Causes Of Confusion

But confusion is apt to arise when,.

(a) there are two substances present with boiling points very near together.

(b) one of the components is present in relatively very small amount; or.

(c) two or more of the substances form mixtures of constant boiling point.

a. Two Components boiling at nearly the same Tempera-ture. - If there are isomeric members of a homologous series present in the mixture, it is very likely that their boiling points may in some cases be very near together.

Pentanes In Petroleum

An instance of this has already been referred to 1 in the case of the light distillate from American petroleum consist ing chiefly of butanes, pentanes, and hexanes. Even with the very efficient still-head shown in Fig. 53 (p. 150), the results of the first distillation seem to indicate the presence of only a single substance between the butanes and. hexanes, boiling at about 33°; and it is only after repeated fractionation that the presence of both normal and isopentane (boiling points 36.3° and 27.95°) is clearly shown. The fact, however, that for the middle fractions the values of dimmish, while for those below and above them they increase, is a clear indication that we are not dealing with a single substance.

1 Young and Thomas, " Some Hydrocarbons from American Petroleum, I, Normal and Isopentane," Trans. Chem. Soc, 1897, 71, 440.

This' may be seen from Table 77, in which the results of the first three fractionations are given ; it will be seen that, in the first fractionation, the highest values of are those for fractions 5 and 6 ; in the second, the fractions that have the highest values are Nos. 5 and 7, and in the third they are Nos. 4 and 7.

Table 77

Considering 5 and 6 as one fraction the values of for the first three fractionations would be 204, 167, 121, respectively; the values for No. 4 in the three fractionations are 58, 123, and 172, and for No. 7 they are 168 and 193 in the second and third fractionations.

The whole course of the separation is, however, better seen in the diagram (Fig. 36, p.. 113). Both the pentanes may be obtained in a pure state by distillation only, or, at any rate, after removal of small quantities of impurity by means of a mixture of nitric and sulphuric acids. It does not appear, however, than any hydrocarbon that boils at a higher temperature than normal pentane can be so separated.

Hexanes In Petroleum

Let us consider, for example, the hexanes.1 A preliminary distillation through an ordinary or fairly efficient still-head appears to indicate the presence of a single substance boiling at about 66°, and after a few fractionations with a very efficient still-head it is seen that a further separation, apparently into two components, as in the case of the pentanes, is taking place, but after long continued fractionation it is found that the separation does not end there. Two series of fractionations were carried out, one of American, the other of Galician petroleum.

1 Young, " Composition of American Petroleum," Trans. Chem. Soc, 1898, 73, 905.