Table 61. Isoamyl Alcohol and Benzene Boiling points - Benzene, 80.2° ; isoamyl alcohol, 132.05° ; middle point, 106.1°

Here the agreement is very satisfactory.

One Component In Large Excess

That a single distillation may be sufficient when the more volatile of two components is present in large excess, while two or more distillations are necessary when it is present in relatively small amount, is further shown by the following results.

A mixture containing 90 grams of benzene and 10 grams of toluene was distilled through an evaporator still-head of three sections.

Weight below middle point: observed, 89.6 ; corrected, 89.9. Percentage of benzene in mixture : taken 90 0 ; found, 89.9.

When 100 grams of a mixture containing only 10 per cent, of benzene was distilled through the same still-head, very little came over below the middle point, 954°, and the quantity was too small to admit of fractional distillation. By twice redistilling all that came over below 110.6°, however, the weight below the middle point rose to 9.0 grams. Allowing a loss of 0.3 gram for each distillation the corrected weight would be 9.9 instead of 10 grams.

With a larger quantity, 250 grams, a fairly satisfactory result was obtained even with a "pear" still-head. In this case the distillate was divided into three fractions, and the following results were obtained :Table 62

The weight below the middle point is clearly approaching a limit, the increase being smaller each time; allowing 0.3 gram for loss by evaporation in each distillation the last weight would be 23.2 + 1.5 = 24.7, and the percentage 9.9 instead of 10.

Advantages Of Efficient Still-Head

A great saving of time is, however, effected and a more certain result is obtained by the use of a very efficient still-head. Thus, on distilling 300 grams of the above mixture through an 18-column Young and Thomas still-head, the first distillation gave 21.4 grams below the middle point, and a total of 76.1 below the boiling point of toluene. On redistillation of the 76.1 grams the weight below the middle point was 29.2 grams, so that with no correction for loss by evaporation the calculated percentage of benzene would be 9.7, and with the larger still-head it would be fair to allow 04 gram for each distillation, which would bring up the weight to 30.0 grams, and the percentage of benzene to 10.0.

Mixtures Of Three Components

The fractionation of a mixture of methyl, ethyl, and propyl acetates with a plain vertical still-head one metre in length has already been referred to (p. 105), and some details have been given. With so large numbers of fractions and of fractional distillations, it would not be possible to distribute the loss by evaporation, and by transference from receiver to still, between the different fractions, but the total loss was ascertained in each fractionation, and we may calculate the percentages on the total quantity of material left at the end of each operation instead of on the original quantity taken.

The boiling points of the three esters are 574°, 7745°, and 101.55° respectively, and the two middle points are therefore 674° and 89.35°. The percentage weights of distillate below 674°, from 674° to 89.35°, and above 89.35° were read from the curves (Fig. 35, p. Ill) and are given below.

Table 63

It will be seen that the numbers remain nearly constant after the first three fractionations, and that the mean percentages calculated from the last nine distillations agree fairly well with those in the original mixture. The actual loss by evaporation must have been greatest for methyl acetate and least for propyl acetate, and the calculated percentages are too low for the first and too high for the second of these esters.