Vapour pressure of solvents

The further development of modern solution theory is connected with three persons, namely the French researcher Raoult (1830-1901) [28], the Dutch physical chemist van t Hoff (1852-1911) [5], and the Swedish scientist Arrhenius (1859-1927) [6]. Raoult systematically studied the effects of dissolved nonionic substances on the freezing and boiling point of liquids and noticed in 1886 that changing the solute/solvent ratio produces precise proportional changes in the physical properties of solutions. The observation that the vapour pressure of solvent above a solution is proportional to the mole fraction of solvent in the solution is today known as Raoult s law [28]. 

Two physical laws are available to express the vapour pressures of solvents in water in dilute solutions, namely Raoult s law and Henry s law. Operating in the very dilute solutions common in waste water treatment, it does not matter which law is used to obtain the system s properties. Unfortunately, the experimental work reported in the technical literature for Henry s law is expressed using a wide variety of units and because the Henry s law constant, H, is variable with temperature this law is less convenient to use. For this reason, the data tabulated here are suitable for use using Raoult s law. 

Table 3.8 Correction factors for vapour pressure of solvents Separation of solvents from water over dilute aqueous solutions 37... 

In addition to the problems of handling residues both during and after solvent recovery, involatile materials cause difficulties by reducing the vapour pressure of solvents, as was shown in the description of steam distillation. 

Boiling point and vapour pressure of solvent mixtures... 

Vapour pressure of solvent for various values of n ztro heat of mixing). 

Raoult s law When a solute is dissolved in a solvent, the vapour pressure of the latter is lowered proportionally to the mole fraction of solute present. Since the lowering of vapour pressure causes an elevation of the boiling point and a depression of the freezing point, Raoult s law also applies and leads to the conclusion that the elevation of boiling point or depression of freezing point is proportional to the weight of the solute and inversely proportional to its molecular weight. Raoult s law is strictly only applicable to ideal solutions since it assumes that there is no chemical interaction between the solute and solvent molecules. 

We may now understand the nature of the change which occurs when an anhydrous salt, say copper sulphate, is shaken with a wet organic solvent, such as benzene, at about 25°. The water will first combine to form the monohydrate in accordance with equation (i), and, provided suflScient anhydrous copper sulphate is employed, the effective concentration of water in the solvent is reduced to a value equivalent to about 1 mm. of ordinary water vapour. The complete removal of water is impossible indeed, the equilibrium vapour pressures of the least hydrated tem may be taken as a rough measure of the relative efficiencies of such drying agents. If the water present is more than sufficient to convert the anhydrous copper sulphate into the monohydrate, then reaction (i) will be followed by reaction (ii), i.e., the trihydrate will be formed the water vapour then remaining will be equivalent to about 6 mm. of ordinary water vapour. Thus the monohydrate is far less effective than the anhydrous compound for the removal of water. 

D = diffusilivity of the solvent vapour from the sample solution (cm s ) o = surface tension of sample solution (erg-cm ) p = vapour pressure of sample solution (mm Hg X 1-359 = g cm )... 

PRESSURE SYSTEM Defined in the Pressure System Safety Regulations 2000 as a system containing one or more pressure vessels of rigid construction, any associated pipework and protective devices the pipework with its protective devices to which a transportable gas container is, or is intended to be, connected or a pipeline and its protective devices which contains or is liable to contain a relevant fluid, but does not include a transportable gas container. Here relevant fluid is steam any fluid or mixture of fluids which is at a pressure of >0.5 bar above atmospheric pressure, and which fluid or a mixture of fluids is a gas, or a liquid which would have a vapour pressure of >0.5 bar above atmospheric pressure when in equilibrium with its vapour at either tlie actual temperature of the liquid or 17.5°C or a gas dissolved under pressure in a solvent contained in a porous substance at ambient temperamre and which could be released from the solvent with the application of heat. 

All the above deals with gases and gas phase processes. We now turn to non-gaseous components of the system. There are many ways of expressing this. Probably the simplest is to consider an ideal solution of a solute in a solvent. If the solution is ideal, the vapour pressure of the solute is proportional to its concentration, and we may write p = kc, where c is the concentration and k is the proportionality constant. Similarly, = Arc , which expresses the fact that the standard pressure is related to a standard concentration. Thus we may write from equation 20.198 for a particular component... 

It follows directly from this that the vapour pressure of a solution is lower, at a given temperature, than that of pure solvent. For, if temperature and pressure are coordinates, a horizontal line through p p = 1 atm. will cut the vapour pressure P curves of solution and solvent at points, the abscissae of which represent temperatures at which both vapour pressures are equal to atmospheric pressure, i.e., the boiling-points. 

Corollary 1.—The vapour pressure of the solution is greater than that of the pure solvent when the concentration of the solute is greater in the vapour than in the liquid phase. 

Corollary 2.—The vapour pressure of the solution is equal to that of the pure solvent when c = c. Since, by Henry s law, c/c depends only on temperature, and since distillation of liquid cannot alter its composition in this case, the solution will distil unchanged at a constant temperature exactly like a pure substance. This holds only within the limits of applicability of Henry s law. 

A. Findlay (1902) found that Ramsay and Young s rule for the vapour pressures of pure liquids ( 89) has an analogue in the case of solutions. If TA, TA1 are two temperatures at which the substance A has the solubilities s, s, and TB, TB two temperatures at which another substance has the same solubilities in the given solvent then ... 

Dolezalek had previously (1903) proposed a very simple relation between the vapour pressures of concentrated salt solutions and their composition the logarithm of the vapour pressure of the solvent is nearly a linear function of the number of mols of salt (x) per mol of water ... 

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. 

Vapour pressure osmometry is the second experimental technique based on colligative properties with importance for molar mass determination. The vapour pressure of the solvent above a (polymer) solution is determined by the requirement that the chemical potential of the solvent in the vapour and in the liquid phase must be identical. For ideal solutions the change of the vapour pressure p of the solvent due to the presence of the solute with molar volume V/1 is given by... 

In the development of physical chemistry, investigations of dilute solutions have been very important. A dilute solution consists of the main constituent, the solvent, and one or more solutes, which are the diluted species. As early as in 1803 William Henry showed empirically that the vapour pressure of a solute i is proportional to the concentration of solute i ... 

More than 80 years later Francois Raoult demonstrated that at low concentrations of a solute, the vapour pressure of the solvent is simply... 

It should be borne in mind that both in the first and last runnings there is some of the main product. The vapour pressure of a distillable substance is so considerable even below the boiling point that its vapours already pass over along with the more volatile constituents (usually residues of solvent) of the original material. On the other hand the boiling point of a substance rises when it is mixed with higher boiling substances. 

For the solubility of TPA in prepolymer, no data are available and the polymer-solvent interaction parameter X of the Flory-Huggins relationship is not accurately known. No experimental data are available for the vapour pressures of dimer or trimer. The published values for the diffusion coefficient of EG in solid and molten PET vary by orders of magnitude. For the diffusion of water, acetaldehyde and DEG in polymer, no reliable data are available. It is not even agreed upon if the mutual diffusion coefficients depend on the polymer molecular weight or on the melt viscosity, and if they are linear or exponential functions of temperature. Molecular modelling, accompanied by the rapid growth of computer performance, will hopefully help to solve this problem in the near future. The mass-transfer mechanisms for by-products in solid PET are not established, and the dependency of the solid-state polycondensation rate on crystallinity is still a matter of assumptions. 

---