Laboratory Operations- Melting point determination and rotary evaporation

What is a substance’s melting point

it is the temperature at which the solid and liquid phases of the substances are in equilibrium at a presence of 1 atm and is a physical characteristic property that is unique for a given substance

What can the melting point of a substance be used for

it can be used to determine the purity of the substance by comparing it with the theoretical melting point and can be used to identify an unknown substance by comparing it with the theoretical melting point

How do you determine a melting point of a substance

it is determined by being pulverized on a watch glass and tapped 1-2mm into the open end of a capillary tube

What is the capillary tube placed into

it is placed into a Mel Temp apparatus and the heating is adjusted with a dial

What is the dial on the Mel Temp apparatus usually set to

it is usually set on 7 to approximated the melting temperature (orientation melting point)

What happens once an approximate melting point is obtained

the machine is cooled to fifteen degrees below the orientation melting point and then slowly heated up again

What two temperature make up the melting point range

the initial temperature where the substance begins to melt and the final temperature when all the solid has turned into liquid

What is the melting range of a pure substance

a substance is pure when it melts with a range of no more than 1-2 degrees celsius

What is the rotary evaporator or rotovap

it is the fastest more efficient and most environmentally friendly way of removing volatile solvents from a non-volatile sample

how doe the rotovap work

it works by increasing the rate of evaporation of the solvent by reducing the pressure to lower the solvent boiling point, rotating the sample to increase the effective surface area and heating the solution

how do you adjust the vertical position of the glass apparatus

you use a manual lift knob control on the right side of the stand so that there is enough space above the water bath

What the of flow is sufficient for the cooling water on to the condenser coils

a gentle flow is sufficient

What does the stopcock hole need to be lined up with

it needs to be lined up with the vent hole

what marks are one the stopcock

it has a blue mark on the knob of the tap indicating the side with the hole

how full should the round bottom flask with the sample solution that need to be evaporated be

it should be no more than half full

what is the evaporation flask attached to

it is carefully attached to the ground glass joint and secured with a slip, do no use grease

What needs to be used if the flask neck size doesn’t match the joint on the rotary evaporate

a reduction adaptor needs to be used

what does closing the stopcock do

it generates a vacuum in the apparatus

how far should you lower the evaporation flask

you should lower until the lower surface of the glass is just submerged into the water bath

what should you do if the solution starts to boil vigorously

you should release the vacuum with the stopcock

what can you do once the flask is stable

you can remove you hand from the tap and leave the solution to evaporate

what should you do if evaporation is slow

turn on the heat to the thermostatic water bath and adjust eat accordingl

Why does the rotovap apparatus not need to be set at the normal boiling point of the solvent

it doesn’t need to be set at the normal boiling point because the apparatus is under a vacuum so 4-5- degrees celsius is usually sufficient