Freezing simple solutions

We applied the MD technique to the simulation of brine rejection from freezing salt solutions employing the rigid 3-site simple point charge (SPC/E) water model to obtain the microscopic picture of this very important natural process. Further, we used a recently... 

A typical pharmaceutical formulation will serve as model, frozen aqueous solutions that are contained in loosely stoppered glass vials. They stand directly on the freeze-drier shelves. Such a model appears deceptively simple but is in fact subject to considerable complexities that bedevil the modelling of optimum processing conditions and the estimation of a realistic drying cycle time. As emphasised in earlier chapters, for the case where the product does not display eutectic behaviour, it is imperative to freeze the solution to below Tg, in order to ensure maximal freeze-concentration and minimise deleterious chemical reactions that have been shown to occur at high rates in part frozen, supersaturated mixtures.By the same token, the temperature during the removal of ice by sublimation must on no account be allowed to rise above T at... 

The data in Table 10.3 suggest that the situation is not as simple as this discussion implies. The observed freezing point lowerings of NaQ and MgS04 are smaller than would be predicted with t = 2. For example, 0.50 m solutions of NaCl and MgS04 freeze at —1.68 and —0.995°C, respectively the predicted freezing point is — 1.86°C. Only in very dilute solution does the multiplier i approach the predicted value of 2. 

There has been an increase in the use of cadaveric heart valves for patients with valvular defects. The valves are best stored by freezing but some success has been achieved by simple cold storage in an antibiotic medium made up of ingredients common to most tissue culture solutions. At a storage temperature of 4 °C there is a continual loss of viability of fibroblasts so that by three weeks there are practically no viable cells and the valves cannot be used. 

Center-Fed Column Crystallizers Two types of center-fed column crystallizers are illustrated on Figs. 20-8 and 20-9. As in a simple distillation column, these devices are composed of three distinct sections a freezing or recovery section, where solute is frozen from the... 

Mann and Mitchell [58] described a simple colorimetric method for estimation of (-D)-penicillamine in plasma. Blood containing 2-50 pg of penicillamine was mixed with 0.1 M EDTA solution in tromethamine buffer solution. 0.1 mL of this solution was adjusted to pH 7.4 and centrifuged. To a portion of the plasma was added 3 M HCL, the mixture was freeze-dried, and a suspension of the residue in ethanol was centrifuged. The supernatant liquid was mixed with tromethamine buffer solution (pH 8.2) and 10 mM 5,5 -dithiobis-(2-nitrobenzoic acid) in phosphate buffer solution (pH 7.0), the mixture was shaken with ethyl ether, and the absorbance of the separated aqueous layer was measured at 412 nm. The mean recovery was 60% (four determinations), and the calibration graph was linear for the cited range. 

Most hydrophobic substances have low solubilities in water, and in the case of liquids, water is also sparingly soluble in the pure substance. Some substances such as butanols and chlorophenols display relatively high mutual solubilities. As temperature increases, these mutual solubilities increase until a point of total miscibility is reached at a critical solution temperature. Above this temperature, no mutual solubilities exist. A simple plot of solubility versus temperature thus ends at this critical point. At low temperatures near freezing, the phase diagram also become complex. Example of such systems have been reported for sec-butyl alcohol (2-butanol) by Ochi et al. (1996) and for chlorophenols by Jaoui et al. (1999). 

Hydrogen cyanide (Table 15.1) is a colorless, flammable liquid or gas that boils at 25.7°C and freezes at minus 13.2°C. The gas rarely occurs in nature, is lighter than air, and diffuses rapidly. It is usually prepared commercially from ammonia and methane at elevated temperatures with a platinum catalyst. It is miscible with water and alcohol, but is only slightly soluble in ether. In water, HCN is a weak acid with the ratio of HCN to CN about 100 at pH 7.2, 10 at pH 8.2, and 1 at pH 9.2. HCN can dissociate into H+ and CN. Cyanide ion, or free cyanide ion, refers to the anion CN derived from hydrocyanic acid in solution, in equilibrium with simple or complexed cyanide molecules. Cyanide ions resemble halide ions in several ways and are sometimes referred to as pseudohalide ions. For example, silver cyanide is almost insoluble in water, as are silver halides. Cyanide ions also form stable complexes with many metals. 

For a solution with a liquid as solvent, the temperature at which it freezes to a solid is slightly lower than the freezing point of the pure solvent. This phenomenon is known as freezing point depression and is related in a simple manner to the concentration of the solute. The lowering of the freezing point is given hy... 

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