Vapour pressure of mixtures

Fractional distillation. The aim of distillation is the separation of a volatile liquid from a non-volatile substance or, more usually, the separation of two or more liquids of different boiling point. The latter is usually termed fractional distillation. The theoretical treatment of fractional distillation requires a knowledge of the relation between the boiling points, or vapour pressures, of mixtures of the substances and their composition if these curves are known, it is possible to predict whether the separation is difficult or easy or, indeed, whether it will be possible. 

Vandoni and Viala examined the vapour pressures of mixtures of nitric acid in acetic anhydride, and concluded that from o to mole-fraction of nitric acid the solution consisted of acetyl nitrate, acetic acid and excess anhydride in equimolar proportions the solution consisted of acetyl nitrate and acetic acid, and on increasing the fraction of nitric acid, dinitrogen pentoxide is formed, with a concentration which increases with the concomitant decrease in the concentration of acetyl nitrate. 

Previous to the researches of Ivonowalow, the vapour pressures of mixtures had been investigated theoretically by G. Kirchhoff (Pogg. Ann. (1858), 103, 104 Ostw. Klass. No. 101), and by Gibbs Scientific Papers, Yol. I.). The latter had established the theorem relating to mixtures with stationary vapour pressures. 

It is fairly generally believed that nitrosulphonic acid plays an important part in the lead chamber process for the manufacture of sulphuric acid (see p. 153). The vapour pressures of mixtures of sulphuric acid with nitrous or nitric acid or with both these acids, within the range occurring in the chambers, increase with the nitrogen acid content and with rise in temperature, and the total pressure is always higher than the sum of the individual pressures, especially when the sulphuric acid is concentrated, for nitric acid - sulphuric acid mixtures this may be explained 1 by the occurrence of the following... 

Fig. 94—Vapour Pressures of Mixtures of Nitric and Sulphuric Acids.

Fig. 13.6. Total vapour pressures of mixtures of carbon disulphide (A) and benzene (B) at 30 °C.

Measurements of water vapour pressures of mixtures of nickel chloride hydrates [16DER7YNG] and [40BEL], and over pure solids [93UVA/TIM] have been reported. Water vapour pressure values reported for the equilibrium ... 

The partial vapour pressures of mixtures of carbon tetrachloride and cyclohexane were measured by Scatchard, Wood, and Mochel (J. Amer. Chem. Soc. 1939, 61, 3206). They applied corrections to convert vapour pressures to fugacities and derived values for the excess Gibbs function G- by means of the formula... 

Fig. 82.—Vapour Pressures of Mixtures of Potassium Chromium in Potassium Aluminium Alums.

A simple and frequently used technique to determine the vaporization enthalpies of volatile liquids is the isoteniscope method [96,97]. The isoteniscope was devised to trap a small amount of the liquid and its vapour in a part of a vessel that is totally immersed in a thermostatted bath, separated from the measuring device by an open-end manometer which is also immersed in the bath. The liquid in this inner manometer is usually the liquid being studied. The pressure in the outside space is measured by conventional methods. When the inner manometer shows that the outside pressure is the same as that inside (the vapour pressure of the liquid at the bath temperature) the outside pressure is registered. This method has been widely employed to obtain vapour pressures of mixtures [98],... 

Gilmour et al. have described an apparatus for measuring the vapour pressure of mixtures at low temperatures. They used to click gauge as the null device... 

Determinations by Dynamical Method.—The vapour pressures of mixtures may be determined by the dynamical... 

Again, the vapour pressures of mixtures may be determined indirectly from their boiling points under a series of pressures. The boiling points of each mixture are mapped against the pressures or, better, the logarithms of the pressures, and from the curves so obtained the pressure at any required temperature can be read off. 

Vapour Pressures.—The lowest member of the series, methyl alcohol, bears the closest resemblance to water, and, as Konowaloff has shown, the vapour pressures of mixtures of these substances are in all cases intermediate between those of the pure components, and the curve representing the relation between vapour pressure and molecular composition does not deviate very greatly from a straight line, as will be seen from Fig. 14, in which the vapour pressures of mixtures of four alcohols with water are given, at the boiling points of the alcohols under a pressure of 400 mm. 

Fio. 14.—Vapour pressures of mixtures of aleohols with water. 

As regards the vapour pressures of mixtures, it has been sho vn that there are two simple cases —... 

Lehfeldt s Results.—The experiments of Lehfeldt (11) show that the vapour pressures of mixtures of toluene and carbon tetrachloride are slightly lower than those calculated from the formula P = while... 

Indirect Evidence.—As the vapour pressures of mixtures of carbon disulphide with carbon tetrachloride and with benzene have not been determined at constant temperatures, it is not possible to give tables in the same form as before, but an estimate of the relative maximum deviations of the vapour pressures of mixtures of these substances from those... 

If the vapour pressures of mixtures of the two substances differ but little from those given by the formula... 

That the vapour pressures of mixtures of two closely related compounds—and rarely of others—are represented... 

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