Isothermic distillation

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. 

Reagents such as water, ammonia, hydrochloric acid, nitric acid, perchloric acid, and sulfuric acid can be purified via distillation (preferably under reduced pressure and particularly with perchloric acid) using an allglass still. Isothermal distillation is convenient for ammonia a beaker containing concentrated ammonia is placed alongside a beaker of distilled water for several days in an empty desiccator so that some of the ammonia distils over into the water. The redistilled ammonia should be kept in polyethylene or parafrin-waxed bottles. Hydrochloric acid can be purified in the same way. To ensure the absence of metal contaminants from some salts (e.g. ammonium acetate), it may be more expedient to synthesise the salts using distilled components rather than to attempt to purify the salts themselves. 

Vapor-phase decomposition and collection (Figs 4.16 to 4.18) is a standardized method of silicon wafer surface analysis [4.11]. The native oxide on wafer surfaces readily reacts with isothermally distilled HF vapor and forms small droplets on the hydrophobic wafer surface at room temperature [4.66]. These small droplets can be collected with a scanning droplet. The scanned, accumulated droplets finally contain all dissolved contamination in the scanning droplet. It must be dried on a concentrated spot (diameter approximately 150 pm) and measured against the blank droplet residue of the scanning solution [4.67-4.69]. VPD-TXRF has been carefully evaluated against standardized surface analytical methods. The user is advised to use reliable reference materials [4.70-4.72]. 

Fig. 111.—Experimental values of the interaction parameter %i plotted against the volume fraction of polymer. Data for polydi-methylsiloxane M =3850) in benzene, A (New-ingi6). polystyrene in methyl ethyl ketone, (Bawn et aV ) and polystyrene in toluene, O (Bawn et alP) are based on vapor pressure measurements. Those for rubber in benzene, T (Gee and Orr ) were obtained using vapor pressure measurements at higher concentrations and isothermal distillation equilibration with solutions of known activities in the dilute range.

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. 

Separation of dibenzothiophene derivatives from petroleum oil fractions has been achieved by integrated approaches in which fractionating processes such as isothermal distillation, vacuum fractionation, and molecular distillation have been combined with spectroscopic methods including mass spectrometry and NMR spectroscopy. " Dibenzothiophenes have also been concentrated in sharp chromatographic fractions obtained, for example, by, alumina gel percolation, and have been detected by gas chromatography. Gas chromatography has also been used to... 

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. 

A simple isothermal distillation apparatus is illustrated in Fig. 9.9. While not specifically designed for air-sensitive compounds, the apparatus may be loaded in a dry box. After introduction of samples and solvent, it is sealed off under vacuum and allowed to equilibrate, with the apparatus arranged so the liquid is in the large bulbs in order to afford a maximum area of exposed solvent. The apparatus is periodically tipped so the two solutions flow into the calibrated legs. When no change in volume is observed from one time to the next, the relative volumes of the solutions are, to a good approximation, proportional to the moles of solute in the two legs. 

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. 

Fig. 9.10. Apparatus for the determination of molecular weights of air-sensitive compounds by the isothermal distillation technique. Temperature fluctuations of the two solvents are minimized in this illustration by a Dewar filled with water. This apparatus is used in the following manner. In a dry box the sample is placed in a tared tube through sidearm A, the sidearm is then sealed off, the tube and remnant of the sidearm are weighed, and the tube is attached to the apparatus by glassblowing at B. A weighed portion of a standard is introduced into the other bulb, and the filling tube is sealed off. After evacuation, opening of the break-seal, and reevacuation, a measured portion of solvent is distilled into both arms of the apparatus. The process of equilibration is followed by periodic removal and measurement of the solvent from one arm. The solvent may be measured volumetricly in the liquid or gas stales, or by weight.

A related technique, vapor phase osmometry, is based on the idea of isothermal distillation. Such an osmometer is shown schematically in Figure 2.4. In effect, the vapor... 

The molar mass of a newly synthesized organic compound was determined by the isothermal distillation method. In this procedure two solutions, each in an open calibrated vial, are placed side by side in a closed chamber. One of the solutions contained 9.3 mg of the new compound, the other 13.2mg of azobenzene (molar mass 182). Both were dissolved in portions of the same solvent. The experiment was untouched for three days, during which solvent distilled from one vial into the other until the same partial pressure of solvent was reached in both vials. After this, there was no further net distillation of solvent. Neither of the solutes distilled at all. The solution containing the new compound occupied 1.72mL and the azobenzene solution occupied 1.02mL. What is the molar mass of the new compound The mass of solvent in solution may be assumed to be proportional to the volume of the solution. 

By methylation end-group assay. 1 Number-average degree of polymerization of methylated polysaccharide by id (isothermal distillation) or op (osmotic pressure) methods. c Viscosity-average degree of polymerization. 

No species with more than two nitrogen atoms was found among the products. The isothermal distillation of the products observed at 77° K. into the mass spectrometer was followed until these products were largely exhausted. The distillation curves are presented in Figure 4, where it is seen that N2, with the highest initial partial pressure, diminishes in concentration (as measured by the mass spectrometric peak intensity) with time whereas F2 appears, and after passing through a maximum, is... 

Preparation of Distillates and Residues. Crude oil samples were topped up to 350° C in an isothermal distillation unit (17). The 350+ ° C residues were treated in a short path distillation unit (18) to prepare 300°-400° C and 400°-525° C or 400°-550° C distillates. The boiling range was checked by simulated distillation using a technique developed in common with ELF and I.F.P. (19). The residues (525+° C or 550+ ° C) used in this work come from the same operations. 

Helmholtz calculated the work done in the isothermal distillation of the solvent into the solution. The reversible dilution by dv of any solution containing in the volume v 1 mol. of solute and X mols. of solvent can be performed as follows. The quantity of pure solvent (dx mols.) which, when mixed with the solution, increases its volume by dv, is evaporated reversibly and isothermally at the temperature T and the vapour pressure jpQ. The vapour is then allowed to expand until its pressure has become equal to the lower vapour pressure p of the solution, and is then condensed so as to dilute the solution by dv. The quantities of work done in the evaporation and in the condensation are practically equal to one another, provided that the saturated vapour obeys the gas laws, and provided that the volume of the hquid is neghgible compared with the volume of the vapour. Hence the total work done dw is equal to the work done by the vapour in expanding from to p. 

AVe can calculate the e.m.f. E of the combination, if we know the maximum work obtainable from the transference of 1 mol. of ZnSO from the concentrated to the dilute solution. (The work done against the atmospheric pressure owing to the change in volume of the solution may be neglected here.) It is sufficient for this purpose to calculate the work done when the transference is carried out reversibly. Helmholtz carried out this calculation for the isothermal distillation of the solvent from the dilute to the concentrated solution. Nern t calculated the osmotic work done in the process of dilution. 

Isothermal distillation. Consider two cells differing in concentration by an indefinitely small amount dc. The... 

The same amoimt of work may be obtained reversibly in a different manner, viz. by the isothermal distillation of the amount dm from the small sphere to the plane surface. Let jp be the ordinary vapour pressure of the liquid at the plane surface and jf its vapour pressure at the small sphere. If the vapour obeys the gas laws, the work done in the transference is... 

Work is done when any solution is diluted in a reversible manner. Let v be the volume and tt the osmotic pressure of the solution. The maximum work done when the solution is diluted isothermally by d-y is tt dv. If we now allow for the fact that the surface of the solution is increased by da during the dilution (which we may suppose to be performed by isothermal distillation from a large volume of the solvent), we must subtract the work yda due to the surface tension. Hence the free energy of the solution diminishes during dilution by... 

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