Sulfur melting point depression

Fig. 7.1 Plot of melting point depression (AT) as a function of fraction of units cross-linked, p, for polymer networks formed from random chains. A molten polyethylene cross-linked by ionizing radiation o natural rubber cross-linked with sulfur natural mbber cross-linked with di-t-butyl peroxide. (Data from Refs. (13) and (14))...

The freezing point depression of a solvent is proportional to the concentration of solute particles and may be used to measure the extent of ionization once the new particles have been identified qualitatively as ions. The method has the obvious disadvantage of not allowing measurements over a range of temperatures in a single solvent. It is almost certainly not worth while to compute an enthalpy of ionization from ionization constants at two different temperatures in two different solvents. Usable solvents are limited not only by the requirement that the melting point be at a convenient temperature but also by the requirement that the solvent be capable of producing ions yet not be sufficiently nucleophilic to react irreversibly with them once they are formed. For this reason most cryoscopic work has been done in sulfuric acid or methanesulfonic acid.170... 

A 2.0-gram sample of the oil was boiled with 50 ml. of N sodium hydroxide for 4 hours under reflux, and then the mixture was distilled to yield 20 ml. of distillate. Ethanol was identified in the distillate by gas chromatography. The alkaline aqueous residue was acidified with dilute sulfuric acid and extracted with ether. The ether was evaporated to yield a solid mass, which was recrystallized from water to yield colorless crystals, melting at 199°-201°C. and not depressing the melting point of a mixture with authentic o-phthalic acid. An infrated spectrum of this compound was identical with that of authentic o-phthalic acid. 

The reactions of triphenylmethanol are dominated by the ease with which it dissociates to form the relatively stable triphenylmethyl carbocation. When colorless triphenylmethanol is dissolved in concentrated sulfuric acid, an orange-yellow solution results that gives a fourfold depression of the melting point of sulfuric acid, meaning that four moles of ions are produced. If the triphenylmethanol simply were protonated only two moles of ions would result. 

In principle, one would also expect the homocyclic Sg to be present in hot sulfur melts but this species may be too unstable to survive the quenching and extraction procedure since never has ever a peak at the expected retention time been observed. However, according to Rau et al. [94] saturated sulfur vapor contains only 0.7 mol% Sg but 20% Sg and 12% Sy at 200 °C. At higher temperatures the melt will also contain traces of S4, S3 and S2 [61] see also the next section. The sum of the concentrations of the non-Sg rings at 116 °C is 4.87%, practically identical to the concentration of r-sulfur as derived from the freezing point depression (see above), thus confirming the reliability of the HPLC analysis. 

Heptasulfur imide melts at 113.5° without decomposition, but the melting point is strongly depressed by small admixtures with sulfur. A eutectic between S7NH and Ss is observed at 91.5° with 46% of sulfur (15). Otherwise the physical behavior of S7NH strongly resembles that of elementary sulfur. Thus, for example, it is not wetted by water but dissolves in many organic solvents—indeed more readily than sulfur itself. 

Infrared spectra from NASA s Galileo mission show distinct absorptions on Jupiter s moon Europa that have been attributed to one or more sulfuric acid hydrates. Sulfuric acid in solution with water causes significant freezing-point depression of water s melting point, down to 210 K, and this would make more likely the existence of liquid solutions beneath Europa s icy crust.The interpretation of the spectra is somewhat controversial. Some planetary scientists prefer to assign the spectral features to the sulfate ion, perhaps as part of one or more minerals on Europa s surface. 

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