Comparative Results With Plain And Improved Still-Head

The great superiority of the "evaporator " still-head is clearly shown by the results given in Table 58.

Table 58

Long plain still-head.

"Evaporator " still-head of 8 sections.

Percentage weight of methyl acetate recovered

48.1 .

76.8

„ ,, ethyl acetate ,,

53.5

79.8

„ ,, propyl acetate ,,

72.5 .

94.2

Number of fractionations required .

32

6

About 70

17

Percentage weight of material(1) Lost by evaporation and transference .

21.0

2.8

(2) Lost by treatment with reagents .

5.6 .

0.0

(3) Left undistilled ....

17.1

13.8

In the long-continued fractionation with the plain still-head, a considerable amount of moisture was absorbed by the esters and some hydrolysis took place. Before the fractionations were completed it was necessary to treat the propyl acetate with potassium carbonate and the methyl acetate with phosphorus pentoxide, and the loss was thus increased.

With the "evaporator " apparatus, on the other hand, the propyl acetate showed no acid reaction whatever, and the methyl acetate was only slightly moist.

The specific gravities were again determined and were found to agree well with those given on p. 112.

Fractional Distillation Under Reduced Pressure

The improved still-heads may be employed for distillation under reduced pressure. A 12-column Young and Thomas still-head, for example, was used by Francis1 for the separation of isoheptyl and normal heptyl bromides from the products formed by the action of bromine on the distillate from American petroleum coming over between 93.5° and 102°. The

1 Francis and Young, "Separation of Normal Isoheptane from American Petroleum," Trans. Chem. ]., 1898, 73, 920.

M pressure in this case was 70 mm. Wade and Merriman 1 employed an "evaporator" still-head for the distillation of mixtures of ethyl alcohol and water under pressures both lower and higher than that of the atmosphere (pp. 17, 59).

1 Wade and Merriman, " Apparatus for the Maintenance of Constant Pressures above and below the Atmospheric Pressure. Application to Fractional Distillation," Trans. Chem. Soc., 1911, 99, 984.