PDF Publication Title:
Text from PDF Page: 108
98 Ralf Steudel mocycles so-called C18 stationary phases have turned out to be the best, e.g., silica gel the surface OH groups of which have been modified by substi- tution of the hydrogen atoms by dimethyloctadecylsilyl groups (Me2C18H37- Si-). While untreated silica gel decomposes metastable sulfur rings such as S7 to S8, the C18 phases are inert provided the surface OH groups have been substituted quantitatively. High-purity methanol is used as a mobile phase. Addition of cyclo-hexane to the methanolic eluent (up to 30 vol.%) reduces the retention time and at the same time enhances the solubility of the larger sulfur rings, while addition of water (up to 5%) increases the retention time [89, 90]. In some cases ethanol has been added as a third component, and gradient techniques helped to reduce the retention time of the larger rings [91]. All homocycles Sn with n=6–28 have been separated and can be deter- mined in this way alongside one another. The rings larger than S28 which are definitely present in p-sulfur (see above) cannot be separated by this tech- nique owing to their low solubility in the polar mobile phase, owing to their low concentration in the quenched sulfur melt and owing to their large re- tention times which result in broad and therefore small peaks due to diffu- sion broadening. The retention times tR of sulfur homocycles systematically increase at identical conditions with the ring size allowing the estimation of tR for new Sn species by inter- or extrapolation. The capacity factors k01⁄4ðtR t0Þ=t0 ð11Þ are linearly dependent on the number n of the sulfur atoms in a semiloga- rithmic plot (t0: dead time of the chromatographic system) [89, 91]. The most sensitive detection technique for the separated components is the mea- surement of the UV absorption at or near 254 nm since all compounds con- taining S-S bonds show a very strong absorbance in this wavelength region [92]. The samples are injected into the flowing mobile phase as very dilute CS2 solutions (1 mg Sn/100 ml CS2). Chromatograms of quenched sulfur melts, of p-sulfur and of Sx demon- strate that sulfur melts contain all homocycles from S6 to at least S28 [91]. There is no reason to assume that larger rings are absent; it is just not possi- ble to detect them by the presently available techniques. If these rings are too large they will be insoluble and in this way become components of the polymeric sulfur S1. To analyze sulfur ring mixtures quantitatively by HPLC requires the deter- mination of the calibration functions A=f(c) by analysis of solutions of dif- fering concentrations c for all the allotropes available as pure materials (A: peak area). In most cases this function has the form A=a·c with the slope de- pending systematically on the ring size or the number of sulfur atoms, re- spectively [89]. The quantitative composition of liquid sulfur as determined by HPLC analysis after quenching in liquid nitrogen, extraction with CS2 and separa- tion from the insoluble Sm is shown in Table 3 [93]. These data are the most reliable information about the composition of liquid sulfur in the tempera-PDF Image | Topics in Current Chemistry
PDF Search Title:
Topics in Current ChemistryOriginal File Name Searched:
Elemental-Sulfur-und-Sulfur-Rich-Compounds-I.pdfDIY PDF Search: Google It | Yahoo | Bing
Sulfur Deposition on Carbon Nanofibers using Supercritical CO2 Sulfur Deposition on Carbon Nanofibers using Supercritical CO2. Gamma sulfur also known as mother of pearl sulfur and nacreous sulfur... More Info
CO2 Organic Rankine Cycle Experimenter Platform The supercritical CO2 phase change system is both a heat pump and organic rankine cycle which can be used for those purposes and as a supercritical extractor for advanced subcritical and supercritical extraction technology. Uses include producing nanoparticles, precious metal CO2 extraction, lithium battery recycling, and other applications... More Info
CONTACT TEL: 608-238-6001 Email: greg@infinityturbine.com (Standard Web Page)