Dissolution media, aqueous processing

Occasionally salts may demonstrate lower aqueous solubilities and dissolution rates than expected due to the common ion effect. For example, the dissolution rate of doxycycline hydrochloride dihydrate salt was determined as four times greater in water than in O.IM HCl solution. Changing the dissolution medium from O.IM HCl to O.IM methanesul-fonic acid resulted in an increase in rate, clearly demonstrating specificity for chloride ions. This effect is due to the presence of chloride ions in the dissolution medium, which disturb the position of equilibrium for the drug solubilization process ... 

Suitable manipulation of the dissolution medium can sometimes inhibit the conversion of one polymorph to another during the dissolution process, thus permitting the measurement of otherwise unobtainable information. In studies on the polymorphs of sulfathiazole and methylprednisolone, Higuchi, who used various alcohols and additives in the dissolution medium to inhibit phase transformations, first employed this approach [87], Aguiar and Zelmer were able to characterize thermodynamically the polymorphs formed by chloramphenicol palmitate and mefenamic acid by means of dissolution modifiers [88], Furthermore, the use of an aqueous ethanol medium containing 55.4%... 

Macheras et al.[56] used an automated FI on-line dialysis spectrophotometric system, incoiporating the dialyzer in a sample loop, to monitor the kinetic process of drug dissolution in low fat milk. The system is similar to that shown in Fig. 6.3, and the dissolution medium is circulated as donor stream through the dialyzer, pumping it from and back to the dissolution tank. Commercial formulations of salicylamide, propantheline bromide, nittbfurahtoin and acetaminophen were used in the study. The results show that the dissolution rate of drugs in milk are lower than in an aqueous buffer. 

It can be seen from Figure 5.18 that the KD values for zirconium are higher than those for hafnium at all nitric acid concentrations. This is because the dissolution of zirconium nitrate (Zr(N03)4) into zirconyl (Zr02+) and nitrate (NOj) ions takes place to a lower extent as compared to the corresponding dissolution of hafnium nitrate in an aqueous medium. Hence, separation is feasible. However, at higher nitric acid concentrations the separation factor is reduced significantly because the dissociation of hafnium nitrate (Hf(NOs)4) decreases sharply with increasing nitric acid concentration, with the result that the separation factor, p, falls off rapidly. Hence, the separation process calls for the adjustment of the nitric acid concentration to a suitably low value. 

Speciation is a dynamic process that depends not only on the ligand-metal concentration but on the properties of the aqueous solution in chemical equilibrium with the surrounding solid phase. As a consequence, the estimation of aqueous speciation of contaminant metals should take into account the ion association, pH, redox status, formation-dissolution of the solid phase, adsorption, and ion-exchange reactions. From the environmental point of view, a complexed metal in the subsurface behaves differently than the original compound, in terms of its solubility, retention, persistence, and transport. In general, a complexed metal is more soluble in a water solution, less retained on the solid phase, and more easily transported through the porous medium. 

Models for the dyeing of polyester fibers with disperse dyes have been developed [8], When the dye is applied from aqueous medium, it is adsorbed from the molecularly dispersed aqueous solution onto the fiber surface and then diffuses into the interior of the fiber. The following parameters determine the rate of dyeing and, to some extent, the leveling properties (1) the dissolution rate during the transition from the dispersed crystalline state of the dye into the molecularly dispersed phase, and (2) the diffusion rate at the fiber surface and, especially, in the interior of the fiber. The rates of both processes vary with temperature. 

Upon conlacl wilh an aqueous medium, most materials acquire a surface electric charge. A variety of processes have charging mechanisms, including ion adsorption, ionization, and ion dissolution. 

These processes occur by precipitation through evaporative concentration of a solute in the aqueous medium until its dissolution capacity is exceeded. Then, a solid is formed and deposited either as a sediment or on a nearby surface. These products are called evaporites. A typical example is the deposition and formation of calcium carbonate stalactites and stalagmites. Evaporation is a major process in arid areas and it influences the chemistry of surface waters. That is why in saline lakes, inland seas, or even in estuaries, evaporites of NaCl or NaCl/KCl and deposits of CaS04 and CaC03 are formed. Here, CaS04 generally precipitates first, and then NaCl. 

Consider the dissolution of an ionic compound such as potassium fluoride in water. Break the process into the following steps separation of the cations and anions in the vapor phase and the hydration of the ions in the aqueous medium. Discuss the energy changes associated with each step. How does the heat of solution of KE depend on the relative magnitudes of these two quantities On what law is the relationship based ... 

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