Common Laboratory Techniques and Apparatus PDF

Activity No. 2 COMMON LABORATORY TECHNIQUES AND APPARATUSES I. COMMON LABORATORY TECHNIQUES The Common Laboratory Techniques describe procedures for safely conducting an experiment. Be sure to read each technique carefully before the labora...

Activity No. 2 COMMON LABORATORY TECHNIQUES AND APPARATUSES I. COMMON LABORATORY TECHNIQUES The Common Laboratory Techniques describe procedures for safely conducting an experiment. Be sure to read each technique carefully before the laboratory session for completing a safe and successful experiment. A. Handling Liquid Chemicals 1. Opening a Reagent Bottle. Grasp the reagent bottle with one hand and using the other hold the glass stopper between fingers and lift (Fig.A.1a). Never lay the stopper from a reagent bottle on the laboratory bench. Impurities may be picked up and thus contaminate the solution when the stopper is returned to the reagent bottle. Flat top glass stoppers should be laid upside down on the laboratory bench. Fig. A.1. Removing a glass stopper from a reagent bottle. 2. Transferring Liquids to Another Container. To transfer a liquid from one container to another, hold a stirring rod against the lip of the container holding the liquid and pour it down the rod which should touch the inside wall of the receiving vessel (Fig.A.2). Try not to transfer more reagent than is needed, and do not return unused chemicals to the reagent bottle. Fig. A.2. Transferring a liquid from a reagent bottle. 5 3. Reading the Volume of a Liquid For exacting the measurements of liquids in graduated cylinders, pipettes, burettes, and volumetric flasks, the solution’s volume is read at the bottom of the meniscus. Read with the eye horizontal to the liquid’s surface (Fig.A.3). A clear or transparent liquid is read more easily by positioning the top edge of a black mark (made on a white card) just below the level portion of the liquid. The black background reflects off the bottom of the lower meniscus and better defines the liquid’s level. Substituting a finger for the black mark on the white card will work, but it is not as effective. Fig. A.3. Reading the volume of the liquid. 4. Testing for the Odor and Taste of Chemicals Chemicals should not be tasted nor tested for odor unless specifically directed to do so. Working with toxic vapors must be done in a ventilating hood. To test the odor of a substance, waft a bit of vapor towards the nose as shown in Fig A.4. Do not inhale the vapor directly from the test tube. Never hold your face directly over vapors! Waft toward your nose Fig. A. 4. Testing for odors. 5. Heating Liquids a. Test tube. The test tube should be no more than one-third full. Move the test tube circularly in and out of the flame, heating the liquid from the top to the bottom. Never fix the flame’s position at the base of the test tube. Never point 6  the test tube to anyone as sudden ebullition (a sudden violent outburst of the liquid) may take place. See Fig. A.5a1 and Fig. A.5a2. Fig. A.5a1. Heating liquid in a test tube. Warning: Remember not to point the hot or heated test tube toward yourself or anyone else. Fig. A.5a2. Improper position of heating liquid in a test tube. b. Erlenmeyer Flask. An Erlenmeyer flask less than one-fourth full may be heated directly over a flame. Hold it with a piece of tightly folded paper or flask tongs (not crucible tongs) and gently swirl. Do not place the hot flask on the laboratory bench; allow to cool by setting the flask on a wire gauze. See Fig. A.5b. Fig. A.5b. Heating liquid in an Erlenmeyer flask. 7  c. Beaker (or flask). Support the beaker (or flask) on a wire gauze. To avoid bumping (sudden formation of superheated liquid near the flame), place a glass stirring rod (or use boiling chips) in the beaker. Position the flame directly under the tip of the stirring rod (Fig. A.5c). Place a second supporting ring around the top of the beaker (or flask) to prevent it from being accidentally knocked off. d. In Hot Water Bath. This is used to heat a small quantity of solution in a test tube that needs to be held at a constant temperature over a period of time (Fig. A.5d). If the solution is to be heated in a beaker or Erlenmeyer flask, use a larger beaker as water bath filled to about ¼-full, and heat to the desired temperature. Fig. A.5c. Heating liquid in a Fig. A.5d. Heating liquid in a beaker. hot water bath. 6. Evaporation of Liquids a. Nonflammable liquids may be evaporated in an evaporating dish with a gentle, direct flame (Fig. A.6a) or over a steam bath (Fig. A.6b), which is more efficient rather than rapid boiling. Avoid inhaling the vapors. b. Flammable liquids may be similarly evaporated substituting a heating mantle for the Bunsen flame. The use of a fume hood or an improvised hood is suggested if large amounts are evaporated in a laboratory with inadequate ventilation; consult with your laboratory instructor. Fig. A.6a. Evaporation of Fig. A.6b. Evaporation of nonflammable liquid over a low, nonflammable liquid over a steam direct flame. bath. 8 B. Handling Solid Chemicals Transferring a Solid Reagent First, read the label on the bottle twice to be sure of using the correct reagent. If the bottle has a glass stopper, place it with the topside down on the bench (Fig. B). If it has a screw cap, remove and set it on the bench, inner side facing up. Hold the bottle with the label against your hand, tilt, and roll back and forth. Try not to dispense more reagent than needed. Do not insert a spatula or other object into the bottle unless your instructor specifically tells you to. If too much reagent is taken, do not return excess to the bottle but rather share it with other students. Recap when finished. Fig. B. Transferring solid reagent. C. Handling Precipitates Decantation. A liquid can be decanted (poured off the top) from a solid if the solid clearly separates from the liquid in a reasonably short period of time. Allow the solid to settle to the bottom of the beaker or test tube. If beaker is used, it may be tilted to allow the solid to settle at the side below the spout of beaker (Fig. C.1a). Transfer the liquid (called the (supernatant or decantate) with the aid of a clean stirring rod to a receiving vessel (Fig. C.1b). Do this slowly so as not to disturb the solid. Fig. C.1a. Decantation Fig. C.1b. Decantation 9 Centrifugation. A centrifuge (Fig. C.2a) spins at velocities of 5,000 to 25,000 revolutions per minute. A liquid-solid mixture in a small test tube or centrifuge tube is placed into the sleeve of the rotor of the centrifuge. By centrifugal force, the solid is forced to the bottom of the test tube or centrifuge tube and compacted. The clear liquid, called the supernatant, is then easily decanted without any loss of the solid. The following precautions are observed in operating a centrifuge: a) Never fill the centrifuge tubes to a height more than 1 cm from the top. b) Label the centrifuge tubes to avoid confusion. c) Always operate the centrifuge with even number of centrifuge tubes containing equal volumes of liquid placed opposite one another in the centrifuge. This balances the centrifuge and eliminates excessive vibration and wear. If only one tube needs to be centrifuged, then balance the centrifuge with a tube containing the same volume of solvent (Fig. C.2b). d) Never attempt to manually stop the centrifuge, let the rotor come to rest on its own. Fig. C.2a. Centrifuge Fig. C.2b. Centrifugation Gravity Filtration. Gravity filtration has the advantage that it is simple and economical. It is most effective when the amount of solid to be isolated is small. If a large amount of solid or a gelatinous solid is to be isolated, this method is not used because the filter paper may become clogged. In this case, vacuum filtration is most suitable. 1. General Procedures for Filtration a. Allow the precipitate to settle to the bottom of the container. b. Wet the filter paper with a small amount of water or a liquid in which the precipitate is insoluble. c. Decant (pour off) the bulk of the solution into the funnel using a stirring rod. This step ensures the filtration will not be slowed by the precipitate clogging the filter paper prematurely since most of the solution will have already filtered through. d. Mix the precipitate with the remaining solution and dump the resulting precipitate slurry into the funnel. Simultaneously, spray a stream of solution from a wash bottle to rinse out any slurry adhering to the walls of the container. (See 10  Fig C.3.1) Remember: the liquid or solution used for washing the precipitate must be one in which the precipitate is insoluble. Fig. C.3.1. Transferring a precipitate. Wet a rubber policeman or glass rod with wash liquid and use it to help transfer any precipitate still clinging to the container walls. Use wash bottle to rinse out whatever precipitate remains. Typically 5– to 10-mL of a wash solution are drawn through the funnel to wash the precipitate. This may be repeated several times to ensure that only pure precipitate remains. Allow air to be drawn through the filter paper several minutes to dry the precipitate partially. e. Usually precipitates are washed or rinsed to remove impurities, unless the filtrate is the desired component. f. Lift a corner of the filter paper with a spatula and then use the corner to pry up the rest of the filter paper. Carefully transfer the filter paper and precipitate to a previously weighed watch glass. Allow the precipitate to air dry, or oven dry (if melting point is above 70oC). 2. Folding a Filter Paper Fold the filter paper in half (Fig. C.3.2). then refold to within about 10° of a 90° fold, tear off the corner unequally, and open. The tear enables a close seal to be made across the paper’s folded portion when placed in a funnel. Fold and crease lightly Tear off corner Fold again Approx. 10o Open out like this Fig. C.3.2. Technique in folding a filter paper. 11 D. Inserting a Glass Tubing into a Stopper To insert glass tubing (including thermometers, long-stemmed funnels, thistle tubes, etc.) through a rubber stopper, first lubricate the tube and stopper with water or glycerol. Hold the tubing with a cloth near the end to be inserted, and push with a twisting motion. (If you twist a long-stemmed funnel or thistle tube by the large end, it is easily broken.) Fig. D. Inserting a glass tube into a stopper. E. Operating the Bunsen burner The Bunsen burner, used for most laboratory heating, produces a cone-shaped flame, as illustrated in Figure E. Ordinary beakers, crucibles, and other objects to be heated are placed just above the hottest portion of the flame. This allows most heat to spread about them. Do not place in the cold inner cone of the flame, which consists of unburned gas. For maximum temperature, have the gas on full pressure, and with the air vents open, adjust the needle valve or the air valve to produce short blue flame of many short cones that are about 0.5 cm high. The object to be heated is placed about 1 cm above the grid. Fig. E. Operating the Bunsen burner. 12 II. A. COMMON LABORATORY APPARATUSES This section details common laboratory apparatus and their appropriate use. Knowing the proper use will help ensure safe laboratory practices. Test Tube Holder test tube with holder tripod 1.Test Tube 2.Test Tube Rack wire gauze beaker reagent bottle iron stand with iron rings Bunsen Burner triple beam balance buret iron stand and clamp tongs spatula crucible graduated cylinder Dilution mark dropper volumetric flask flat bottomed flask Erlenmeyer Flask evaporating dish clay triangle safety glasses filter funnel mortar and pestle (used for grinding paste or powder) plastic wash bottle rround bottomed flask 13 B. USES OF COMMON LABORATORY APPARATUSES 1. Balance - for measuring mass 2. Beaker - to hold, mix, and heat liquids 3. Bunsen burner - as heat source in the absence of flammable materials 4. Buret - for dispensing accurate volume of a liquid 5. Clay triangle - to support a crucible during heating 6. Crucible - for holding chemicals during heating to very high temperatures 7. Crucible tongs - to hold crucibles 8. Erlenmeyer flask - to hold and mix chemicals. The small neck is to facilitate mixing without spilling 9. Evaporating dish - to heat liquids for evaporation 10. Funnel - for filtration; also to transfer liquids or fine-grained materials into containers with small openings. 11. Graduated cylinder - to measure a precise volume of a liquid 12. Mortar and pestle - to crush and grind materials 13. Iron clamp - used with a ring stand to hold glassware, such as beaker or funnel 14. Iron stand - to support laboratory glassware, water bath, etc. 15. Scoopula/spatula - to transfer solids 16. Stirring rod - for stirring and mixing 17. Test tube - to hold and mix liquids 18. Test tube holder - to hold a test tube during heating 19. Test tube rack - to hold several tubes at one time 20. Thermometer - to measure temperature 21. Utility/iron clamp - to secure glassware to a ring stand 22. Volumetric flask - to prepare solutions of accurate volume 23. Volumetric pipet - to measure small amounts of liquid very accurately; draw in or suck the liquid using an aspirator (never use your mouth) 24. Wash bottle - to rinse parts of glasswares and to add small quantities of water 25. Watch glass - to hold solids while weighing or to cover a beaker 26. Wire gauze - to support a container, such as beaker, on an iron ring during heating 14