Selection of Solubility Data

Selection of Solubility Data Solubility values determine the liquid rate necessaiy for complete or economic solute recoveiy and so are essential to design. Equihbrium data generally will be found in one of three forms (1) solubility data expressed either as solubility in weight or mole percent or as Heniy s-law coefficients, (2) pure-component vapor pressures, or (3) equilibrium distribution coefficients (iC values). Data for specific systems may be found in Sec. 2 additional references to sources of data are presented in this section. 

Several data sets for the solubilities of poorly soluble substances in mixed solvents were selected from Solubility Data Series (Acree, 1995) ... 

Apart from analytical errors, a possible explanation for the difference between the two sets of solubility data would be that the specimen used by Gelbach and King was not perfectly crystalline and hence had a higher solubility. As there is no obvious ground for rejecting one of the values, the review selects the mean of the two solubility products with a large uncertainty ... 

The objective of the present review is to present an assessment of the sources of published thermodynamic data in order to decide on the most reliable values and their uncertainties that can be recommended for modelhng purposes. Experimental measurements pubhshed in the scientific literatrrre are the main somce for the selection of recommended data. Previous reviews are not neglected, since they form a valuable source of critical information on the qrrality of primary publications. When necessary, experimental source data are re-evalrrated by using chemical models which are either foimd to be more realistic than those used by the original author, or are consistent with subsequent information, or with side-reactiorts discussed in another section of the review (for example, data on carbonate and hydroxide solubilities might need to be reinterpreted to take into accormt the crystal stractrrre and particle size of the phases actually investigated). 

Figure A-32 Comparison of solubility data measured by Moon [1989MOO] with Th(OH)4(am) in 0.5 M NaC104 and with Th02(cr) in 0.1 M NaC104 at 18°C. The solubility curve for Th02(am, hyd) is calculated using the hydrolysis constants and SIT coefficients selected in the present review, in combination with logj Kl,= 9.5 and log, ° =-(8.1+0.3).

The most critical decision to be made is the choice of the best solvent to facilitate extraction of the drug residue while minimizing interference. A review of available solubility, logP, and pK /pKb data for the marker residue can become an important first step in the selection of the best extraction solvents to try. A selected list of solvents from the literature methods include individual solvents (n-hexane, " dichloromethane, ethyl acetate, acetone, acetonitrile, methanol, and water ) mixtures of solvents (dichloromethane-methanol-acetic acid, isooctane-ethyl acetate, methanol-water, and acetonitrile-water ), and aqueous buffer solutions (phosphate and sodium sulfate ). Hexane is a very nonpolar solvent and could be chosen as an extraction solvent if the analyte is also very nonpolar. For example, Serrano et al used n-hexane to extract the very nonpolar polychlorinated biphenyls (PCBs) from fat, liver, and kidney of whale. One advantage of using n-hexane as an extraction solvent for fat tissue is that the fat itself will be completely dissolved, but this will necessitate an additional cleanup step to remove the substantial fat matrix. The choice of chlorinated hydrocarbons such as methylene chloride, chloroform, and carbon tetrachloride should be avoided owing to safety and environmental concerns with these solvents. Diethyl ether and ethyl acetate are other relatively nonpolar solvents that are appropriate for extraction of nonpolar analytes. Diethyl ether or ethyl acetate may also be combined with hexane (or other hydrocarbon solvent) to create an extraction solvent that has a polarity intermediate between the two solvents. For example, Gerhardt et a/. used a combination of isooctane and ethyl acetate for the extraction of several ionophores from various animal tissues. 

Sample preparation for the common desorption/ionisation (DI) methods varies greatly. Films of solid inorganic or organic samples may be analysed with DI mass spectrometry, but sample preparation as a solution for LSIMS and FAB is far more common. The sample molecules are dissolved in a low-vapour-pressure liquid solvent - usually glycerol or nitrobenzyl alcohol. Other solvents have also been used for more specialised applications. Key requirements for the solvent matrix are sample solubility, low solvent volatility and muted acid - base or redox reactivity. In FAB and LSIMS, the special art of sample preparation in the selection of a solvent matrix, and then manipulation of the mass spectral data afterwards to minimise its contribution, still predominates. Incident particles in FAB and LSIMS are generated in filament ionisation sources or plasma discharge sources. 

A wide variety of solubilities (in units of g/m3 or the equivalent mg/L) have been reported. Experimental data have the method of determination indicated. In other compilations of data the reported value has merely been quoted from another secondary source. In some cases the value has been calculated. The abbreviations are generally self-explanatory and usually include two entries, the method of equilibration followed by the method of determination. From these values a single value is selected for inclusion in the summary data table. Vapor pressures and octanol-water partition coefficients are selected similarly. 

In summary, the physical-chemical and environmental fate data listed result in the tabulated selected values of solubility, vapor pressure, Kow, dissociation constant where appropriate and reaction half-lives at the end of each chapter. These values are used in the evaluative environmental calculations. 

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