Melting depression

The basis of any calorimetric purity method is the relationship between the melting depression of a substance and the level of impurities according to van t Hoffs law. The purity is readily calculated from the DSC curve of a single melting event of a few milligrams of the substance, without the need for reference standard of the drug substance and its impurities. 

Then as the temperature increases, pure A melts between and T. is the temperature at the end of the melting. For the corresponding DSC curve, an endotherm at the eutectic temperature is observed, then the melting of crystals A occurs. The effect of impurity on the DSC curve is a melting depression and a broadening of the melting curve (Fig. 13). 

The kinked dicarboxy compounds obviously has considerable capability for decreasing the melt transitions with increasing substitution for the terephthalic acid. A decrease to below 300°C is already achieved with the diphenyl ether derivatives-(3,4 -0 and 4,4 -0) at a degree of substitution corresponding to 30%. For the corresponding diphenyl ketone derivatives (3,4 -K and 4,4 -K) a somewhat higher degree of substitution is required, 35-40%, to get a comparable melt depression. The more stiff structure of the ketones compared to the ethers may be connected with this kind of behaviour. However, further increase in the amounts has a very drastic effect on the melt transition. 

PBSA and PVPh are miscible crystalline/amorphous polymer blends. Miscibility of PBSA/PVPh blends was evidenced by the single composition-dependent glass-transition temperature over the entire blend compositions. The negative polymer-polymer interaction parameter, obtained from the melting depression of PBSA, indicates that PBSA/PVPh blends are thermodynamically miscible. 

L.H. Liang, J.C. Li, Q. Jiang, Size-dependent melting depression and lattice contraction of Bi nanocrystals. Phys. B-Condens. Matter 334(1-2), 49-53 (2003)... 

The substance is pure, but on warming undergoes slight thermal decomposition before the melting-point is reached, and the decomposition products then act as impurities and depress the melting-point. 

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. 

The cryoscopic constant Kj gives the depression of the melting point AT (in degrees Celsius) produced when 1 mol of solute is dissolved in 1000 g of a solvent. It is applicable only to dilute solutions for which the number of moles of solute is negligible in comparison with the number of moles of solvent. It is often used for molecular weight determinations. 

Garbodiimide Formation. Carbodiimide formation has commercial significance in the manufacture of Hquid MDI. Heating of MDI in the presence of catalytic amounts of phosphine oxides or alkyl phosphates leads to partial conversion of isocyanate into carbodiimide (95). The carbodiimide (39) species reacts with excess isocyanate to form a 2 + 2cycloaddition product. The presence of this product in MDI leads to a melting point depression and thus a mixture which is Hquid at room temperature. 

Random insertion of ethylene as comonomer and, in some cases, butene as termonomer, enhances clarity and depresses the polymer melting point and stiffness. Propylene—butene copolymers are also available (47). Consequendy, these polymers are used in apphcations where clarity is essential and as a sealant layer in polypropylene films. The impact resistance of these polymers is sligbdy superior to propylene homopolymers, especially at refrigeration temperatures, but still vastiy inferior to that of heterophasic copolymers. Properties of these polymers are shown in Table 4. 

Sodium sulfate decahydrate melts incongmently at 32.4°C to a sulfate Hquid phase and an anhydrous sulfate soHd phase. The presence of other salts, such as NaCl, can depress the melting poiat to 17.9°C. 

In more recent times, naphthalene has been used in condensation products from naphthalene sulfonic acids, utili2ing formaldehyde as additives to improve the flow properties of concrete these are referred to as superplastici2ers. Another newer appHcation is the production of diisopropylnaphthalenes. The mutual depression of the melting points in the mixture gives a Hquid which is used as a solvent for dyes in the production of carbonless copy paper. 

The effect of different types of comonomers on varies. VDC—MA copolymers mote closely obey Flory s melting-point depression theory than do copolymers with VC or AN. Studies have shown that, for the copolymers of VDC with MA, Flory s theory needs modification to include both lamella thickness and surface free energy (69). The VDC—VC and VDC—AN copolymers typically have severe composition drift, therefore most of the comonomer units do not belong to crystallizing chains. Hence, they neither enter the crystal as defects nor cause lamellar thickness to decrease, so the depression of the melting temperature is less than expected. 

The polarity of the polymer is important only ia mixtures having specific polar aprotic solvents. Many solvents of this general class solvate PVDC strongly enough to depress the melting temperature by more than 100°C. SolubiUty is normally correlated with cohesive energy densities or solubiUty parameters. For PVDC, a value of 20 0.6 (J/cm (10 0.3 (cal/cm ) has been estimated from solubiUty studies ia nonpolar solvents. The value... 

Batchwise fractional distillation can be used to adjust the ratio of isomers in the mixture. For example, the heat-transfer composition of Table 4 is obtained by collecting all of the ortho- some of the meta- and excluding most of the i ra-terphenyl present in the natural mixture. Economics and considerations of melting point depression favor inclusion of lower melting quaterphenyl isomers. 

In the flux-growth method, crystals of the desired ceramic are precipitated from a melt containing the components of the product phase, often in addition to additives used to suppress the melting point of the flux. These additives remain in solution after crystal growth is complete. Crystals are precipitated onto seeds by slowly cooling the melt or the seed, or occasionally by evaporating volatile components of the melt such as alkaH haHdes, depressing the solubiHty of the product phase. 

C, is the concentration of impurity or minor component in the solid phase, and Cf is the impurity concentration in the hquid phase. The distribution coefficient generally varies with composition. The value of k is greater than I when the solute raises the melting point and less than I when the melting point is depressed. In the regions near pure A or B the hquidus and solidus hues become linear i.e., the distribution coefficient becomes constant. This is the basis for the common assumption of constant k in many mathematical treatments of fractional solidification in which ultrapure materials are obtained. 

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