DRYING SAMPLES

A number of techniques are employed to remove water from samples and to protect them from atmospheric moisture.
To dry solids, first remove as much water as possible by suction
If the solid is thermally stable compound, heat can be used to dry off the remaining water by placing the solid in an oven set at 20deg Celsius below its melting temperature and drying it to constant weight.
Ovens and drying cabinets are also used to dry apparatus which will be used to handle moisture sensitive compounds.
Remember that solid dried in an oven must be transferred to a desiccators to cool down.

Otherwise it will pick up moisture on cooling
The desiccators contain a drying agent which absorbs moisture from the air in the desiccators.
There are a range of drying agents which provide the means of drying gases, liquids, and those solid compounds which are not thermally stable enough to be dried in an oven.
All of these can be used in desiccators.
Its self-indicating silica gel has advantages which make it the usual choice.

It contains a cobalt salt indicator that is blue when the silica gel is active and turns pink when the silica gel as absorbed so much water that it is no longer an effective drying agent.
A further advantage of this drying agent is that once wet, it can be placed in an oven set at 125deg Celsius to dry off the water.
The return of the blue color shows that the active form had been regenerated.
Phosphorus pent oxide is more efficient in removing water than silica gel and it’s sometimes used when very high hygroscopic compound are stored.

But it is extremely reactive and should be used with caution
To keep out moist air, the desiccators’ lid must form an air tight seal
In this simple atmospheric desiccators, the ground glass face must be greased to make the seal air tight
Such simple atmospheric desiccators are best used for storing hygroscopic materials and equipment such as infra-red plates and cells
And of course they are also used to prevent oven dried materials absorbing atmospheric moisture while they cool.
Silica gel is also used in desiccator’s cabinet. These can be used for the long time storage of moisture sensitive items such as thin layer chromatographic plates.
And other larger pieces of equipment that need to be kept dry.
Vacuum desiccators provide a mild means of drying samples without the risk of decomposition at a high temperature of an oven
Before using a desiccators, place some fresh drying agent in the base. Replace the tray.
Ensure that the O- ring is clean and in good condition whether the ground glass flange is greased.

Put the sample inside the desiccators and fit the lid.
Glass vacuum desiccators should always be place in a safety cage before evacuating it and left there while it is under vacuum.
Attach the holes from the vacuum line and open the tap on the lid.
Then with the vacuum line fully opened or with the water pump at maximum flow, slowly closes the bleed valve on the trap to evacuate the desiccators.
Modern plastic vacuum desiccators do not present the same risk of explosion and can be evacuated safely without the cage.
Evacuate the desiccators for at least 15minutes and then close the desiccators tap.

Leave the desiccators under vacuum for a minimum of 1hour.
To simply and safely return the desiccators to atmospheric pressure place the filter paper against the inner tube and gently open the tap.
When the filter paper falls off, the desiccators are at atmospheric pressure and are ready to open.

If the tap is open quickly there will be a violent in rush of air which may have disastrous consequences for the sample.
With very fine samples additional protection can be afforded by covering it with a watch glass.
To ensure that the sample is dry weigh the sample on the watch glass, repeat the drying procedure and re- weigh.
The sample is dry when successive weighing are constant.
Liquid samples can be dry by weighing mixing them with solid drying agent.

It is of cause essential that the drying agent does not react with the liquid and is not soluble in it.
The usual choices for organic liquids are anhydrous magnesium sulphate or powdered molecular sieve.
Add the drying agent in small quantities and swirl the flask,
Initially if the liquid is very wet, the drying agent will form lumps which stick to the flask, add more drying agent until free powder is suspended on the content of the flask is swirled it settled on standing  to give a clear solution.

Stopper the flask and leave it to stand for half an hour for with occasional agitation to ensure thorough drying.
Then remove the drying agent by filtration through glass or cotton wool plug, or on the suction using a harsh fan.
Glasses may be dried by passing them through column of solid drying agent such as molecular soap.

The drying tray should consist of a drying tower or tube packed with cold granules of drying agent which will not block the flow of gas.
a bubbler containing paraffin oil to indicate the gas flow and trap any fine particles of drying agent add an empty threshold bottle  to ensure that no oil droplet is carried over  into the reaction flask  and to prevent the possibilities of reaction mixture been sucked back into the oil buckler.

If the gas source is a cylinder, first ensure that the middle valve is closed and the adjoining taps to the reaction vessel is open.
Then open the cylinder head valve and adjust the regulator valve to give a pressure of 3-5 pants per square inches.
Only then, open the delivery middle valve slowly until there is a steady stream of bubbles in the buckler.

As the reaction is to be under dry conditions the reaction flask should be fitted with the bubbler which allows excess gas to exceed, gives a further check on the flow rate that prevent air entering the apparatus.
Another way of keeping moisture out of an apparatus without totally sealing it, is to use a drying tube filled with desiccant such as molecular soap or an hydrous calcium chloride.
Calcium chloride soap should be used with caution since when saturated with moisture it is liable to block with might cause a pressure build up inside the apparatus.

CONTINOUS FLOW EXTRACTION

When a substance is to be extracted from as solvent in which it is not very soluble. Into one in which it is, then the use of the separating funnel with 3 batches of fresh solvent is usually sufficient.
A method of continuous extraction is used when solubility are less favorable or when it is necessary to extract all the material as in reaction mechanism ………

Two types of apparatus are used. One for extraction from a less dense solvent into a more dense solvent and the other for extracting from a more dense into a less dense solvent.

The most common method is to extract from a relatively dense solvent such as aqueous alcohol into a less dense solvent such as pentane
Whichever method is used, the solvent chosen for the extraction must not be miscible with the solution that is to be extracted.

To set up the apparatus for extraction with the less dense solvent, start by adding the material to be extracted to the sample reservoir flask.
Add the extracting solvent to the reflux flask .
And place the flask in the heating mantle.
Add a few anti-bumping granules then connect the two flasks to the extraction apparatus
The solvent delivery tube on the right should reach close to the bottom of the sample reservoir.
Clamp the apparatus and secure the flasks with clips.
Finally add a reflux condenser .
The water should be secure with wire or clips since the apparatus may be left refluxing for long periods.
Then start heating.

When the solvent in the reflux flask is boiling, its vapor passes across the side arm up into the condenser above the sample flask.
Condensate from the condenser drips into the funnel shaped top of the solvent delivery tube.
There should be a small space between the funnel and the base of the condenser to allow the vapor free access to the condenser
The extracting solvent collects in the delivery tube until a head is formed which forces droplets of solvent out into the bottom of the sample reservoir.
Here the light droplet of solvent rise up through the denser sample solution extracting solute as they do so and forming an upper solute rich layer.

This upper layer accumulates until it reaches the side arm which acts as a ware controlling the return of  solvent to the reflux flask.
As the extracting solvent circles through the system, the solute is concentrated to the reflux flask
The apparatus can be left running for long periods to ensure complete extraction.
In difficult cases, it can be left running for several days
The time needed depends on relative solubility of the material in the two solvents.

The apparatus for extraction with the solvent that which is denser than the sample solution, is designed to produce a continuous descending stream of solvent droplets.
Commonly, these denser solvents are halogenated compounds such as dichloromethane
First, assemble then extracting apparatus and reflux flask then add the extracting solvent to the sample reservoir until the level is a couple of centimeters below the side arm connecting it to the reflux flask which should already contain a few anti-bumping granules
Then, carefully add the material to be extracted on top of the solvent
As the sample layer builds up, it starts to dry the denser lower layer of solvent cross into the reflux flask.

Take care not to add too much sample
Some solvent should always remain at the bottom of the sample reservoir to prevent the sample solution from entering g the tube to the reflux flask.
The solvent delivery tube has holes at the top to allow vapor access to the condenser.
At its base, it has  a glass fret to break the descending solvent into small droplets.
Finally, add the condenser.
Note that the condenser water supply tube must be secured as the apparatus will often need to be left working for long period of time.
Turn on the water, and start heating.
When the solvent in the reflux flask is boiling, the vapor passes up across the upper side arm into the condenser..
Drops of condensate collect in the delivery tube and are channeled to the frit.

A fret forms small droplets with its relatively large surface area increases the rate of solute extraction as the solvent descends through the sample solution in the reservoir.
For solute rich solvent collects at the bottom and then returns through the side arm to the reflux flask where the extracted material is concentrated.
So in both types of apparatus, on completion, all the extracted material will be in the reflux flask together with some solvent.
Solvent in the sample reservoir should be disposed off in the appropriate waste solvent container.