Negative azeotropes

For a solution or mixture of two or more distinct liquid components, an azeotrope is that composition (typically measured in mole fractions or percent weight and referred to as the azeotropic solution) with which there is either a maximum point (a negative azeotrope) or a minimum point (a positive azeotrope) in a boiling point versus composition diagram at constant pressure. 

Maximum boiling point azeotrope (negative azeotrope)... 

The vapour pressure curves, on the other hand, show that the curve is always concave, with or without a minimum (the existence of a minimum implies negative azeotropism) if the molecules of both components are polar or partially ionized. In this case an association of the different kinds of dipole takes place, resulting in a decrease in the free constituents hence the decrease in the vapour pressure, which means a concave curve. 

A similar inverse rule is obtained if a is negative and we are dealing with a negative azeotrope. Essentially similar rules, but making no reference to the relative magnitude of the vapour pressures of the two substances, were discovered empirically by Roozeboom.f... 

We define A as the absolute difference between the boiling points of the two components, and S as the absolute difference between the boiling point of the azeotrope and that of the more volatile component, for positive azeotropes, and the less volatile for negative azeotropes. The quantity S is called the azeotropic deviation, (figs. 28.5 (a) and (6)). 

Toluene and acetic acid form a negative azeotrope which contains 28 wt % of the acid and boils at 100.6 °C at 1 atm vs. 110.7 and 118.1 °C for pure toluene and acetic acid, respectively. 

Only in a few cases partially miscible mixtures present a positive or negative azeotropic point in the single-phase region, outside the miscibility gap, similar to the azeotropic points of homogeneous mixtures (Fig. 1, Types IV and VI). 

The AICI3-PQ5 system exhibits a negative azeotrope, dose to the composition AlCl3,PCl5. Considerable deviations were found in the molar volumes from those predicted by the additivity rule. ... 

Liquid mixtures having an extremum (maximum or minimum) vapor pressure at constant temperature, as a function of composition, are called azeotropic mixtures, or simply azeotropes. Mixtures that do not show a maximum or minimum are called zeotropic. Azeotropes in which the pressure is a maximum are often called positive azeotropes, while pressure-minimum azeotropes are called negative azeotropes. The coordinates of an azeotropic point are the azeotropic temperature t, pressure, and liquid-phase composition, usually expressed as mole fractions. At the azeotropic... 

An example of such a negative azeotrope is hydrochloric acid with a mass fraction whci =20 %(= 200 g kg ) that boils at 382 K (at standard pressure) without its composition changing (Experiment 14.4). 

A few binary mixtures exhibit negative azeotropes these are caused by negative deviations from ideality as reflected in activity coefficients that are less than unity. An example is the azeotrope formed by mixtures of acetone and chloroform, shown in Figure 9.10. Now both the Px and Py curves pass through minima at the same composition, while the corresponding Tx and Ty curves pass through maxima. Hence nega-... 

Here is the azeotropic pressure at absolute temperature T, while A and B are parameters whose values are obtained by fits to azeotropic data. This correlation works well for both positive and negative azeotropes, as shown in Figure 9.12. The principal drawback to (9.3.22) is that the azeotropic compositions remain implicit to find those compositions, we must solve the phase-equilibrium problem. 

Figure 9.12 Tests of the Clausius-Clapeyron equation (9.3.22) for correlating azeotropic pressures and temperatures. Circles are positive azeotropes formed in mixtures of methyl acetate and methanol (see Figure 9.11). Triangles are negative azeotropes that occur in mixtures of acetone and chloroform (see Figure 9.10). Lines are least-squares fits.

Binary homogeneous (single-phase) liquid mixtures having an extremum (maximum or minimum) vapor pressure P at constant temperature T, as a function of composition, are called azeotropic mixtures, or simply azeotropes. The composition is usually expressed as mole fractions, where for component 1 in the liquid phase and i for component 1 in the vapor phase are identical. Mixtures that do not show a maximum or minimum are called zeotropic. A maximum (minimum) of the P xi) or P(yi) curves corresponds to a minimum (maximum) of the boiling temperature T at constant P, plotted as a function of. Vj or yi [see T xj) and T(yi) curves. Types 1 and 111, in Fig.l]. Azeotropes in which the pressure is a maximum (temperature is a minimum) are often called positive azeotropes, while pressure-minimum (temperature-maximum) azeotropes are called negative azeotropes. The coordinates of an azeotropic point are the azeotropic temperature Taz> pressure arid the vapor-phase composition, which is the same as the liquid-phase composition... 

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