Chem 23.1 Common Laboratory Techniques Study Guide.pdf

University of the Philippines Visayas Department of Chemistry College of Arts and Sciences...

University of the Philippines Visayas Department of Chemistry College of Arts and Sciences Chem 23.1 Inorganic Analytical Chemistry Laboratory STUDY GUIDE Common Laboratory Techniques Introduction This experiment aims to refresh and determine your common laboratory skills and techniques. The techniques can be grouped into measuring principles: weighing, transferring of liquids (pipetting, titration, etc.), methods of separation (filtration, decantation, evaporation). Weighing is a fundamental part of most laboratory activities, whether used to prepare solutions or to evaluate samples. Liquid transfer is another important laboratory technique that varies on the vessel used. Titration is frequently performed in analytical chemistry that employs chemical reactions to quantify the target analyte. This technique can be used to determine chemical compounds that are performed routinely that use the buret, beaker or Erlenmeyer flasks. This is an experiment to review your basic knowledge and skills on common laboratory techniques. We hope to enhance your skills and capability to better equip you when you graduate as a chemistry student in this university. The following series of activities were adapted from the general chemistry laboratory techniques. A. Using a Bunsen burner Laboratory burners come in many shapes and sizes, but all accomplish one main purpose: a combustible gas-air mixture yields a hot, efficient flame. The combustible gas used to supply the fuel for the Bunsen burner in most laboratories is natural gas. Natural gas is a mixture of gaseous hydrocarbons, but primarily the hydrocarbon methane, CH4. If sufficient oxygen is supplied, methane burns with a blue, nonluminous flame, producing carbon dioxide and water as combustion products: CH4 (g) + 2O2 (g) ⇌ CO2 (g) + 2H2O (g) With an insufficient supply of oxygen, small carbon particles are produced, which, when heated to incandescence, produce a yellow, luminous flame. The combustion products may, in addition to carbon dioxide and water, include carbon monoxide. Lighting the Bunsen burner 1. Check the availability of gas in the gas outlet. 2. Connect the burner (gas inlet) to the gas outlet using a rubber tubing (Figure A.1). 3. Close the gas control valve at the base of the burner and fully open the gas valve at the outlet. 4. Use a match or striker to ignite the gas while gradually opening the gas control valve. Adjusting the Bunsen burner Adjust the height of the flame using the gas control valve. Then, open the air holes until the flame exhibits three distinct cones (bluish-green with a light blue cone and burns quietly) (Figure A.2). This type of flame is the ideal working flame – non-luminous flame. A clean burning flame requires more air than what is available from the gas exhaust. Too little air produces a sooty, orange-yellow flame that is quite noisy. Too much air supply may cause the Note that the course pack provided to you in any form is intended only for your use in connection with the course that you are enrolled in. It is not for distribution or sale. Permission should be obtained from your instructor for any use other than for what it is intended. Page 1 of 10  University of the Philippines Visayas Department of Chemistry College of Arts and Sciences Chem 23.1 Inorganic Analytical Chemistry Laboratory flame to blow itself out. Burner barrel Blue Flame Air holes Hottest part of flame Bright blue flame “cool” inner cone Gas Gas control inlet Figure A.1. Bunsen Burner Figure A.2. Flame of a properly adjusted Bunsen Burner B. Transferring Liquids or Solutions When a liquid or solution is to be transferred from a bottle, remove the glass stopper and hold it between the fingers of the hand used to grasp the reagent bottle. Never lay the glass stopper on the laboratory bench; impurities may be picked up and thus contaminate the liquid when the stopper is returned. If the reagent has a screw cap, place the top side down on the lab bench (Figure B.a). To transfer a liquid from one vessel to another, hold a stirring rod against the lip of the vessel containing the liquid and pour the liquid down the stirring rod, which, in turn, should touch the inner wall of the receiving vessel. Return the glass stopper or screw cap to the reagent bottle (Figure B.b- c). Do not transfer more liquid than is needed for the experiment. Do not return any excess or unused liquid to the original reagent bottle. a b c Figure B. Transferring of liquid or Solutions (Decanting). C. Heating Liquids or Solutions Note that the course pack provided to you in any form is intended only for your use in connection with the course that you are enrolled in. It is not for distribution or sale. Permission should be obtained from your instructor for any use other than for what it is intended. Page 2 of 10  University of the Philippines Visayas Department of Chemistry College of Arts and Sciences Chem 23.1 Inorganic Analytical Chemistry Laboratory Many procedures call for a solution to be heated. Heating a mixture either accelerated the rate of a chemical reaction or causes the formation of larger crystals of precipitate, allowing its separation to be more complete. Liquids and solutions can be heated directly either using a Bunsen burner or hot plate. Hot liquids and solutions can be cooled down by placing the glass vessel under flowing tap water or in an ice bath. Using a beaker or flask 1. Support the beaker or flask on a wire gauze that is centered over an iron ring attached to an iron stand (Figure C.1.a) 2. Caution must be taken not to heat the liquid too rapidly as “bumping” – sudden formation of bubbles from the superheated liquid, may occur. To avoid or minimize bumping, position the flame directly beneath the tip of the stirring rod followed by a constant stirring or add boiling chips to the liquid. 3. Nonflammable liquids in a beaker or flasks that are more than one-fourth can be slowly heated directly with a hot plate. Note: Hot plates are hot. Do not touch! (Figure C.1.b). 4. If a stirring hot plate is used, place the stir bar into the liquid and turn on the stirrer (Figure C.1.c). b a c Figure C.1. Heating of liquids or solutions in a beaker or flask using a (a) Bunsen burner and (b) hot plate. Using a test tube 1. A liquid or solution in a test tube can be heated directly using a burner (Figure C.2.a). 2. The test tube should be less than one-third full of liquid. 3. Hold the test tube with a test tube holder at an angle of about 45°C with the cool flame. A cool flame is a non-luminous flame supplied with a reduced amount of fuel. If you can feel the heat of the flame with the hand that is holding the test tube holdser, the flame is too hot! 4. Move the test tube circularly in and out of the flame, heating from top to bottom, mostly near the top of the liquid. (Caution: Never fix the position of the flame at the base of the test tube, and never point the test at anyone; the contents may be ejected violently if the test tube is not heated properly). 5. Small quantities of liquids in test tubes that need to be maintained at a constant, elevated Note that the course pack provided to you in any form is intended only for your use in connection with the course that you are enrolled in. It is not for distribution or sale. Permission should be obtained from your instructor for any use other than for what it is intended. Page 3 of 10  University of the Philippines Visayas Department of Chemistry College of Arts and Sciences Chem 23.1 Inorganic Analytical Chemistry Laboratory temperature over a period of time can be placed in a hot water bath. The heat source may be a hot plate or a direct flame, depending on the chemicals being used. The setup is the same as that for heating a liquid in a beaker (Figure C.2.b). 6. To heat one or several solutions in test tubes in a hot water bath, a 250 mL beaker containing ~200 mL of tap water is satisfactory. The test tubes can be placed directly into the bath, supported by the of the beaker. Maintain a warm-water bath with a hot plate or a cool flame. a b Figure C.2. Heating of liquids or solutions in a test tube using a (a) bunsen burner and (b) hot plate. D. Measuring Mass The laboratory balance is perhaps the most used and abused piece of equipment in the chemistry laboratory. Therefore, because of its extensive use, there are several guidelines that be must followed to maintain the longevity and accuracy of the balance. There are different electronic balances available for use in the laboratory, each having varying degrees of sensitivity (Figure D.1-2). It is important to know (by reading the Experimental Procedure) the precision required to make a mass measurement and then to select the appropriate balance. It may save you time during the data analysis. Record mass measurements that reflect the precision of the balance—that is, the correct number of significant figures. When using an analytical or top loading balance, it is important to remember the following: 1. Handle the balance with care. It is expensive. 2. If the bubbles in the balance is not centered, or the balance itself is not leveled, see your laboratory instructor. Note that the course pack provided to you in any form is intended only for your use in connection with the course that you are enrolled in. It is not for distribution or sale. Permission should be obtained from your instructor for any use other than for what it is intended. Page 4 of 10  University of the Philippines Visayas Department of Chemistry College of Arts and Sciences Chem 23.1 Inorganic Analytical Chemistry Laboratory 3. Use weighing paper, a watch glass, a beaker, or some other container to measure the mass of chemicals. Do not place chemicals directly on the balance pan. 4. If the balance is not operating correctly, see your laboratory instructor. Do not attempt to fix it yourself. 5. Clean the balance and balance area of any spilled chemicals. The mass measurement of a sample can be completed in two ways: (a) the traditional method, and (b) the modern electronic balances. In the traditional method, the mass of weighing paper or a clean, dry container is first measured and recorded. The sample is then placed on the weighing paper or in the container and the combined mass is measured. The mass of the weighing paper or container is then subtracted from the combined mass to record the mass of the sample. On modern electronic balances, the mass of the weighing paper or container can be tared out— that is, the balance can be zeroed after placing the weighing paper or container on the balance, in effect, subtracting its mass automatically. The sample is then placed on the weighing paper or in the container, and the balance reading is the mass of the sample. For either method, the resultant mass of the sample is the same and is called the tared mass of the sample. After completing the mass measurement, return the mass settings to the zero position. Figure D. Top-loading balance used for rough weighing [left], and analytical balance used for accurate measurements [right] E. Heating Solids Solids are heated to dry them or to test their thermal stability. A drying oven is often used for low temperature heating, and porcelain crucibles for high temperature heating. Beakers and test tubes can be used for moderately high temperature heating. Using a Drying Oven Note that the course pack provided to you in any form is intended only for your use in connection with the course that you are enrolled in. It is not for distribution or sale. Permission should be obtained from your instructor for any use other than for what it is intended. Page 5 of 10  University of the Philippines Visayas Department of Chemistry College of Arts and Sciences Chem 23.1 Inorganic Analytical Chemistry Laboratory When solid chemicals are left exposed to the atmosphere, they often absorb moisture. If an exact mass of a solid chemical is required, the absorbed water must be removed before measuring the mass. The chemical is often placed in an open container in a drying oven (Figure E.1) set at a temperature well above room temperature (most often at ~110ºC) for several hours to remove the adsorbed water. The container is then removed from the drying oven and placed in a desiccator (Figure E.2) for cooling to room temperature. Cooling in a Desiccator When the container containing a solid chemical is cooled, moisture tends to condense on the outer surface, adding to the total mass. To minimize this mass error, substances and mixtures that may tend to be hygroscopic are placed in a desiccator until they have reached ambient temperature. A desiccant, typically anhydrous calcium chloride, CaCl 2, absorbs the water vapor from within the desiccator - forming its hydrated form: CaCl2 2H2O. The absorbed water molecules can be easily removed with heat, and the calcium chloride can be recycled for subsequent use. F. Measuring Volumes of Liquids or Solutions Using a graduated cylinder Graduated cylinder is typically used for measuring the volume of a liquid when extreme accuracy is NOT required. When reading the volume of a liquid, the eye should be level with the meniscus (a curved surface). For colored liquids or solutions, the upper meniscus is used (Figure F.1) Figure D. Reading volumes and liquids or solutions. The eye should be level with the meniscus (a curved surface). Using a pipet Note that the course pack provided to you in any form is intended only for your use in connection with the course that you are enrolled in. It is not for distribution or sale. Permission should be obtained from your instructor for any use other than for what it is intended. Page 6 of 10  University of the Philippines Visayas Department of Chemistry College of Arts and Sciences Chem 23.1 Inorganic Analytical Chemistry Laboratory 1. A pipet is used to deliver a definite volume of a liquid. To draw liquid to the pipet, use a rubber bulb aspirator. Attach the bulb to the pipet, compress the bulb and insert the jet end into the liquid. Admit the liquid by releasing the bulb slowly (Figure F.2.a) 2. Control the delivery of the liquid with your FOREFINGER. Not your thumb (Figure F.2.b). 3. Deliver the liquid in a vertical position, with the pipet tip touching the wall of the receiving flask. 4. Hold the pipet constantly in a vertical position until it is empty, or the desired volume is reached. 5. Remove any hanging droplet of liquid by touching the tip to the side of the container. 6. Do not blow out the small amount of liquid that remains on the tip because the pipet is calibrate with the remaining liquid considered. Figure D. Using a pipet. Liquid is drawn out using an aspirator [left] and dispensed in a receiving container [right]. Using a buret 1. Completely fill a clean buret with distilled water with the stopcock closed. Let it stand for a few minutes and check for leaks. 2. Check whether the stopcock turns freely. Do not force stuck stopcocks. Call the attention of your instructor to aid you. 3. Rinse the buret twice with small amounts of the solution to be used (titrant). 4. Fill the buret beyond the zero mark with the titrant. Remove any trapped bubbles by allowing the titrant to run rapidly through the buret tip. 5. Discard any solution left in the buret unless otherwise directed by your instructor. 6. Wash the buret before storing it away. Note that the course pack provided to you in any form is intended only for your use in connection with the course that you are enrolled in. It is not for distribution or sale. Permission should be obtained from your instructor for any use other than for what it is intended. Page 7 of 10  University of the Philippines Visayas Department of Chemistry College of Arts and Sciences Chem 23.1 Inorganic Analytical Chemistry Laboratory For right-handers Figure F. The titration set up [left]. During titration, one hand should be used to control the stopcock while the other hand is holding flask {middle]. End point of titration is achieved once the solutions changes color [right]. G. Separating a Liquid from a Solid Filtration 1. Prepare the filter paper to be used for gravity filtration. Cut out a 5 x 5 piece of filter paper. 2. Fold the filter paper in exact halves and fold it again crosswise into two. 3. Make a small tear in one corner. This tear seals the paper against the inflow of air to the underside of the filter paper. 4. Open the folded paper to form a cone (Figure G.1.a). Figure G. Folding a paper to be used in filtration. 5. Place the filter paper in a funnel. Moisten it with a little water and press it against the top wall of the funnel to form a seal. The filter paper must always be smaller than the funnel. Support the funnel with a clamp or a funnel rack (Figure G.1) 6. Using one portion of mixture formed in (Precipitation), carefully transfer the precipitate by pouring the mixture with the aid of a stirring rod into the filter paper. The liquid that Note that the course pack provided to you in any form is intended only for your use in connection with the course that you are enrolled in. It is not for distribution or sale. Permission should be obtained from your instructor for any use other than for what it is intended. Page 8 of 10  University of the Philippines Visayas Department of Chemistry College of Arts and Sciences Chem 23.1 Inorganic Analytical Chemistry Laboratory passes through the filter paper is called the filtrate. 7. The tip of the funnel should touch the wall of the receiving beaker to reduce any splashing of the filtrate. Fill the bowl of the funnel until it is less than two-thirds full. Always keep the funnel stem full with the filtrate; the weight of the filtrate creates a slight suction on the filter in the funnel, thus this hastens the filtration process. 8. Wash traces of precipitate in the beaker using small portions of water using your wash bottle. 9. Set aside both the precipitate and filtrate for the next part. Decantation 1. Transfer the precipitate formed in Technique G.1 (Filtration) retained in the filter paper into a beaker by rinsing the filter paper with jets of water from a wash bottle. 2. Allow the solid to completely settle at the bottom of the vessel for several minutes. 3. Transfer the liquid called supernatant into another container with the aid of a clean stirring rod. Do this slowly so as not to disturb the solid. 4. Rinse the precipitate with water and decant again. Evaporation 1. Pour the other portion of mixture in (Precipitation technique) into the evaporating dish. 2. Place the evaporating dish on a wire gauze supported on an iron ring clamped to an iron stand. Heat the dish over a cool flame. 3. Continue heating until crystals begin to appear. Cover the dish with a watch glass and allow the contents to cool. 4. The solid remaining after evaporation is called the residue. Figure G.2. Decantation set-up. Figure G.3. Evaporation set-up. Figure G.1. Filtration set-up. Centrifugation Note that the course pack provided to you in any form is intended only for your use in connection with the course that you are enrolled in. It is not for distribution or sale. Permission should be obtained from your instructor for any use other than for what it is intended. Page 9 of 10  University of the Philippines Visayas Department of Chemistry College of Arts and Sciences Chem 23.1 Inorganic Analytical Chemistry Laboratory 1. Never fill the centrifuge tubes to a height more than 1 cm from the top. 2. Label the centrifuge tubes to avoid confusion of samples. 3. Always operate the centrifuge with an 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. 4. If only one tube needs to be centrifuged, then balance the centrifuge with a tube containing the same volume of solvent (Figure G.4). 5. Never attempt to manually stop a centrifuge. When the centrifuge is turned off, let the rotor come to rest on its own. Figure G.2. Centrifugation (https://microbiologynote.com/types-of-centrifuge/). Note that the course pack provided to you in any form is intended only for your use in connection with the course that you are enrolled in. It is not for distribution or sale. Permission should be obtained from your instructor for any use other than for what it is intended. Page 10 of 10