REVIEW. Basic Laboratory Techniques 1,2

1 REVIEW Basic Laboratory Techniques 1,2 The techniques described in the following sections should have been covered in the prerequisite organic chemistry labs (Chem 261 or equivalent). You are therefore expected to be fully familiar with these techniques before you enter the lab. The only potentially new techniques discussed are in sections E (Heating Methods) all heated liquids/solutions are stirred magnetically in Chem 241 (by adding a Teflon coated bar to the flask and using a magnetic stir motor to spin it); boiling stones are not used. H (Using Ground-Glass Jointware) silicone grease should be used when making most joint connections and must be used for inert atmosphere and vacuum operations. A. Useful Calculations and Significant Figures It is assumed that you will be familiar with the basic calculations used to determine the theoretical yield of a product and to express final calculated values to the correct number of significant figures. As a brief review, if you want to calculate the amount of a substance, use mass of a liquid, use concentration of a solution, use m n = MW 1000 mmol mol m d = m = d V V n = amount; mmol d = density; g ml -1 C = concentration, M (mol L -1 ) m = mass; g m = mass; g n = amount; mmol MW = molecular weight; g mol -1 V = volume, ml V = volume, ml To determine the theoretical yield of a product, you must first examine the balanced chemical reaction and consider its stoichiometry. In synthetic chemistry, it is rare for reagents to be combined in exactly the correct theoretical ratio. Usually, one reagent is limiting and the other reagent(s) is (are) present in excess. This means that the reaction will only produce as much product as is allowed by the lowest relative amount of reagent present, after stoichiometry is taken into account. For example, in the chemical reaction A + b B + c C P if one is to determine which reagent is limiting, a comparison should be made as follows n A n B b 1 nc c 1 C = n V

2 with whichever amount is lowest being the limiting reagent. A quick verification of the above can be made by expressing the mole ratio for the reaction by dividing the amounts of each excess reagent by the amount of the limiting reagent after dividing by its stoichiometry coefficient. The result expresses the actual molar ratios of the reagents that can be compared back to the theoretical stoichiometry of the reaction. The excess reagents should have molar ratios greater than their theoretical stoichiometry coefficient (if not, an error has been made somewhere in the calculations). The following example illustrates the process described above. The final consideration is to express the final answer to the correct number of significant figures. Significant figures are important in experimental chemistry so that we do not overstate the precision of any calculations that are made, as this precision is ultimately limited by how well we can measure things in the lab. For this course, it will suffice to express calculated values so that they have the same number of significant figures as the least precise value (i.e., smallest number of significant figures) in the calculation. Leading zeros preceding a decimal place do not

3 count as significant figures, but zeros following another digit or following a decimal place do. Thus, 0.0020 g has two significant figures while 2.00 mg has three and 0.002 g has only one. For example, in the hypothetical reaction described on the previous page, the molecular masses of the reagents are all known to four significant figures, the volume of AgNO 3(aq) is known to three significant figures but the measured quantities of mass and concentration are only known to two significant figures. Therefore, all calculated quantities should are expressed to only two significant figures (as each uses either mass or concentration in this case). However, doing so at each step of the sequence in determining the theoretical yield would result in unacceptable errors due to rounding. Therefore, we retain at least one insignificant figure at each intermediate step (which, for clarity, are often subscripted; in practice, it is best to retain the result of each intermediate calculation in your calculator s memory and use it in the next step). When all is said and done, the final theoretical yield of Ag 2 CO 3 is 2.6 g; this makes inherent sense since we only weighed 1.0 g of Na 2 CO 3 to begin with, and cannot know the expected yield to a greater precision than what we started out with. B. Transferring Chemicals Solid and liquid reagents are kept in designated areas in the laboratory. Take a container (e.g., beaker or Erlenmeyer flask) to the reagent bottle and obtain the desired quantity of reagent. Do not take reagent bottles to your lab bench. Take care not to transfer reagents into vessels that are suspended above equipment (e.g., above a Thermowell when assembling a reflux) as spills may case irreparable damage. Solids should never be transferred directly onto a container resting on the balance pan as the risk of spillage is high (take a look at the balance table at the end of the first practical lab period to see how many spills occur!) Avoid handling solid reagents with your hands, even if you are wearing disposable gloves. Spills of any kind must be cleaned up as soon as possible. Tip: To better reach the inaccessible inner walls of round-bottomed flasks, bend the flat blade of your spoonula (with a pair of pliers) near the tip and at about half its length as shown to the right (Figure I.1). Figure I.1. Using a bent spoonula to reach the inside wall of a round-bottomed flask.

4 Tip: Use powder funnels to transfer solids into flasks with narrow necks. Use an offset powder funnel to add reagents to flasks with ground-glass joints. Liquids and solutions may be poured directly from the reagent bottle providing that the container they are being poured into has a large enough opening. Take note of the weight of the reagent bottle - do not try to manipulate a heavy bottle when pouring into a small container. It is better to pour a small amount of the liquid into a larger beaker, and then use the beaker to pour the liquid into the smaller vessel. Due to the risk of contamination, previously used disposable pipets must never be inserted directly into a reagent bottle. Never return unused chemicals to the reagent bottle. Instead, ask a classmate if they can use them or dispose of in the correct manner. Tip: Use a funnel to transfer liquids into flasks with narrow necks. It is often convenient to use a powder funnel to transfer liquids if the neck of the flask is wide enough. C. Using an Electronic Balance Electronic balances with automatic taring functions and digital readouts are one of the most common instruments in any chemistry laboratory. Always use an appropriate container when weighing chemicals and never place chemicals directly on the balance pan. Small beakers are convenient for weighing solids and sealed glass flasks should be used for liquids; if you are weighing a liquid in a round bottomed flask, use a cork ring or beaker to support the flask on the balance pan. The balances are expensive and delicate instruments, and every effort must be made to keep the balances and balance table clean. In particular, never transfer chemicals while the receiving container is on the balance as spilled chemicals will cause damage. To weigh a chemical: 1. Zero the balance by pressing the TARE key briefly. If the balance is in a powersaving mode, this will also turn the balance on. 2. Place an empty container on the weighing pan and allow the balance to stabilize. Wherever possible, try to use glass containers. Plastic weigh boats are expensive, cause unnecessary waste, and are attacked by organic solvents (so don't rinse a plastic weigh boat with solvents). 3. Press the TARE key again; this will zero (tare) the weight of the container and the display should now show zero. 4. Remove the container from the balance. Never transfer chemicals while the receiving container is on the balance as spilled chemicals will cause damage.

5 5. Return the sample container to the balance. The display will now show only the weight of the sample. Always wait for motion to disappear before taking a reading. 6. Clean any spilled material from the balance and balance table immediately. D. Filtration Filtrations are generally used to separate a solid from a liquid. Depending on the application, either the solid or the liquid supernatant may be retained. The simplest method is the gravity filtration (Figure I.2). The mixture is passed through a piece of folded filter paper suspended inside a fluted, short-stemmed funnel. The solid is retained by the filter paper and the supernatant is collected in the Erlenmeyer flask. Figure I.2. Carrying out a gravity filtration with filter paper folded into a cone. Tip: The liquid will be drawn through the filter paper only so long as the paper remains in contact with the funnel. Place a small watch glass on top of the filter paper to weigh it down. Tip: Fan-folded fluted filter paper (Figure I.3) will often provide a faster flow rate because a larger surface area of the solution is in contact with the paper. A piece of fan-folded 12.5 cm filter paper works best with a 6.5 cm diameter funnel. Figure I.3. How to make fan-folded fluted filter paper.

6 If you need to filter a small volume of liquid (less than ~10 ml) to remove an unwanted precipitate, it is more convenient to carry out the filtration using a filter that has been assembled by pushing a small plug of cotton or glass wool into the open end of a Pasteur pipet (Figure I.4). Note that you will not be able to recover the solid easily if you use this method. Tip: Use the smallest plug of cotton or glass wool that will seal the stem of the pipet. Tip: Use a bulb to squeeze air through the pipet to recover the final traces of liquid from the cotton or glass wool plug. Figure I.4. A Pasteur pipet filter. Suction filtrations (also called vacuum filtrations) are carried out using a porcelain Buchner funnel that rests on top of a rubber adapter which is inserted into a filtering flask (Figure I.5). The filtering flask is attached to a trap that is in turn connected to a vacuum source (most often a water aspirator). The house vacuum line should only be used to provide suction when aqueous solutions are being filtered and must not be used for filtering volatile, flammable solvents. Tip: Use the small (4.25 cm) Buchner funnel to filter ~1 g or less of solid. If you use the large (9.0 cm) funnel for a small quantity, you will find it difficult to scrape the solid from the large surface area of the filter paper. Tip: The water aspirator should be turned on full to achieve maximum suction. Always open the relief valve on the trap before turning off the water (if you don t, water will be sucked into the trap). Tip: Filter paper has a slight curve at the edge. To ensure a good seal, place the filter paper so that it is convex (i.e., ). The filter paper should be large enough so that all of the holes in the Buchner funnel are covered without the paper curling up the sides of the funnel. You should pour the solution slowly and attempt to collect the solid in the center of the filter paper, not at its edges.

7 Tip: Just before you pour the solution onto the filter, you should wet the filter paper with a small amount of the same solvent that is in the flask. This will ensure a good seal and will prevent solid from passing between the filter paper and funnel. Tip: If the suction is poor, check that the relief valve is tightly closed and that the water aspirator is turned on fully. Then, gently press down on the top of the funnel to create a good seal and check that the filter paper has been wetted with solvent as described above. Tip: The easiest way to recover the solid is to use the tip of a spoonula or spatula to remove the entire cake (filter paper + compound) from the Buchner funnel. Use a large piece of filter paper to collect the solid, and then transfer the compound to a labelled vial by folding the larger piece of filter paper and using it as a funnel. Figure I.5. Carrying out a suction filtration with a Buchner funnel. E. Heating Methods Combination stirrer/hot plates are provided in your communal equipment. Mixtures heated to the boiling point must be stirred to avoid bumping (a sudden, violent boiling of a superheated solvent); this is conveniently achieved by adding a Teflon -coated magnetic stir bar to the flask. Tip: Only one flask should be stirred at a time on a stir plate.

8 Tip: 100 ml or smaller round-bottomed flasks should be equipped with ½ stir bars; 1 stir bars should only be used with flat bottomed containers (i.e., beakers or Erlenmeyer flasks > 100 ml) and 250 ml round bottomed flasks (in a 100 ml rbf, the 1 stir bar will often swirl erratically around the sides of the flask). Care must be taken to avoid overheating mixtures placed on the hot plate. Different models of stirer/hot plates produce different rates of heating. As an example, consider that Corning reports that the following temperatures are achieved with its stirer/hot plates: 3 Dial Setting Maximum Temperature LO 55 C 2 180 C 4 350 C 6 440 C HI 460 C Heating mantles (Thermowells) will generally be used to heat round-bottomed reaction flasks. The heating mantle is plugged into a variable transformer called a Variac (not directly into the wall socket); the Variac is then set to provide the desired temperature (see Figure I.6). In order to avoid bumping, a stir plate is placed beneath the heating mantle and a magnetic spin bar is placed in the flask. At the end of a reaction, the Thermowell should be lowered away from the flask using the lab jack. The mixture should continue to be stirred until it has cooled to below the boiling point; if this is not done, the solvent may still bump. Since hot plates are communal equipment, always check that the hot plate is OFF before you plug it in. Do not turn on the heat if the stirrer is under a Thermowell. Tip: When you are assembling the apparatus, the lab jack should be used to raise the Thermowell until it is in direct contact with the round bottomed flask. That way, if the rate of heating becomes to great at any point during the reaction, the jack can be quickly lowered to cool the flask. Always keep an eye on your reaction, and do not leave it unattended for long periods of time. Tip: Different Variac/heating mantle combinations will produce different results. The rate of heating depends on other factors such as whether or not the glass vessel is in direct contact with the heating mantle. If the Variac has a 120/140V switch, use the 120V setting.

9 Tip: Hot glassware should not be transferred immediately into an ice-water bath as even Pyrex may crack during rapid cooling. A warm water bath or a piece of paper towel soaked in water and wrapped around the flask will speed cooling. Figure I.6. A plot of Variac setting versus temperature achieved. * Do not use a dial setting greater than 60 unless instructed to do so as dangerously high temperatures may result. F. Recrystallization Recrystallization is used extensively for the purification of soluble inorganic solids. The impure compound is dissolved in an appropriate solvent, and conditions are established that encourage the regrowth of crystals of the compound while leaving the undesirable impurities dissolved in the supernatant solution. Erlenmeyer flasks are the best vessels for recrystallizations (and should generally be used unless otherwise specified). The most common recrystallization technique is to slowly dissolve the solid in a minimum of the hot solvent, gravity filter to remove any insoluble impurities, and then set the saturated solution aside to crystallize as it cools. In this case, the solid must be soluble in the hot solvent, but insoluble in the cold solvent. Note that the terms hot and cold are relative - hot may be room temperature if the crystallization is carried out in an ice bath. Tip: Make sure that you securely clamp any flasks that are set aside to cool in an icewater bath. Do not rely upon piled ice to hold the flask upright. Tip: To figure out what the minimum volume of solvent required is, add the solvent in small quantities and heat while swirling the flask. When only a small amount of solid remains, add the solvent dropwise until the solid just dissolves * The temperatures were obtained by heating a 100 ml round bottomed flask containing high temperature oil in a Thermowell at the indicated settings on three Variacs chosen at random. On Variacs with a 120V/140V switch, the temperatures reflect the 120V setting (which is the one you should use). The temperature achieved in the flasks for a particular Variac dial setting generally agreed to within ± 10 C.

10 (remember that there may be insoluble impurities, so don t continue to add solvent if a small amount of solid refuses to dissolve). Tip: The quickest way to add solvent in small portions is to "roughly calibrate" a disposable pipet to deliver ~1 ml at a time. To do this, draw 1 ml of the solvent (measured in a graduated cylinder) into the disposable pipet, and mark the solvent level with a felt-tipped pen. Tip: If you have to gravity filter a hot solution, use a funnel with the shortest stem possible - if the filtration is slow, you may find that your product crystallizes on the filter paper or in the stem of the funnel. The recrystallization is then completed by collecting the product by suction filtration. The product is often washed with a small quantity of the cold recrystallization solvent to remove the final traces of any impurities. Depending on the nature of the solvents used, a second wash solvent may be used to remove traces of the first (and thus must be miscible with the first solvent). This is often done if the first solvent is high boiling and does not evaporate readily. Recrystallization from a mixed solvent can be carried out if a single suitable solvent cannot be found to recrystallize a compound. The compound will generally be quite soluble in the first (more polar) solvent, but insoluble in the second (less polar) solvent; the solvents must be miscible. The solid is dissolved in the minimum amount of the first solvent, and the second solvent is then added until the product begins to precipitate (as a result of the decrease in solution polarity). Typically, the mixture is then cooled to complete the crystallization. The product is collected by suction filtration, and is washed with the solvent in which it is not soluble. Tip: The trick to a proper mixed solvent recrystallization is to use the minimum amount of solvent to initially dissolve the compound. The more solvent you use at this point, the more of the second solvent you will have to add to crystallize the compound; sometimes, if the volume of the mixed solvent is large, the compound will not crystallize at all. G. Recording the Melting Point of a Product The melting point of a compound is a useful criterion of purity and is defined as the range of temperature at which the first crystal begins to melt to the temperature when the final crystal in the sample has melted. Therefore, the melting point is really a melting range; avoid the urge to report a single number for the melting point of your compound. For example, triphenylphosphine, P(C 6 H 5 ) 3, melts over the range 79-81 C. It would not be correct to say that

11 the melting point was 80 C. If we synthesized triphenylphosphine in the laboratory, the melting point of our sample would be an indication of its purity. Impure compounds tend to melt at lower temperatures and over a broader range than pure compounds. In practice, melting points are determined using an apparatus (e.g., Gallenkamp) that applies heat to a sample contained in a glass capillary. Typically, a thermometer is inserted close to the sample to measure the temperature. A magnifying glass built into the apparatus allows you to see the sample as it is heated. Tip: When loading the capillary, first ensure that the compound is dry. The solid must be moved from the open end of the capillary to the closed end to provide ~ 1-2 mm sample height. In practice, this is best done by dropping the capillary into a long tube and letting it strike a hard surface (it will not break). Tapping the capillary on a bench top may work, but more force is exerted in this manner and the capillary is easily broken. Tip: The trick to obtaining an accurate melting point is to heat the sample slowly over the melting range. If the temperature is increasing rapidly as the melting point is reached, a larger melting range will be observed than if the temperature was increasing slowly. A stable compound will not change composition after it has melted. Many compounds decompose as they melt. If the compound decomposes as it melts, the temperature range is recorded with the notation (dec) following it. For example, if a white compound turned black and liquefied over the temperature range 154-158 C, we would record its melting point as 154-158 C (dec). H. Using Ground-Glass Jointware Two sizes of ground-glass jointware are provided in your equipment kit. The sizes are designated 14/20 (smaller) and 24/40 (larger) to describe the size of the ground-glass joint; the first number describes the diameter of widest part of the joint while the second describes its height. Ground-glassware is very expensive and should thus be handled with due care. The joints of ground-glassware must be greased for any operations that are performed under an inert atmosphere or that are carried out at the vacuum manifold. In most other applications, the joints do not require greasing, but a greased joint will generally be easier to separate than an ungreased one. The grease should be applied lightly so that an uninterrupted film is achieved (rotate the assembled joint to check the grease film). To avoid contaminating the contents of the reaction vessel, only the upper half of the joint should be greased (see Figure I.7). Consult your T.A. if any ground glass joints become stuck together.

12 Figure I.7. The proper way to grease a ground-glass joint. Tip: Always separate ground-glass joints following use. Degrease all joints using a lab towel that has been dipped in a hydrocarbon solvent (e.g., pentane). You may have to repeat the procedure to remove all of the grease (you should be able to see the rough texture of the ground glass when the joint is clean). I. Carrying Out a Reaction under Reflux Many chemical preparations require heating to increase the rate of reaction. The temperature of a reaction can be conveniently controlled by heating the solvent to its boiling point. In order to avoid solvent loss through evaporation, the reaction vessel is capped by a water-cooled condenser. As the solvent boils, it strikes the cold walls of the condenser and drips back into the flask, thereby maintaining the solvent volume. The simplest design for a reflux is shown in Figure I.8; modifications employing addition funnels and drying tubes can be made easily by using a reaction vessel with more than one neck. Heating mantles (Thermowells) will generally be used to heat reaction flasks under reflux (see Part E, Heating Methods). Tip: The water should always flow into a condenser at its lowest point and exit from its highest point. This avoids trapping air in the water jacket.

13 Figure I.8. A simple reflux apparatus. J. Carrying Out a Simple Distillation Distillation involves the separation or purification of liquids by vapourization and condensation. The liquid is vapourized by heating it (see Part E, Heating Methods), and is then condensed in a water-cooled condenser arranged so that the liquid drips into a collecting vessel (and not back into the heated vessel as was the case for a reflux). The apparatus for a simple distillation is shown in Figure I.9. All ground-glass joints must fit tightly; if any of them do not seal (especially the ones at the distilling flask), the distillation will likely fail. Tip: Note the arrangement of clamps in Figure I.9. You may use a clamp to support the condenser while you are assembling the apparatus, but do not clamp the

14 condenser too tightly in the final set-up as this tends to encourage the separation of joints at the adapters connected to it. As with a reflux, the liquid being distilled must be stirred. It should be heated slowly until a gentle boil is achieved and the liquid begins to drip slowly into the receiving flask. The temperature of the distilling liquid is measured using a thermometer inserted into the 3-way connecting tube; the temperature is measured as the liquid condenses on the bulb of the thermometer and drips back into the distilling flask. The distillation is complete when the temperature either rises (meaning that there is a higher boiling liquid remaining in the distilling flask) or falls after the liquid stops collecting in the receiving flask. Figure I.9. A simple distillation apparatus.

15 K. Using a Separatory Funnel A separatory funnel is used to separate two immiscible liquids. It may be used in procedures that are referred to as extractions or washings (but don t confuse this with washing a solid precipitate that has been collected by filtration). Before using a separatory funnel, it is important to consider the densities of the two immiscible liquids. The denser liquid phase will be found on the bottom and the less dense liquid will be found on top. Generally, the bottom layer is drained through the stopcock and the top layer is collected by pouring the remaining liquid from the top. Do not discard either layer until you are sure which is which! 1. Place the separatory funnel in an iron ring. 2. With the stopcock closed, add the two liquid phases (do not fill past half capacity). 3. Replace the stopper. 4. Hold the separatory funnel and stopper tightly and invert as shown in Figure I.10. 5. Carefully open the stopcock to vent any pressure that may have built up (preferably into a fume hood). Point the tip away from all workers, including yourself. 6. Close the stopcock and shake gently using a swirling motion. 7. Repeat steps 5 and 6 until no further pressure build up is detected. 8. Replace the funnel in the iron ring, remove the stopper, and drain bottom layer through the open stopcock. 9. Close the stopcock and pour out the upper Figure I.10. Holding a separatory funnel layer through the top of the funnel. (Figure 56 in Ref. 1). 10. Do not discard either layer until you are sure which is which (consider their densities). Tip: Do not shake the funnel too vigorously as this may promote the formation of an emulsion. If the interface between the two liquids is cloudy, consult your T.A. L. Drying a Product That is Dissolved in an Organic Solvent The most frequent use of a separatory funnel (see Part J, above) is to wash an organic liquid or solution with an aqueous solution. This may be done for many reasons, including washing with a basic solution to remove an acidic impurity in the organic phase (or vice-versa), to use the aqueous solution to destroy or quench a particular excess reagent in the organic phase,

16 or to remove a water-soluble impurity from the organic phase. The key part of the process is to understand that the desired product generally remains dissolved in the organic solvent. Typically, the organic phase is placed in an Erlenmeyer flask and an excess of a solid drying agent is added. After the drying agent is given time to work, the mixture is gravityfiltered to remove the solid drying agent (see Part D). Usually, the drying agent is an anhydrous (meaning without water ) inorganic salt that forms a hydrate as it adsorbs the water. Anhydrous sodium sulfate (Na 2 SO 4 ) and anhydrous magnesium sulfate (MgSO 4 ) are commonly used drying agents. Sodium sulfate is almost universal in its application, but is fairly slow and therefore should be left for at least 5-10 minutes before continuing it also has the advantage of clumping when it absorbs water, so the presence of free crystals of Na 2 SO 4 is a good indication that the solution has dried. Magnesium sulfate is incompatible with some mixtures, particularly acidic compounds, and is also a very fine powder that can sometimes bypass filter paper, so some care should be exercised when using MgSO 4 as a drying agent. Tip: After decanting off the dried organic phase, wash the drying agent with several small portions of the same organic solvent. Invariably, some of the organic phase (and your product!) remains adhered to the surface of the drying agent and your yield will be diminished accordingly if you skip this step. References 1. Zubrick, J.W. The Organic Chem Lab Survival Manual, 2nd ed.; Wiley: New York, 1988, Chp. 4, 7, 9, 10, 11, 15, 16. 2. Szafran, Z.; Pike, R.M.; Singh, M.M. Microscale Inorganic Chemistry: A Comprehensive Laboratory Experience; Wiley: New York, 1991, pp. 67-69, 74-78. 3. Information sheet accompanying Corning model PC-300, PC-320, PC-500, and PC-520 combination stirrer/hot plates, Corning, Inc., New York, 1987.