Mixtures of constant boiling point

By treatment of the alcohol with a mixture of constant boiling point hydrobromic acid and concentrated sulphuric acid the presence of sulphuric acid results, as a rule, in more rapid reaction and improved 3delds. A typical example is ... 

In the case of the fusion of a compound of two components with formation of a liquid phase of the same composition, the temperature is a maximum in the case of liquid mixtures of constant boiling-point, the temperature may be a minimum. 

In the distillation of petroleum, such difficulties are of common occurrence and are due to one or other of three causes —(a) to the presence of two substances, the boiling points of which are very close together (b) to the presence of one or more components in relatively very small quantity (c) to the formation of mixtures of constant boiling point. 

Data, more or less complete, for eighteen liquids are given in the following table (3). Where the value of Dg can only be approximately ascertained, the number is enclosed in a bracket. In a few cases, the boiling point and molecular composition of the mixture of constant boiling point only can be ascertained, and these values are given under d and w. 

List of Known Mixtures of Constant Boiling Point. 

Generally, if K is positive and r is less than unity, there must be a particular value of q for which log = loggr, and there must therefore be a possible mixture of constant boiling point. 

It has been stated (p. 71) that when a mixture of two substances is heated, the vapour is richer than the liquid in the more volatile of the two components into which the mixture tends to separate, whether these components are the original substances which were mixed together or a mixture of constant boiling point and one of the original substances.. ... 

Unless a mixture of constant boiling point, of the same composition as that of the original mixture, is formed, in which case the composition of the vapour would be the same as that of. the liquid. 

Mixtures of Constant Boiling Point.—Eeference has been made in Chapter IV. to the formation of mixtures of constant (minimum or maximum) boiling point. When a liquid contains two components which are capable of forming a mixture of constant boiling point, it is not possible to separate both components all that can be done is to... 

When a liquid contains three components which are not closely related to each other it may happen that both a ternary and a binary mixture of constant boiling point arc formed on distillation. In that case it is only possible to separate one of the original components in a pure state. These points will be considered more fully in Chapters XIII. and XV. 

By middle point is to be understood in aU cases tbe temperature midway between the boiling points of the two components", whether single substances or mixtures of constant boiling point, into which the original mixture tends to separate or, in the case of more complex mixtures, the temperature midway between the boiling points of any two consecutive fractions of constant boiling point. 

Mixtures of Constant Boiling Point.—For the sake of brevity, a mixture of constant boiling point containing two components will be referred to in this chapter simply as a binary mixture, and a mixture of constant boiling point containing three components as a ternary mixture. 

The quantity of a mixture of constant boiling point may be estimated by the distillation method in exactly the same way as that of a single substance. The methods of experiment and of calculation are similar in all respects. ... 

Determination op the Composition op Mixtures of Constant Boiling Point by Distillation... 

Binary Mixtures.—Since a mixture of constant boiling point behaves like a single substance on distillation, it is possible, if we know the composition of the mixture distilled, to calculate that of the binary mixture. 

In both cases mixtures of minimum boiling point, very rich in the more volatile component, are formed, so that the separation would be that of the mixture of constant boiling point from that component which is in excess. 

It appears to be only when the separation of the components (either simple substances or mixtures of constant boiling point) by distillation is exceedingly difficult that the method is inapplicable. 

Distillation Method.—In the last chapter it has been shown how the composition of a mixture of constant boiling point may be ascertained from the weight of distillate that comes over below the middle point, when a mixture of known composition is distilled. This method is generally, but not universally applicable. 

There are several other methods by which the composition of a mixture of constant boiling point may be determined. 

By Separation of Pure Mixture.—The most accurate method—applicable, however, only to those mixtures for which the first method can be employed—is to separate the mixture of constant boiling point in a pure state by fractional distillation and to determine its composition either (a) by chemical analysis, (6) by the removal of one component, (c) from its specific gravity, or (d) from its refractive power. 

This method was employed by Eyland (6) to ascertain the approximate composition of the large number of mixtures of constant boiling point examined by him. 

C. Rylaud, Liquid Mixtures of Constant Boiling Point, Amer. Chem. Joum., 1899, 22, 384. 

It has been pointed out that there are many cases in which the two components of a mixture cannot be separated by fractional distillation, owing to the formation of a mixture of constant boiling point. It is, however, sometimes possible to eliminate one of them by adding a third substance and then distilling the mixture. 

Formation of Ternary Mixture of Minimum Boiling Point.—The substance added frequently forms a ternary mixture of constant boiling point with the two components of the original mixture, but the relative weights of these components in the ternary mixture differ from those in the binary mixture of constant boiling point which they themselves form. 

Amount of Alcohol Recovered.—No doubt the quantity of alcohol actually recovered would be somewhat less than this, but it should be at least 20 grams, giving a total of, say, 54 grams out of 103-7. Moreover, the remainder of the alcohol, except the small amount actually lost by evaporation, could be recovered in the form of the alcohol-water mixture of constant boiling point. 

Methyl alcohol and benzene cannot be separated from each other by distillation, because their properties are so dissimilar that a mixture of minimum boiling point is formed, and, as its boiling point is much lower than that of the binary benzene-water mixture, no separation can bo effected by adding water and distilling. A ternary mixture does not come over, but the first fraction still consists of the benzene-alcohol mixture of constant boiling point. 

Proofs of Purity.—Even, however, if the true boiling point of the pure substance is known, and if the observed boiling point agrees with it, we cannot be absolutely sure of the purity of the liquid unless we know—from the method of preparation—that there is no possibility of the formation of a mixture which distils without change of composition at practically the same temperature as the pure liquid. If we can be sure that the original mixture contains only substances which are very closely related to each other, such as members of a homologous series, the problem will be much simplified, for it may then be concluded with certainty that no mixture of constant boiling point can be formed. 

Ethyl Alcohol and Water.—Ethyl alcohol containing, say, 16 or 20 per cent, of water would behave in the manner indicated by the same curve (Fig. 65), but, in this case, even svith the most efficient still-head, we should not have pure alcohol in the first part of the distillate, nor even the pure mixture of constant boiling point, but a mixture containing at least 5 and probably. as much as 7 or 8 per cent, of water, for the mixture of constant boiling point which contains 95 57 per cent, of alcohol is extremely difficult to separate from water, although there is a wide difference between the two boiling points. 

---