Table 42

* Specific gravity at 0° compared with that of water at 4°.

The esters were redistilled over phosphorus pentoxide before their specific gravities were determined.

Complex Mixtures

The procedure in the case of complex mixtures is similar to that adopted when there are three components, but the number of fractions must be increased and the time required is longer.

If, in a complex mixture, there are two liquids boiling at temperatures not very far apart, together with others which have boiling points relatively much lower and much higher, the early fractionations do not appear, as a rule, to indicate the presence of the two liquids, but of a single substance with a boiling point between those of the two which are actually present, and it is only after some progress has been made with the fractionations that the presence of both liquids is clearly shown.

Separation Of Pentanes From Petroleum

A striking instance of this is afforded by the separation of isopentane (b.p. 27.95°) and normal pentane (b.p. 36.3°) from the light distillate from American petroleum.1 After treatment with a mixture of concentrated nitric and sulphuric acids to remove impurities, the "petroleum ether " consists chiefly of butanes and probably a little tetramethyl methane, all boiling below 10°, the two pentanes referred to, and hexanes and other hydrocarbons boiling higher than 60°, with very much smaller quantities of per.ta-methylene and trimethylethyl methane, both of which boil at about 50° ; there are no substances present except the two pentanes with boiling points between about 10° and 50°.

In order to effect the separation of the pentanes it is necessary to employ a very efficient still-head (Chapter XII (Modifications Of The Still-Head (Continued)).), but the method of arranging the fractions is the same as when an ordinary still-head is used.

Graphical Representation and Interpretation of Results.

- The fractionations will be referred to more fully later on (pp. 149 and 205), but it may be well here to consider the curves showing the results of the 1st, 4th, 7th, 10th and 13th fractionations (Fig. 36).

Fig. 36. - Separation of normal and isopentane from American petroleun.

The first curve, I., seems to indicate 2 that a single substance boiling at about 33° is being separated from others boiling considerably lower (butanes) and higher (hexanes), but, if that were really the case, the curves for subsequent fractionations should become more horizontal (indicating greater purity) at about that temperature, and more vertical at, say, 25° and 40°.

Instead of this, they become less horizontal at about 33° and the next curve, IV., approximates to a straight line between about 30° and 37°. This change is a sure sign that there are at least two substances present boiling at temperatures not very far apart, but little or no light is thrown on the actual boiling points.

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

2 Young;" Experiments on Fractional Distillation," Journ. Soc. Chem. Industry, 1900, 19, 1072.

Curve VII. is fairly straight from about 28.5° to about 35.5° but then becomes much more horizontal, terminating at 364°. This seems to indicate that the less volatile component boils at a temperature not far from 36° (the hexanes having now been almost completely eliminated).

Curve X. is distinctly more vertical in the middle ; it becomes nearly horizontal above and terminates at 36.3°, showing that the higher boiling point is a little over 36.0°. As the curve is more horizontal below than No. VII. there is evidence that the second component must boil not far from 28°.

The upper extremity of the curve XIII. is perfectly horizontal at 36.3° and the true boiling point of the less volatile component (normal pentane) is thus established ; the form of the lower part of the curve indicates that the boiling point of the more volatile component (isopentane) must be very close to 28° and further fractionation showed that it is really 27.95°.

Hexamethylene In American Petroleum

When, in a complex mixture, one component is present in relatively very small quantity, it may very easily be overlooked, and it is only by keeping a careful record of the results of the fractionations and especially of the values , or by plotting the weights of distillate against the temperature, that the presence of such substances can be detected.

Table 43

As an example, consider the distillate from American petroleum coming over between 66° and 91°.1 In this case, again, no satisfactory result could be obtained without the use of a very efficient still-head. The results of the 5th, 6th, and 7th fractionations between the above-named limits of temperature and of the 12th fractionation between 74° and 85° are given in Table 43. The 5th fractionation gave high values of from 61.5° to 70°, with a maximum at 67° to 68°.

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

Above 70° the ratios fell to a minimum at 78° to 84° and then rose again. Now, if there were no substance present between the hexanes and heptanes, the ratio would become smaller each time at the intermediate temperatures, and notably at about 80°; but it will be seen that in the next fractionation the ratio was slightly higher instead of lower. The number of fractions in the neighbourhood of 80° was therefore increased, and in the 7th fractionation a maximum value of was observed at about this temperature in addition to that due to normal hexane near 69°.

Further fractionation of the distillates coming over between 72° and 88° showed that there was really a substance present boiling not far from 80°, as will be seen from No. 12. It might be supposed that this substance was benzene (b.p. 80.2°); but that could not have been the case, for a 10 or 20 per cent solution of benzene in hexane boils almost constantly at nearly the same temperature as hexane itself, and, when American petroleum is distilled, almost the whole of the benzene comes over below 70°, mostly from about 63° to 68°. Moreover, in this case the aromatic hydrocarbons had been removed by treatment with nitric and sulphuric acids.

The quantity of liquid was too small to allow of a fraction of quite constant boiling being obtained, though considerable improvement was effected.