Rate Of Distillation

The mixture must now be slowly distilled : for laboratory purposes the drops of distillate should fall at the rate of about 1 per second, but on the large scale a much greater rate would be necessary. The slower the distillation the better is the separation and, although each distillation takes longer, time will on the whole be saved, because the same result will be attained with a smaller number of distillations. Unless otherwise stated, all the laboratory distillations recorded in this book have been carried out at the rate of 1 drop of distillate per second.

Systematic Fractional Distillation

The results of the fractionation 2 of benzene and toluene with an ordinary distillation bulb, now being described, are given in Tables 38 and 39; the read temperatures are not stated but only the true temperatures under normal pressure.

A flask of about 270 c.c. capacity was used for the first distillation and the bulb of the thermometer was covered with a little cotton-wool to prevent super-heating (p. 13).

First Fractional Distillation

In the first distillation the temperature rose almost at once to 86° and the first portion of the distillate was therefore collected in the 4th receiver. On the other hand the temperature reached 110.6°, the boiling point of toluene, before the whole of the liquid had come over. The distillation was therefore stopped, the apparatus allowed to cool and the residue in the flask weighed; this residue, amounting to 10.9 grams, consisted of pure toluene and did not require to be distilled.

1 When a mixture of unknown composition is to be distilled, we cannot decide upon the temperature ranges of the fractions beforehand. In that case, the height of the barometer must be noted and the thermometer must be read when fractions of suitable bulk have been collected. The thermometer readings may be corrected afterwards.

2 For the results obtained with an improved still-head, see Table 55 (p. 158).

Table 38

Second Fractionation

The flask of 270 c.c. capacity was now replaced by a smaller one - about 80 c.c. and the first fraction, that in receiver No. 4, was redistilled. A small quantity came over below 83.2° and was collected in flask No. 2 ; this receiver was removed and No. 3 substituted as soon as the temperature reached 83.2°, and No 4 was put in the place of No. 3 when the temperature had risen to 86.2°. At 89.2° the flame was removed and, after the cotton-wool on the thermometer had become dry, the second fraction from the first distillation - that in No. 5 - was added to the residue in the still. On recommencing the distillation it became clear that the temperature would not reach 86.2° for a considerable time and the first portion of the distillate was therefore collected in receiver No. 3. The process was continued as before ; flask No. 4 was substituted for No. 3 at 86.2°, No. 5 for No. 4 at 89.2° and the distillation was stopped at 92.2°. The contents of receiver No. 6 were now added to the residue in the still and the distillate was collected in Nos. 4, 5 and 6 ; the remainder of the fractionation was carried out in this manner except that, after the 11th fraction from the first distillation had been placed in the still, no distillate was collected in No. 9 because the temperature rose at once to 104.6° ; so also, after the addition of the last fraction, no distillate came over below 107.6°.