Static solubility measurements

Figure 16. High-pressure optical cell for static solubility measurements (1 = soft-iron wire, 2 = Bridgman piston, 3 = screw cap, 4 = thrust collar, 5 = teflon packing, 6 = inner Bridgman piston, 7 = cap of autoclave, 8 = sliding piston, 9 = body of autoclave, 10 = window fitting, 11 = sapphire window, 12 = sample container, 13 = sealing screw, 14 = stirring rod for details see [45,76]).

Various procedures have been reported in literature to investigate the sol-ubihty of compoimds in SCCO2 [34], They can be divided into static [34, 35] and dynamic [36,37] methods. Most recently, also parallel techniques have been developed for solubility measurements [38]. All methods, except for the static synthetic one (see below), require analytical measurements to quantify the amount of solute dissolved in SCCO2. Mainly spectroscopic, chromatographic, and gravimetric techniques are applied for the analytical measurements. 

In this paper solubility measurements of synthetic and natural dyestuffs are presented using VIS-spectroscopy. The investigations concentrate on two different methods. I. P-carotene was measured as a function of temperature and pressure in near- and supercritical C02 (289 to 309 K, 10 to 160 MPa) and CC1F3 (297 to 326 K, 12 to 180 MPa), respectively, using a static method. II. Additionally, the solubilities of l,4-bis-(n-alkylamino)-9,10-anthraquinones (with n-alkyl = butyl, octyl) were determined with a dynamic method in temperature and pressure ranges from 310 to 340 K and 8 to 20 MPa, respectively this method permits a continuous purification from better soluble impurities as well as the measurement of solubilities at the same time. For both anthraquinone dyestuffs intersection points of the solubility isotherms were found in the plot of concentration versus pressure. This behavior can be explained by a density effect. 

SOLUBILITY MEASUREMENTS OF p-CAROTENE WITH A STATIC METHOD... 

FIGURE 5 Apparatus for solubility measurements using a static/analytical method. V symbols represent valves to turn flows on or off, and SW represents switching valves to divert flows. (From Bristow, S., Shekunov, B. Y., and York, P., Ind. Eng. Chem. Res., 40(7), 1732-1739, 2001, reproduced with permission from the American Chemical Society.)... 

FIGURE 6 Variable-volume view cell for use in the static/synthetic method of solubility measurement. Components follow variable volume ceU (1), screw pump (2), magnetic stirrer (3), magnetic bar (4), upper sampling line (5), lower sampling line (6), sapphire window (7), thermostated bath (8), light som-ce (9), video camera (10), and monitor (11). (From Crampton, C., Charbit, G., and Neau, E., J. Supercrit. Fluids, 16(1), 11-20, 1999, reproduced with permission from Elsevier.)... 

To check the validity of the time-lag method for estimating solubility coefficients in these systems, static sorption measurements were conducted with CO2. Such measurements are difficult owing to the very low level of sorption. The solubility coefficient obtained is given in parenthesis in Table 2. Clearly, the agreement with those obtained from transient permeation experiments is not perfect however, the values obtained by the two approaches are close enough to confirm the general conclusions discussed above. 

The flow method ts the simplest and the most straightforward. In the flow method, the solvent fluid is supplied to a compressor by a pressure cylinder. At the desired pressure, the fluid passes into the thermostatted extractor cell that contains the solute present in appropriate matrix (e.g., multiple layer of glass wool). The fluid dissolves the solute in the extractor and, on expansion through a heated metering valve, precipitates solute into a series of collection vessels to be measured gravimetrically. The volume of the decompressed fluid is totaled by a wet or dry gas test meter. Static or equilibrium solubility measurement methods are used to eliminate the need to sample the supercritical fluid solution. A high-pressure flow cell is placed in the flow circuit to monitor the dissolution process by spectrophotometry. 

Figure IL Conditions for solubility measurements with the static method.

Surface tension methods measure either static or dynamic surface tension. Static methods measure surface tension at equilibrium, if sufficient time is allowed for the measurement, and characterize the system. Dynamic surface tension methods provide information on adsorption kinetics of surfactants at the air-liquid interface or at a liquid-liquid interface. Dynamic surface tension can be measured in a timescale ranging from a few milliseconds to several minutes [315]. However, a demarkation line between static and dynamic methods is not very sharp because surfactant adsorption kinetics can also affect the results obtained by static methods. It has been argued [316] that in many industrial processes, sufficient time is not available for the surfactant molecules to attain equilibrium. In such situations, dynamic surface tension, dependent on the rate of interface formation, is more meaningful than the equilibrium surface tension. For example, peaked alcohol ethoxylates, because they are more water soluble, do not lower surface tension under static conditions as much as the conventional alcohol ethoxylates. Under dynamic conditions, however, peaked ethoxylates are equally or more effective than conventional ethoxylates in lowering surface tension [317]. 

Static test results may be evaluated by measurement of change of mass or section thickness, but metallographic and X-ray examination to determine the nature and extent of attack are of greater value because difficulty can be encountered in removing adherent layers of solidified corrodent from the surface of the specimen on completion of the exposure, particularly where irregular attack has occurred. Changes in the corrodent, ascertained by chemical analysis, are often of considerable value also. In view of the low solubility of many construction materials in liquid metals and salts, changes in mass or section thickness should be evaluated cautiously. A limited volume of liquid metal could become saturated early in the test and the reaction would thus be stifled when only a small corrosion loss... 

In the sorption experiments of Icopini et al. (2004), the measured isotopic contrast between Fe(II)aq and the goethite starting material was -0.8%o after Fe(II) had sorbed to the surface over 24 hours in this case, the isotopic fractionation between sorbed Fe(II) and Fe(II)aq is not the 0.8%o measured difference, but is approximately +2.1%o based on an inferred 8 Fe value for the sorbed component as calculated from Fe mass balance (Fig. 4), as was noted in that study. Measured differences in Fe isotope compositions between ferric oxide/hydroxide and Fe(II)aq during dissimilatory Fe(III) reduction and photosynthetic Fe(II) oxidation have been proposed to reflect fractionation between soluble Fe(II) and Fe(III) species, where the soluble Fe(III) component is postulated to be bound to the cell and is not directly measured (Beard et al. 2003a Croal et al. 2004). In the case of dissimilatory Fe(III) reduction, assuming a static model simply for purposes of illustration, if 50% of the Fe in a pool that is open to... 

---