结晶时油状物的处理

结晶时油状物的处理

(in english)

treat about the oil just few ways ,followed :

The separation of a second liquid phase, commonly known as an

expected

crystalline

solid,

sometimes

occurs

during

recrystallisation.

The

oil

often

solidifies

on

standing,

although

at

times

a

considerable

period

may

elapse

before

crystallisation occurs. The resulting crystals will probably occlude some of the mother

liquor, and the purity will therefore not be high. If the substance has a low melting

point, it must not be assumed that this apparently abnormal behaviour is due to the

presence

of

impurities

;

it

is

probably

an

example

of

a

system

in

which

the

solid

phases consist of the pure components and the components are only partially miscible

in

the

liquid

state

(see

Section

1,18).

The

separation

of

the

oil

may

be

avoided

by

diluting the solution considerably, but this will lead to large losses. It is probably best

to

re-heat

the

mixture

until

a

clear

solution

is

obtained,

and

allow

it

to

cool

spontaneously; immediately the oil commences to separate, the mixture is vigorously

stirred

so

that

the

oil

is

well

dispersed

in

the

solution.

Eventually,

crystals

will

separate and these will grow in the bulk of the solution and not in a pool of oil, so that

occlusion

of

the

mother

liquor

is

considerably

reduced.

When

all

the

oil

has

disappeared,

stirring

may

be

stopped

and

the

crystals

allowed

to

accumulate.

Sometimes the addition of a minute quantity of the crude compound in order to

may facilitate the

initial

crystallisation.

Occasionally substances form

supersaturated solutions from which the first crystals separate with difficulty this is

sometimes

caused

by

the

presence

of

a

little

tar

or

viscous

substance

acting

as

a

protective

colloid.

The

following

methods

should

be

tried

in

order

to

induce

crystallisation:

—

1. By scratching the inside of the vessel with a glass rod. The effect is attributed to the

breaking

off

of

small

particles

of

glass

which

may

act

as

crystal

nuclei,

or

to

the

roughening of the surface, which facilitates more rapid orientation of the crystals on

the surface.

2.

By

inoculating

(

the

solution

with

some

of

the

solid

material

or

with

isomorphous

crystals,

crystallisation

frequently

commences

and

continues

until

equilibrium is reached. The

film

of

liquid

to

a

low

temperature.

Several

drops

of

the

solution

are

placed

in

a

test-tube or beaker and spread into a thin film by rotating the container the latter is

then cooled in a mixture of ice and salt or in some other suitable freezing mixture. A

better procedure, which avoids the necessity of subsequently scraping the surface to

remove the

low melting point, is to moisten a small glass bead with the supersaturated solution,

place it in a test-tube, cool

the latter in a freezing mixture and thus form crystals on the surface of the bead. The

glass bead can then be rolled out of the tube into the vessel containing the main bulk

of the solution.