General Laboratory Procedures, Chemistry tutorial

Introduction:

It could be a very conquering experience to walk into a laboratory lacking understanding the use of the apparatus or equipment for your use. Thus before you begin the practical classes it will be convenient to know the use of some of the apparatus you shall meet.

Balances and weighing:

Most of the experiments in chemistry comprise weighing at different steps. Much time can be lost throughout weighing procedures, and one of the principal time wasters is the habit of weighing to a degree of precision in surplus of the requirements of the experiment. For synthetic work, weighing to 0.1 g or 0.01 g is quite adequate. Only for analytical work, is greater accuracy requisite, on the order of 0.001 g or 0.0001 g.

Even if weighing is merely effected to the necessary degree of accuracy, time can be wasted in the real process, and unless some process is employed whereby weighing is taken out rapidly, most of the experiments can't be done in the time generally available.

At no time are chemicals to be weighed directly to the pans of the Analytical Balances.

Accurate weighing technique: weighing by difference

To weigh a correct amount of solid (that is, to the nearest 0.001 g or better), put a weighing bottle (and cap) on a balance, tare it, take away the bottle from the balance, and put an estimated amount of material into the bottle. If the solid includes large crystals or lumps it must be lightly ground in a mortar before weighing.

The weighing bottle with contents is now capped, wiped clean and weighed by employing the correct method on the analytical balance. If the weight is considerably dissimilar from desired, remove the bottle from the balance and repeat the above method until close. Instantly record the weight in your note-book.

Return to the laboratory and tip the solid into your flask, utensil or whatever is appropriate, no attempt being made to eliminate the traces of solid which will cling to the weighing bottle. Return to the balance room and re-weigh the almost empty bottle accurately. The loss in weight is the precise weight of solid taken. This avoids the rather awkward method of washing the whole solid from the bottle and is fast and more precise. This process is frequently used, as it is rarely essential to weigh out a precise amount. This is bad practice to weigh out, for illustration -1.25 g of a solid to make an exact 0.10 M solution. It is better to make use of the above method, finish up with a weight of 1.32 g and state the solution as:

(1.32g⁄12.6L)M = 0.0105M

This ignores the very messy practice of removing and adding odd crystals to try to get a precise weight.

By employing the above method it is never essential to have any loose chemicals near a balance, as only a closed bottle is employed on the balance.

Setting up apparatus:

Whenever ground-glass joints are utilized, it is not essential to lubricate them apart from whenever high temperatures or vacuum are involved or an inert atmosphere is very important. If a joint becomes seized, try the given methods of loosening it:

(i) Rock the cone in the socket

(ii) Tap the joint softly by a block of wood

(iii) Warm the joint in a small flame and then tap smoothly

(iv) Soak the joint in penetrating oil and then try tapping.

A general cause of seizure is a caustic alkali. Try to keep alkalis off the ground-glass, and if they do get on it, wash carefully as soon as possible.

Seizures can generally be avoided via disassembling the apparatus instantly after use. Where needed, the procedures call for lubricating the joints by silicone grease (that is, high vacuum grease).

CAUTION: Silicone grease might cause corneal damage. In order to ignore accidental transfer of grease to your eyes, be sure to thoroughly wash by soap and water to get rid of residual silicone grease from your skin.

Care must for all time be taken, whenever glass apparatus is set up, to avoid strain. It is best to begin with one piece, and builds from there. Take a distillation apparatus as an illustration:

a) Lightly clamp or fix the flask at a height suitable for heating.

b) Attach the still-head, screw-cap adapter and thermometer (that is, no more clamps are required for these)

c) Attach the rubber tubing to the condenser, then place a clamp and stand in such a way that the condenser will rest on the lower, fixed, side of the clamp. Attach the condenser to the still-head, and clamp lightly.

d) Attach and support the receiver adapter and the receiver.

The similar process must be followed for the other assemblies.

Reflux and distillation:

Dissimilar to ionic reactions, which are often extremely rapid, reactions between the covalent substances tend to be slow. Specifically in main-group and Organometallic reactions, it might be essential to keep a reaction mixture hot for the matter of hours. This, coupled by the fact that volatile and inflammable solvents should be used, makes it essential for special equipment to be employed.

Reflux:

The utilization of a reflux condenser is often required. It is employed whenever a reaction mixture has to be kept boiling for the appreciable time and the solvent is volatile. The water condenser might be employed for solvents boiling up to around 130 °C, and for higher boiling-point solvents an air condenser is enough. The flask should never be filled more than half way; the size of flask must thus be selected by consideration of the total volume of the reaction mixture. The boiling stone or similar substance must be employed to promote even boiling for all the reflux methods which don't make use of magnetic stirring.

The main purpose of the apparatus is to keep the solution hot with no loss of solvent. It is pointless to boil viciously; the heating must be controlled in such a way that the solution is just simmering. The flask might be heated by an electric heating mantle controlled via a Variac (never plug a heating mantle directly to the electrical outlet), or by employing an oil bath on an electric hot plate.

Distillation:

The main purpose of distillation is to purify a liquid, or to eliminate a solvent from the solution. The flask should never be more than half full, a boiling stone or magnetic stirring should for all time be employed, and the choice of condenser is similar as for reflux work. The heating of the flask might be accomplished by using any of the usual means. Purification of a liquid through distillation is best performed at a rate not exceeding two drops of distillate per second. On the other hand, eliminating a large quantity of solvent might be done much more quickly.

Fractional distillation:

The aim of fractional distillation is to separate two liquids of different boiling-point. As by other forms of distillation, the flask should never be more than half full, and a boiling stone or magnetic stirring should for all time be used. In order to get the good separation of liquids, it is necessary that the distillation be carried out very slowly. The slower the distillation, the better the separation. A rate of one drop of distillate per second must be the aim.

As the efficiency of the method based on the fractionating column reaching thermal equilibrium (there must be a gradual increase in temperature from the top to the bottom of the column), best outcomes are obtained if drafts are excluded. Moreover, the source of heat must be steady.

Use of the separatory funnel:

The separatory funnel is employed for several vital procedures. Apart from care is taken; its use can be one of the main causes of mechanical loss. The choice of size is specifically significant and, as with flasks in distillation, the smallest that will correctly do the job, is best. 

Separating two immiscible liquids:

The liquid mixture is poured to the funnel and the funnel is smoothly agitated to assist in the separation into layers. The funnel must for all time be stoppered, however if a specifically volatile substance, such as ether, is present, the funnel must be vented occasionally via the stopcock (hold it slightly inverted while doing this) to evade the possible buildup of pressure.

Whenever separation into layers has occurred, the stopper is detached and the lower layer drained into a small flask. Swirling the funnel and once again allowing separation to take place often provides a further small sample of the lower layer.

The top layer is poured from the top of the funnel to a second flask. This is a wise safety measure to always keep both liquids, even if one of them is to be rejected. It is surprising how frequently the wrong layeris thrown away.

Washing a crude liquid:

One of the most general methods comprising of shaking a crude liquid product by an aqueous solution to remove some of the impurities. The reagents must for all time be employed in small quantities, and the procedure repeated if essential. Mechanical loss is for all time greater whenever large volumes of washing solutions are employed.

Gases are often made up in substantial quantities throughout the cleaning process; therefore, it is necessary to release the pressure regularly. This is best done via inverting the well-stoppered funnel and turning on the tap.

If the requisite substance is the top layer, then allowing the bottom layer to run off is quite simple. The whole bottom layer of waste must not be run off each time. It is better to leave a small quantity of the aqueous solution, and add further fresh reagent. Careful separation is completed only when running off the last of the different washing solutions. This avoids the risk of unintentionally losing a few drops of the treated product.

Whenever the required substance happens to be the bottom layer, avoiding mechanical loss becomes harder. If the product is run off between each and every wash and then returned to the funnel for the next, the loss can become extremely great. The best compromise is acquired by employing rather large volumes of washing solutions, and decanting the spent solution from the top of the funnel. In this manner the product never leaves the funnel till the final wash is over. It is then run out into its receiver, leaving the final washing solution in the funnel. 

Liquid extractions:

The separatory funnel is frequently used to extract a solute from one solvent by means of a second solvent immiscible by the first. The elimination of a solute from water by means of ether is one of the most general illustrations of this application. The size of the funnel is selected to accommodate the whole of the aqueous solution. This saves time that would otherwise be spent in repetition. A sequence of extractions by a small quantity of ether is much more efficient than one with a huge amount of ether. In practice the volume employed is that which provides the smallest manageable top layer, bearing in mind that the ether solution should be decanted from the top of the funnel. If the layer is too small, decantation becomes hard. The solution is generally extracted around three times by fresh quantities of ether, and all of the ether extracts are decanted into one flask. Subsequent to the final extraction the aqueous layer is run off and the last ether layer decanted fully.

Filtration methods:

There is a diversity of methods used for the separation of a liquid or solution from a solid.

Simple filtration:

The usage of a filter funnel and a piece of filter paper folded into four is generally reserved for the ionic substances (example: NaCl) precipitated from the aqueous solution. Precipitates obtained in the qualitative analysis and inorganic experiments are often instead fine, and can't be well filtered by using a pump. Covalent solids, though, are usually separated from the volatile solvent, and the relative slowness of simple filtration leads to the complications caused due to evaporation.

It is vital in simple filtration to make sure that the paper is carefully folded. The paper should be fixed cautiously into the funnel and wetted completely with water, or the appropriate solvent, prior the filtration.

The contents of the filter paper must remain at least 1.5 cm from the top of the paper. Such simple precautions can make an extreme difference in the time needed for a filtration to be completed, and must never be overlooked.

Buchner funnel and filter pump:

This technique of filtration is the most widely utilized whenever dealing with recrystallized substances. The Buchner funnel might be joined to the flask by means of a cork, however a much more helpful device includes of a flat piece of rubber by a hole in the center capable to receive the funnel stem and forming a good seal.

The disc of rubber allows, reasonably, any size funnel to be fitted to any size flask. If this process is adopted, then the size of the funnel chosen is the least that will include the solid, and the flask is as well chosen to be the least which will contain the liquid, if both solid and liquid are essential. If the solid is to be rejected, then a large funnel can be employed to raise the rate of filtration. On the contrary, if the liquid is to be rejected, then the flask might be large adequate to contain the whole liquid in addition to the washings. The choice of size is extremely vital, as mechanical loss throughout filtration can be important.

The filter-paper disc is positioned in the funnel, and wetted by the solvent present in the solution to be filtered. It is vital that the funnel and flask be completely dry. If the solvent involved is ethanol, then the paper might be wetted by water. If available, connect the suction flask to a Woulff bottle. The pump is then turned on and the paper pressed to place. Throughout filtration, the pump should never be turned off, as this might cause water from the pump to be drawn back to the filtrate. Whenever all of the material has been filtered, turn on the stopcock of the Woulff bottle to ambient pressure (or detach the pump from the flask) as the pump is still running. If a few solid has not been transferred to the funnel, a part of filtrate can be retrieved and employed for rinsing the residue to the funnel.

The solid is washed free of filtrate via pouring a small part of chilled fresh solvent to the funnel while the pump is disconnected.  Finally, the solid is drained as dry as possible via suction from the pump as applying pressure by a clean glass stopper.

Gravimetric filtration:

In quantitative work it is significant that entire solid be transferred and retained in the filter funnel. A filtering crucible having a porous sintered-glass bottom is the most proper apparatus to use. Porosities from 0 (coarse) to 5 (very fine) are available; however for most purposes a porosity of 3 is best; some fine precipitates will need a porosity of 4.

Before usage, the sintered-glass crucible is dried in an oven, cooled and precisely weighed. To gather the solid the pre-weighed crucible is set in the mouth of the Buchner flask by means of a firm rubber cone. The pump is turned on, and as much supernatant as possible is decanted off via the crucible. The liquid must be directed into the crucible through a glass rod.

The solid is then transferred, by using a gentle jet of the suitable solvent to swill out all particles. Whenever solid clings to the apparatus, it can be collected by using a glass rod protected by a rubber 'policeman'. The pump suction at this phase must be as gentle as possible; or else the porous glass might clog. Lastly, the solid and crucible are washed repeatedly to get rid of all soluble materials, and dried to constant weight.

Drying methods:

The drying of liquids:

Extreme drying is not generally essential by organic liquids and drying agents like anhydrous calcium chloride or anhydrous sodium sulphate are sufficient. Of the two, calcium chloride is the more efficient, however as well the more messy.

As calcium chloride will eliminate water and ethanol, it is used when both require removing. If, though, the drying only requires removing water; anhydrous sodium sulphate is usually used. Sodium sulphate will only work at temperatures under 30 °C and must usually be employed at room temperature. It is capable to remove its own weight of water, however the use of too much drying agent must not be done too often as this will cause the drying agent to become 'wetted' by the product and a big mechanical loss will be required.

In order to dry the organic liquid, whether a product or a solution having the product, the liquid must be placed in an appropriate sized conical flask, fitted by a good stopper or cork, and the drying agent added. The corked flask must be shaken at intervals, and left for at least five minutes, if possible longer.

If adequate drying agent has been employed some must remain unchanged in appearance: that is, a fine opaque powder of sodium sulphate or firm granules of calcium chloride.

The drying of solids:

Different methods exist for drying the solid materials. Whenever deciding which process to use it is significant to know somewhat about the physical properties of the material. For illustration: if dehydration of a hydrate or melting of the organic solid takes place, Recrystallisation will have to be repeated resultant in further loss of material and time.

However, the process of air drying takes longer than the others; it is one of the safest for non-deliquescent solids. The damp solid, drained as dry as possible on the filter, is transferred to the watch-glass and spread out uniformly. The solid can then be left to dry overnight in a place free of dust and drafts. As an added precaution against the dust, a second, bigger watch-glass must be positioned over the product in such a fashion that free evaporation remains possible. However, the desicator is ideal for drying many solids; care should be taken whenever drying hydrates. It is quite likely to lose some water of crystallization if the dehydrating agent is too efficient. Therefore, samples to be dried must be spread out on a watch-glass and labeled by their name and date. The desicator should be regularly recharged by fresh desiccant, and the ground-glass seal kept greased by a minimum quantity of silicone grease, as a result it appears transparent. 

It is significant to keep in mind that after opening, a desicator takes at least (2 hours) two hours to re-establish a dry atmosphere.

A vacuum desicator is employed to speed the drying of a sample. The sample should be covered by a second watch-glass and the desicator emptied and filled slowly to avoid blowing the sample about. In order to guard alongside implosion, a vacuum desiccator should be covered by strong adhesive tape, or be enclosed in the special cage whenever being evacuated and de-evacuated.

Recrystallisation and purification of solids:

Inorganic solids, whenever first prepared, are hardly ever pure. The original solid should be recrystallized from a suitable solvent. If the solvent is a flammable liquid, as it frequently is, it is better to carry out a Recrystallisation in reflux, till more experience has been gained. With ethanol, an extremely common solvent, it is quicker and neater to make use of a conical flask, however this does require a risk of fire.

Reflux method:

The solid is positioned in an appropriate sized flask, preferably a conical flask as it can be simply put aside to cook, and a condenser attached. The small volume of solvent is poured down the condenser and the mixture is increased to its boiling point. If the complete solid has not dissolved, a bit more solvent must be added after eliminating the mixture from the hot plate. Repeat this method till the solid just dissolves at the boiling-point. If there are no insoluble solid impurities, the solution will be clear. The mixture must then be eliminated from the hot plate and slowly allowed to cool to room temperature. Once the solution reaches room temperature, it might be essential to gently swirl the flask in order to prime crystallization. The solid generally crystallizes on cooling; however, if crystallization is slow to begin, scratching the inside of flask by a glass rod often assists crystals to form. The flask must be cooled to at least room temperature, or if possible lower, by positioning it in either iced water or a refrigerator.

The pure product is filtered off at the pump. It is necessary for both the filter flask and funnel to be clean and dry, apart from for the solvent concerned. The mixture to be filtered is poured on to the filter paper and the solid remaining in the flask is washed out by the filtrate. This is significant. The filtrate is, obviously, a saturated solution of the required solid, and therefore the filtrate can't decrease the yield by dissolving some of the crystals. The filtrate must be employed for washing out the flask quite a few times, until the whole solid has been transferred to the filter. On no account should fresh solvent be employed for transferring the solid to the filter. The recrystallized solid is then dried in an appropriate manner, bottled and labeled.

Open flask method:

This is basically the same as the prior method, however is carried out directly on the hot plate by an open conical flask. The solvent is only just permitted to come to a boil and then the flask is taken away from the heat. At this point, it must be possible to view the vapor condensing within the flask, and there must not be a risk of fire if care is taken. The obvious benefit of this process is speed. This approach is not appropriate for low-boiling solvents like ether or pentane.

Recrystallisation requiring hot filtration:

If, throughout Recrystallisation, there is an insoluble solid impurity, it becomes essential to filter the hot solution. Care must be taken that no crystallization takes place during the procedure as this would block up the filter funnel and cause great complexity. To avoid this, the given method is employed:

The crude solid is dissolved in the solvent in normal manner, and when the complete solute has just dissolved at the boiling-point, a further small quantity of solvent is added. This makes sure that the solution is not quite saturated. This solution is kept hot whereas a separate sample of solvent is heated to boiling and then poured via the prepared Buchner funnel. This method heats up the funnel and flask. The filter paper that should be in position is held in place via a glass rod. The chosen funnel must be reasonably large as this will retain the heat better and the filtration will be faster.

The hot solution is quickly filtered by the pump on full. As soon as all of the solution is throughout the funnel, the pump is disconnected and the funnel removed. At this phase, the solute will almost for all time have started to crystallize in the receiving flask. To save mechanical loss, the solution must be kept in the flask and cooled in the normal manner. The final filtration to gather the crystals thus needs another flask.

The use of activated charcoal:

At times there are colored impurities present in the crude material to be recrystallized. These are eliminated from the solution whereas hot by adsorption onto activated charcoal. The Recrystallisation is taken out generally till the crude material is dissolved. At this point, a little additional solvent is added, and the mixture cooled to some extent. A small amount of activated charcoal is added to the cooled solution. This is vital to cool the solution before adding the charcoal, as this material tends to promote boiling. If the solution is not adequately cooled prior to the addition of the charcoal, the whole mixture will generally boil over violently.

The mixture by the charcoal is allowed to boil gently for a few minutes, and is then filtered hot. It is significant to make sure that the paper is well fitted or charcoal might get around the edges and contaminate the product. As in the hot filtration, the funnel must be large in such a manner that the filtration is as fast as possible. The flask must be of an appropriate size for the volume of purified solution obtained.

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