Disappearing-phase method

Determination of solvus curves (disappearing-phase method) 377... 

DETERMINATION OF SOLVUS CURVES (DISAPPEARING-PHASE METHOD)... 

Since the curve of Ipjl vs. weight percent B is not linear, high accuracy in the extrapolation depends on having several experimental points close to the phase boundary which is being determined. The accuracy of the disappearing-phase method is therefore governed by the sensitivity of the x-ray method in detecting small amounts of a second phase in a mixture, and this sensitivity varies widely... 

Whichever technique is used to detect the second phase, the accuracy of the disappearing-phase method increases as the width of the two-phase region decreases. If the (a + P) region is only a few jjercent wide, then the relative amounts of a and P will vary rapidly with slight changes in the total composition of the alloy, and this rapid variation of WJW will enable the phase boundary to be fixed quite precisely. This is true, for the x-ray method, even if the atomic numbers of A and B are widely different, because, if the (a + P) region is narrow, the compositions of a and P do not differ very much and neither do their x-ray scattering powers. 

As we have just seen, the disappearing-phase method of locating the boundary of the a field is based on a determination of the composition at which the P phase just disappears from a series of (a -f- P) alloys. The parametric method, on the other hand, is based on observations of the a solid solution itself. This method depends on the fact, previously mentioned, that the lattice parameter of a solid solution generally changes with composition up to the saturation limit, and then remains constant beyond that point. 

Recalling the earlier discussion of the disappearing-phase x-ray method of locating a solvus line (Sec. 12-4), we note from Eq. (14-12) that the intensity ratio /y// is not a linear function of the volume fraction Cy, or, for that matter, of the weight fraction h. ... 

Special techniques have been developed to measure critical temperature, pressure and density. The most common manner to observe the critical temperature is to heat a sample in a closed tube and measure the temperature at which the boundary (meniscus) between liquid and vapor disappears. This method produces an accuracy of about 0.5 degree in most cases. More sophisticated methods for detecting the merging of the two phases are available, but achieving a reproducibility of better that 0.1 degree is difficult. Some properties of a substance change rapidly in the vicinity of the critical point and many organic compounds decompose at or below the critical temperature. Rapid methods of observation have been developed for these compounds. 

Qualitative HPLC methods, using area percent, are used to monitor the disappearance of starting material and the formation of byproduct. Without the inclusion of an internal standard and the calculation of response factors, it is not possible to establish with certainty whether all of the starting material can be accounted for. An internal standard must be stable in the reaction mixture, must not co-elute with any of the components, and must be stable in the mobile phase. Ideally, the internal standard has a retention time about half that of the total analysis time. Internal standardization is extremely useful for kinetic studies. Added to the reaction vessel, samples that are withdrawn at various times will contain identical concentrations of internal standard, and chromatograms can be directly compared or adjusted to identical scales to correct for variation in injection volume. 

Method B. Bromination TMA-Br3 (1.54 g, 10 mmol) and (PhC0)202 (0.24 g, 1 mmol) is added to the alkylarene (10 mmol) in PhH (20 ml) at room temperature and the mixture is stirred until the colour disappears and the evolution of HBr stops. H20 (20 ml) is added and the organic phase is separated, washed well with H20 and aqueous Na2CO, (sat. soln.), dried (Na2S04), and evaporated to yield the brominated product. 

Method A The 0,5-dialkyl dithiocarbonate is prepared by procedure 4.1.14 from O-alkyl potassium dithiocarbonate (50 mmol). The mixture is cooled to 50 °C and, without isolation of the ester, KOH pellets (14 g, 0.25 mol) are added portionwise at <80 °C. The mixture is stirred at 80 °C until GLC analysis indicates the complete disappearance of the ester (ca. 30 min). Petroleum ether (b.p. 40-60 °C, 150 ml) is added and the organic phase is separated, dried (Na2S04), filtered through silica, and fractionally distilled to give the thioether. 

Method G (typical procedure). NaBH, (5%) in 5% aqueous NaOH, is added dropwise under N2 to a solution of 5a, 6)3-dibromocholestan-3)3-ol (1.50 g, 2.73 mmol) and bis(2-thienyl) ditelluride (0.10 g, 0.24 mmol) in EtOH (20 mL) at room temperature until the red colour of the ditelluride disappears. Air is then introduced to the system to oxidize the catalyst to the ditelluride. The mixture is diluted with ether and H2O, and the organic phase is washed with H2O, dried and evaporated. Chromatography of the residue on SiOj (eluting with CH2Cl2/MeOH, 95 5) furnishes cholesterol (0.95 g (90%) m.p. 149-150°C). 

Method C2 (typical procedurep A 5% solution of NaBH in 5% NaOH is added dropwise to a stirred suspension of bis(2-thienyl) ditelluride (0.75 g, 1.78 mmol) in EtOH under N2, until disappearance of the red colour of the solution. 2-Bromocholestan-3-one (0.80 g, 1.72 mmol) in EtOH (5 mL) is then added dropwise over 10 min, causing an immediate red colouration. After 30 min the mixture is poured into HjO/diethyl ether, and the organic phase is washed several times with HjO and dried (CaClj). Evaporation of the solvent and chromatography (Si02/CH2Cl2) gives cholestan-3-one (0.62 g (93%) m.p. 127-128°C). 

TLC experiment can be carried out in controlled conditions, but then the appealing characteristics of simplicity, cheapness and fastness disappear. In order to make the TLC method simple and robust against temperature and humidity changes, it is possible to select a mobile phase that minimizes the harmful effects of these changes. This is described in Chapter 6. 

Another variation of the stoichiometric method involves loading known amounts of gas and IL into the cell and then increasing the pressure (at constant temperature) until all the gas dissolves in the liquid and, consequently, the vapor phase disappears. Using different loadings of the gas, one can determine the solubility at various different pressures and temperatures. Mercury was used as the pressurization fluid by Peters and coworkers to determine gas solubilities in ILs [4]. Maurer and coworkers used a similar method, but they introduced and withdrew additional known amounts of the IL to pressurize or depressurize the mixture and observe the phase change [5]. 

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