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.