a good criterion of purity because the />-toluenesulfonic acid salt of anthranilic acid has about the same value. The neutral equivalents are widely different 154 for the salt and 291 for p-toluenesulfonylanthranilic acid. The compound obtained in this preparation gives a negative test for anthranilic acid on diazotization and treatment with alkaline /3-naphthol solution. The probable impurity is the sodium salt of -toluene-sulfonylanthranilic acid. 

Such data, in addition to the observed boiling and melting points, furnish a better criterion of purity than the analysis for silicon or bromine, since a small quantity of tetra-bromide, if present, will not alter appreciably the percentage composition. 

Step 3 assumes natural convection. This may be difficult, because at 10 °C above the melting point, the viscosity may be too high for an efficient natural convection. This can be checked by calculating the Rayleigh criterion. The height of the tank is 1.8 m, but since it is not always full, a height of 0.9 m can be taken. With a dynamic viscosity of... 

Pure thiosemicarbazide is a white crystalline mass, which is very soluble in water and in ethanol. The reported melting point of the material is 181°, but this is not a reliable criterion of purity. The melting point appears to depend on the kind of crystal obtained by re crystallization. The crystals obtained range from those of micro size to long... 

Small fluctuations in melting behavior appear to be an intrinsic property of the complexes even when they are prepared and purified in the absence of air. Thus, melting-point data are of no value as a criterion of identity or purity for these complexes and are therefore not recorded below. 

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