UNIT 2 General Laboratory Procedures 1- INORGANIC CHEMISTRY PRACTICALS

UNIT 2 General Laboratory Procedures (1)

1.0 Introduction

2.0 Objective

1. Main Content
2. Balances and weighing
3. Setting up apparatus
4. Reflux and distillation
5. Use of the separatory funnel

4.0 Conclusion

5.0 Summary

6.0 Tutor Marked Assignments

7.0 References/Further Reading

1.0
Introduction In the previous unit, you were made acquainted to some important rules you need to follow to make your work in the chemistry laboratory a wonderful experience. Nevertheless, it could be a very defeating experience to walk into a laboratory without understanding the use of the equipment for your use. Therefore before you start the practical classes it will be expedient to know the use of some of the equipment you shall meet.

2.0
Objective At the end of this unit, you should be able to explain the use of some simple equipment in your laboratory.

3.0
Main Content

3.1Balances and weighing

Many experiments in chemistry involve weighing at various steps. Much time can be lost during weighing procedures, and one of the principal time wasters is the habit of weighing to a degree of accuracy in excess of the requirements of the experiment. For synthetic work, weighing to 0.1 g or 0.01 g is quite sufficient. Only for analytical work, is greater accuracy required, on the order of 0.001 g or 0.0001 g.

Even if weighing is only effected to the necessary degree of accuracy, time can be wasted in the actual process, and unless some method is used whereby weighing is carried out rapidly, many experiments cannot be done in the time normally available.

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

3.1.1 Accurate weighing technique: weighing by difference

To weigh an accurate amount of solid (i.e. to the nearest 0.001 g or better), place a weighing bottle (and cap) on a balance, tare it, remove the bottle from the balance, and place an estimated amount of material into the bottle. If the solid contains large crystals or lumps it should be lightly ground in a mortar before weighing.

The weighing bottle (with contents) is now capped, wiped clean and weighed using the correct procedure on the analytical balance (if the weight is significantly different from desired, remove the bottle from the balance and repeat the above procedure until close. Immediately record the weight in your notebook.

Return to the lab, and tip the solid into your flask, vessel or whatever is suitable, no attempt being made to remove the traces of solid which will cling to the weighing bottle. Return to the balance room and re-weigh the nearly empty bottle accurately. The loss in weight is the accurate weight of solid taken. This avoids the rather awkward process of washing the entire solid from the bottle and is quicker and more accurate. This method is often used, as it is rarely necessary to weigh out an exact amount. It is bad practice to weigh out, for example, 1.25 g of a solid to make an exact 0.10 M solution. It is better to use the above method, finish up with a weight of 1.32 g and express the solution as:

M

This avoids the very messy practice of adding and removing odd crystals to try to get an exact weight.

By using the above method it is never necessary to have any loose chemicals near a balance, as only a closed bottle is used on the balance.

3.2 Setting up apparatus

When ground-glass joints are used, it is not necessary to lubricate them except when high temperatures or vacuum are involved or an inert atmosphere is vital. If a joint becomes seized, try the following methods of loosening it: (a) rock the cone in the socket, (b) tap the joint gently with a block of wood, (c) warm the joint in a small flame, then tap gently, (d) soak the joint in penetrating oil, then try tapping. A common cause of seizure is a caustic alkali. Try to keep alkalis off the ground-glass, and if they do get on it, wash thoroughly as soon as possible. Seizures can usually be avoided by disassembling the apparatus immediately after use. Where required, the procedures call for lubricating the joints with silicone grease (“high vacuum grease”). CAUTION: silicone grease may cause corneal damage. In order to avoid accidental transfer of grease to your eyes, be sure to thoroughly wash with soap and water to remove residual silicone grease from your skin.

Care should always be taken, when glass apparatus is set up, to avoid strain. It is best to start with one piece, and build from there. Take a distillation apparatus as an example:

1. Lightly clamp the flask at a height convenient for heating,
2. Attach the still-head, screw-cap adapter and thermometer (no more clamps are needed for these),
3. Attach the rubber tubing to the condenser, then position a clamp and stand so that the condenser will rest on the lower, fixed, side of the clamp. Attach the condenser to the still-head, and clamp lightly,
4. Attach and support the receiver adapter and the receiver.

A similar procedure should be followed for the other assemblies.

3.3 Reflux and distillation

Unlike ionic reactions, which are frequently extremely rapid, reactions between covalent substances tend to be slow. Particularly in main-group and organometallic reactions, it may be necessary to keep a reaction mixture hot for a matter of hours. This, coupled with the fact that volatile and inflammable solvents must be employed, makes it necessary for special equipment to be used.

Reflux The use of a reflux condenser is often necessary. It is used whenever a reaction mixture has to be kept boiling for an appreciable time and the solvent is volatile. A water condenser may be used for solvents boiling up to approximately 130 °C, and for higher boiling-point solvents an air condenser is adequate. The flask must never be filled more than half way; the size of flask should therefore be chosen by consideration of the total volume of the reaction mixture. A boiling stone or similar substance should be used to promote even boiling for all reflux procedures which do not employ magnetic stirring.

The purpose of the apparatus is to keep the solution hot without loss of solvent. It is pointless to boil violently; the heating should be controlled so that the solution is merely simmering. The flask may be heated by an electric heating mantle controlled by a Variac (NEVER plug a heating mantle directly into an electrical outlet!), or by using an oil bath on an electric hot plate.

Distillation The purpose of distillation is to purify a liquid, or to remove a solvent from a solution. The flask must never be more than half full, a boiling stone or magnetic stirring must always be used, and the choice of condenser is the same as for reflux work. The heating of the flask may be accomplished using any of the usual means. Purification of a liquid by distillation is best performed at a rate not exceeding 2 drops of distillate per second. Alternatively, removing a large quantity of solvent may be done much more rapidly.

Fractional distillation The purpose of fractional distillation is to separate two liquids of different boiling-point. As with other forms of distillation, the flask must never be more than half full, and a boiling stone or magnetic stirring must always be used. In order to get good separation of the liquids, it is essential that the distillation be carried out very slowly. The slower the distillation, the better the separation. A rate of 1 drop of distillate per second should be the aim. Since the efficiency of the process depends on the fractionating column reaching thermal equilibrium (there should be a gradual increase in temperature from the top to the bottom of the column), best results are obtained if drafts are excluded. In addition, the source of heat should be steady.

3.4 Use of the separatory funnel

The separatory funnel is used for several essential procedures. Except care is taken, its use can be one of the major causes of mechanical loss. The choice of size is particularly important and, as with flasks in distillation, the smallest that will properly do the job, is best.

Separating two immiscible liquids The liquid mixture is poured into the funnel and the funnel is gently agitated to assist in the separation into layers. The funnel should always be stoppered, but if a particularly volatile substance, such as ether, is present, the funnel should be vented occasionally through the stopcock (hold it slightly inverted while doing this) to avoid the possible buildup of pressure.

When separation into layers has occurred, the stopper is removed and the lower layer drained into a small flask. Swirling the funnel and once again allowing separation to occur frequently provides a further small sample of the lower layer.

The top layer is poured from the top of the funnel into a second flask. It is a wise precaution to always keep both liquids, even if one of them is to be discarded. It is surprising how often the wrong layer is thrown away!

Washing a crude liquid One of the most common procedures consists of shaking a crude liquid product with an aqueous solution to remove some of the impurities. The reagents should always be used in small quantities, and the process repeated if necessary. Mechanical loss is always greater when large volumes of washing solutions are used.

Gases are often formed in substantial quantities during the cleaning process, thus, it is essential to release the pressure frequently. This is best done by inverting the well-stoppered funnel and “turning on” the tap.

If the required substance is the top layer, then allowing the bottom layer to run off is quite simple. The entire bottom layer of waste should 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 various washing solutions. This avoids the risk of inadvertently losing a few drops of the treated product.

When the required substance happens to be the bottom layer, avoiding mechanical loss becomes more difficult. If the product is run off between each wash and then returned to the funnel for the next, the loss can become very great. The best compromise is obtained by using rather large volumes of washing solutions, and decanting the spent solution from the top of the funnel. In this way the product never leaves the funnel until 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 often used to extract a solute from one solvent by means of a second solvent immiscible with the first. The removal of a solute from water by means of ether is one of the most common examples of this application. The size of the funnel is chosen to accommodate the whole of the aqueous solution. This saves time which would otherwise be spent in repetition. A series of extractions with a small quantity of ether is much more effective than one with a large amount of ether. In practice the volume used is that which gives the smallest manageable top layer, bearing in mind that the ether solution must be decanted from the top of the funnel. If the layer is too small, decantation becomes difficult. The solution is usually extracted about three times with fresh quantities of ether, and all of the ether extracts are decanted into one flask. After the final extraction the aqueous layer is run off and the last ether layer decanted completely

4.0
Conclusion In this unit you have studied some general laboratory procedures you will need for a better appreciation of your laboratory. I hope it will guide your use of the laboratory.

5.0 Summary. In this unit you have not performed any experiment, however you have learnt the following; the use of balances and accurate methods of weighing, techniques involved in setting up apparatus, the use of reflux and distillation sets and finally the use of the separatory funnel.

6.0 Tutor Marked Assignments (TMA)

1
Which of the techniques mentioned in this unit would be appropriate in the separation of a mixture of ethanol and water?

1. In which of the techniques named in this unit is density necessary for the separation of liquid mixtures?
2. Which technique in this unit will be suitable for prolonged heating involving organic solvents?