Isopiestic isothermal distillation

The isopiestic (isothermal distillation) method for the determination of molecular weights is closely related to the vapor pressure depression method.10 A weighed amount of standard is introduced into one leg of an apparatus and a weighed portion of the unknown is placed in the other leg. Solvent is introduced into the apparatus, which is then evacuated and thermostated. The solvent will distill from one solution to the other until the vapor pressures (and therefore mole fractions) of the two have equalized. If the solutions are ideal, or if the deviations from ideality are similar, equilibrium will occur when the mole fraction of the known equals that of the unknown. 

Since the polymer solution remains quasi unehanged in eoneenlration, this modified VPO-method is faster than isopiestic isothermal distillation experiments with organic solvents and polymer solutions. Difficulties with the increasing viscosity of concentrated polymer solutions set limits to its applicability, because solutions should flow easily to form drops. 

Reference solutions can be made with the same organic solutes that are used for calibration. In the case of water, NaCl or KCl solutions may be applied as it is done for many isopiestic (isothermal distillation) measurements with aqueous solutions. 

Isopiestic or isothermal distillation. This technique can be useful for the preparation of metal-free solutions of volatile acids and bases for use in trace metal studies. The procedure involves placing two beakers, one of distilled water and the other of a solution of the material to be purified, in a desiccator. The desiccator is sealed and left to stand at room temperature for several days. The volatile components distribute themselves between the two beakers whereas the non-volatile contaminants remain in the original beaker. This technique has afforded metal-free pure solutions of ammonia, hydrochloric acid and hydrogen fluoride. 

Vapour pressure osmometer is a variation of the isopiestic or of the isothermal distillation techniques by which a solvent and a solution in that solvent are placed side by side in a closed container. It measures the difference in temperature created by the condensation of solvent on a sensitive thermistor containing a solution of the solute whose Molecular weight is to be determined. 

The isopiestic method measures a difference in vapour pressure while the isothermal distillation technique depends upon a difference in volume. Despite the specific changes being measured in the techniques each change is proportional to the colligative property of the solution - the lowering of the vapour pressure. 

Since equilibration sometimes takes days or weeks, the isopiestic method is not suitable for unstable compounds or compounds with appreciable volatilities. Also, long equilibration times require leak-free and grease-free apparatus. Another design for an isothermal distillation apparatus is given in Fig. 9.10. 

Isopiestic sorption/desorption methods, i.e., gravimetric sorption, piezoelectric sorption, or isothermal distillation... 

Concentration lowering under isothermal conditions is the classical isopiestic technique, sometimes also called isothermal distillation. A number of solutions (two as the minimum) are in contact with each other via their common solvent vapor phase and solvent... 

Isothermal (or isopiestic) distillation is another method for purifying very volatile acids such as HCl, HF or CH3COOH (see Fig. 12-4). This procedme can be performed in a desiccator at room temperature. T vo beakers, one filled with concentrated add (a.g.), the other with ultrapure water, are placed close to each other. The final acid concentration depends on the time of exposure and on the volume ratio of acid to water and should be determined experimentally. In our laboratory =0.1 mol/L HCl was obtained when pladng 25 mL of concentrated acid 5 cm apart from 25 mL of distilled water in a quartz beaker for 2 d. The purification of hydrofluoric acid by isothermal distillation requires inert materials such as FIFE. 

Concentration lowering under isothermal conditions is the classical isopiestic technique, sometimes also called isothermal distillation. A number of solutions (two as the minimum) are in contact with each other via their common solvent vapor phase and solvent evaporates and condenses (this is the isothermal distillation process) between them as long as the chemical potential of the solvent is equal in all solutions. At least one solution serves as reference system, i.e., its solvent activity vs. solvent concentration dependence is precisely known. After an exact determination of the solvent concentration in all equilibrated solutions (usually by weighing), the solvent activity in aU measured solutions is known from and equal to the activity of the reference solution. This method is almost exclusively used for aqueous polymer solutions, where salt solutions can be applied as reference systems. It is a standard method for inorganic salt systems. 

The other main application of FST to open systems has involved the study of preferential interactions. This is particularly important for understanding the effects of a cosolvent on the properties of biomolecules—the urea-induced denaturation of proteins, for example. The major source for thermodynamic data on small molecule binding to proteins has been equilibrium dialysis studies (Timasheff 1998a). More recently, this has been complemented by isopiestic distillation studies (Anderson, Courtenay, and Record 2002). Both approaches provide thermodynamic descriptions of cosolvent interactions with proteins that can be used to rationalize the effects of cosolvents on protein stability obtained from isothermal-isobaric studies (Smith 2004). 

The isopiestic method is a highly accurate but time-consuming relative method. In a constant-temperature enclosure the solvent distills isothermally from a reference solution of known activity to a solution of unknown activity or vice versa, which entails a change in concentration. At equilibrium the solvent activities of both samples are equal and (R denotes reference solution)... 

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