LABORATORY PRACTICE Non-Instrumental Analysis

Transcription

1 LABORATORY PRACTICE Non-Instrumental Analysis Introduction This module will look at analysis that does not include sophisticated instrumentation. It will build on concepts and skills gained from Laboratory Tests and Procedures, including all safe working procedures, correct equipment selection etc. Methods to be examined include: Paper chromatography Column chromatography Advanced gravimetric analysis Boiling point determination Melting point determination Refractive index determination Recrystallisation Distillation: steam/ fractional, vacuum Manual and automatic extraction 1

2 CHROMATOGRAPHY Chromatography refers to separation of complex mixtures by the use of differences in interactions with an external medium. There are a large number of chromatographic techniques but they all have two common factors: a stationary phase which the components of a mixture interact with by surface adsorption or solubility a mobile phase which carries the mixture through the stationary phase. Schematic for a 3 component mixture separation using chromatography The diagram represents the separation, with time, into three discrete components. The mixture was initially placed at the top of the column as shown in A. Column D shows the separation along the column into triangles, squares and circles. Chromatography is now applicable to the separation of many coloured and colourless solutions by a range of methods including: 2

3 adsorption partition ion-exchange gel chromatography using techniques including: paper thin layer column gas-liquid high-pressure chromatography Adsorption chromatography Stationary phase: Mobile phase: solid liquid or gas The substances less easily adsorbed onto the stationary phase will be eluted first. Partition chromatography Stationary phase: Mobile phase: film of liquid adsorbed onto the surface of a solid support. gas or liquid Separation depends on how firmly the substances to be separated are adsorbed onto the stationary phase and their solubility in the mobile phase. Ion exchange chromatography Charged molecules adsorb to ion exchange resins reversibly so molecules can be bound or eluted according to their ionic charge. Gel chromatography (molecular-sieve chromatography) Separates substances on the basis of their molecular size. 3

4 Terminology Column: cylindrical tube through which solutions pass over the mobile phase Tank: container in which the chromatogram is placed during the separation. Stationary phase: solid or liquid adsorbent Mobile phase: liquid or gas that flows over the stationary phase to separate the mixture Elution: the use of a solvent to separate components of a mixture Development: movement of the solvent front along the paper or column Solvent front: the level reached by the elution fluid Resolution: the degree of separation of the components Eluate, fraction: the solvent containing the component that has been separated R f value: the ratio of the distance the substance being identified travels from the origin to the distance travelled by the solvent front. PAPER CHROMATOGRAPHY The stationary phase is the water bound to the cellulose of the paper. Partitioning occurs between the developing solvent and the bound water molecules as the mobile phase moves over the substance to be separated. Substances that favour the mobile phase over the stationary phase will move closer to the solvent front as development proceeds. To identify each substance, the relative fraction value, R f, of each species is compared to the R f value of similar standard substances. It is preferable to run a set of standards of known composition at the same time. 4

5 THIN LAYER CHROMATOGRAPHY Also uses partition chromatography with some adsorptive effects. It has the advantages of paper chromatography and overcomes the limitation that paper is not of great value for separating non-polar materials. The stationary phase is a mm layer of sorbent, such as silica gel or cellulose spread uniformly over the surface of a glass plate. COLUMN CHROMATOGRAPHY Uses a tube filled with the stationary phase and a solvent as the mobile phase. A small amount of the mixture is added to the top of the column and the separation occurs as solvent passes through the column. Various fractions are collected at the bottom of the column. GAS CHROMATOGRAPHY Mobile phase is an inert gas and the stationary phase is either a liquid distributed on a solid support or a solid. 5

6 At the end of the column a detector provides a response to the varying composition of the mobile phase. HIGH PRESSURE CHROMATOGRAPHY The mobile phase is pumped under pressure through a fine column which contains the stationary phase. PRACTICAL 1 In this session you will: use a technique known as paper chromatography to separate the components of a food dye and make decisions on the choice of the unknown dyes. In this procedure the stationary phase is water molecules absorbed onto the cellulose fibres of the paper. The mobile phase is a mixture of organic solvents. Procedure PART 1 Analysis of Food Dyes by Paper Chromatography To put your work with food dyes into a practical context, consider the following example. Your company makes a wide range of icecream and other dairy products. You produce a number of flavoured products, which contain real fruit purchased at great cost. Unfortunately the production process dulls the true colours. To enhance the appearance of each of your products you need to add the correct colour or blend of food colours green for lime, different yellow blends for mango, banana or apricot and different red blends for raspberry, strawberry or cherry. 6

7 You have just received some trial samples of three new food colours called unknowns 1, 2 and 3. These are more expensive than your usual colours (the standards) but are claimed to be far superior. This task you will perform, asks you to compare whether the new dyes are related to the old ones in any way. Is your company being asked to pay more money for new blends of the old products (something you can do for free) or are they really new and improved ones? PROCEDURE 1. Place a piece of chromatography paper in the template provided. 2. Place as small a dot as possible of each dye solution and mixture on the paper. Note the identity of each solution at the top of the paper in pencil or in your notes. 3. Mark a line in pencil denoting the starting line of the dyes. 4. Dry the spots with a hair dryer. 5. Place another small dot of the dye on top of the dot on the paper and dry again with the hair dryer. 6. Place the paper in the tank as directed. 7. Allow the solvent to travel up the paper until about 1cm from the top of the paper. Mark the solvent front with a pencil before it dries. (This step may be performed by the laboratory staff). 8. After the paper has dried identify which colours may be in the unknown dyes 9. Identify which of the dyes make up the unknown dye solutions. RESULTS and CALCULATIONS Composition of mobile phase: Distance moved by mobile phase: Sample Distance from origin R f Value 7

8 Questions Observe the food dye chromatogram and answer the following questions: 1. Do any of the new samples unknowns 1, 2 and 3 definitely contain any standard dye in current use? List them here: and explain your reasoning. Unknown 1 definitely contains Unknown 2 definitely contains Unknown 3 definitely contains 2. Do any of the new samples unknowns 1, 2 and 3 definitely not contain any standard dye in current use? List them here and explain your answer Unknown 1 definitely does not contain Unknown 2 definitely does not contain Unknown 3 definitely does not contain 3. Do any of the new samples unknowns 1, 2 and 3 contain components not in common to the standard dye in current use? List the number of unexplained components here and explain your reasoning Unknown 1 Unknown 2 Unknown 3 8

9 ASSIGNMENT You are asked to identify ions in a mixture that you have reason to believe contains cobalt, copper and nickel. You set up your chromatogram with samples of cobalt, copper and nickel in positions 1, 2 and 3 and your test sample in position 4. You run the chromatograms in a solvent mixture of ethylmethyl ketone : water: hydrochloric acid 45 : 45: 10 and obtain the following results. Note that the test sample has separated into two constituents. Sample Distance travelled by sample Distance travelled by solvent R f value (x 100) Cobalt 4.5cm 10 cm Copper 6.5 cm 10 cm Nickel 1.0 cm 10 cm Test spot 6.3 cm 10 cm 3.5 cm 10 cm Calculate the R f values for each spot and indicate the possible composition of the test mixture. Explain your reasoning. 9

10 Distillations The boiling point of a liquid is defined as the temperature at which the vapour pressure of the liquid is equal to the external pressure. Vapour pressure tells you how easily a liquid evaporates; those which evaporate easily have lower boiling points. The boiling point of a compound is used as a way to identify the substance Different substances have different boiling points. Liquids are frequently characterised by distillation, a technique that allows both purification and determination of the boiling point. A distillation apparatus is used to determine the boiling point (see the diagram below). The impure liquid is placed in the round bottom flask. A boiling chip must be added to this flask. The water to the condenser must be turned on before heat is applied, this is to ensure the gas vapour becomes condensed and doesn t evaporate into the laboratory. The flask is heated by a heating mantle. A bunsen burner is not used most organic liquids that are distilled are flammable and will catch on fire in the presence of a naked flame!!! The liquid is heated, it starts to boil, the vapour passes by the thermometer (which is used to measure the boiling point), into the cooling chamber called the condenser. The condenser has cold water running through it. The hot vapour hits the cold surface and becomes condensed (ie becomes a liquid). The pure liquid is collected in a receiver flask. When setting up the equipment it is necessary to lightly grease the joints to ensure that they don t become fused together during the distillation. The liquid should not be heated too fast, too high a temperature may cause bumping to occur, in which case the whole distillation would have to be set-up again, why? The distillation is stopped before the entire sample has become distilled; there are two reasons for this. The last few mls of the liquid is usually very impure and may also contain peroxides or other explosive compounds. If these compounds are overheated, an explosion will ensue. 10

11 Boiling Chips: Boiling chips must always be used during a distillation. When liquids are heated strongly in contact with a smooth surface such as glass, the liquid does no always boil smoothly. Rather it forms large bubbles. These bubbles erupt violently and can mechanically lift and agitate great quantities of the remaining liquid and may actually cause the hot liquid to spurt out of the apparatus. This is called bumping. The boiling chips can prevent this from occurring by forming a steady stream of small bubbles. Boiling chips must not be added to hot liquids. If the liquid is already at its boiling point the chip will cause it to boil at once and bumping may occur. Allow the liquid to cool, add the chips and reheat. Always use fresh boiling chips; they cannot be trusted to work more than once. 11

12 SEPARATION OF A MIXTURE BY STEAM DISTILLATION Steam distillation is widely used to separate (or extract) a volatile, water insoluble compound from non-volatile impurities. Or to distil volatile, water insoluble compounds that decompose when heated to their boiling points. Steam distillation consists simply of passing a stream of steam through the impure compound and condensing the steam and compound. The theory behind the separation is that the vapour pressure above the mixture is equal to the sum of the vapour pressures of the constituents. Therefore the boiling point of the mixture will be lower than the boiling point of the analyte. Apparatus for Steam distillation. a. A copper can to produce steam. The can has a glass tube, about 1 meter long, that reaches to the bottom of the can. This tube acts as a safety valve; any pressure in the system will cause the water in the can to blow out the tube. The steam-can is connected to the distillation flask by as short a piece of rubber tubing as possible, to cut down condensation of the steam. b. A round bottomed distillation flask is used to hold the impure compound. Before distillation, it should only be about one third full. Steam is introduced into the liquid by a piece of glass tubing that reaches to the bottom of the distillation flask. c. A splash head to prevent splashing of the contents of the distillation flask into the condenser. If a standard distillation flask is used (with a side arm), the flask is usually clamped at an angle to prevent splashing. d. A water cooled condenser. In this practical you will use steam to assist in the isolation of components of a mixture. 12

13 Techniques involved: Steam distillation Salting out of an organic liquid Separation of immiscible liquids Simple distillation Determination of refractive index. Procedure 1. With assistance from your teacher set-up the steam distillation apparatus. You will need to note carefully the purpose of each of the components and ensure that you able to quickly shut-down the apparatus in an emergency 2. Make an accurate sketch of the apparatus 3. Ensure the steam can is about three-quarters full of water before you light the Bunsen 4. Obtain your impure mixture and add it to the distillation flask. It may be a solid or liquid and all of it should be used 5. Start the distillation by Boiling the water in the steam can with a Bunsen burner Heating your sample using a heating mantle with sufficient water to ensure the flask is half-filled. When both the steam can and the flask are close to boiling, carefully connect the steam line and make sure the cooling water is flowing adequately 6. Collect the distillate until a separate phase or oily layer is no longer produced. 7. Stop the steam distillation by removing the safety tube from the steam can. Do not remove the Bunsen before you disconnect the receiving flask, or your distillate may be sucked back. 8. If you have a liquid product, you should pour the distillate into a separatory funnel supported in a retort ring. If you have a solid product, go to step Add about 20g NaCl to the funnel and agitate gently to dissolve. 10. When the layers separate completely, run of the organic layer into a 250 ml conical flask and extract the aqueous layer twice with 25 ml portions of trichloromethane. Add the trichloromethane washings to the organic phase. 11. Remove any obvious water residues using the separatory funnel. Add about 3g of anhydrous MgSO 4 to the organic phase, swirl and stopper the flask and leave it for 10 minutes. 12. Sep up a simple distillation apparatus. Pour the organic phase through a small wad of cotton wool set in the stem of a simple glass funnel into the distillation flask. 13. Distil the organic phase, collecting the distillate up to 120 o C. This fraction should be discarded in the trichloromethane residue bottle. 14. Change the receiving vessel and collect the product that distils in the range suggested by the teacher. Record the boiling point range of the product 15. Measure the refractive index of the product 16. The solid product should be cooled in ice and then filtered by suction using a buchner funnel. Place the moist product on a large watch glass to dry at room temperature. When dry determine the melting point of the material. 13

14 Discussion Compare the boiling point and refractive index or the melting point of your product to the values available in the literature Comment on the purity of your product Why would a compound be steam-distilled in preference to normal distillation? What is the purpose of the splash head in the steam distillation? Why was NaCl added to the separatory funnel? DIAGRAM OF STEAM DISTILLATION APPARATUS SEPARATION OF A MIXTURE BY FRACTIONAL DISTILLATION Simple distillation is useful to separate liquids that have a boiling point difference of greater than 70 0 C. It does not satisfactorily separate liquids that have close boiling points, for example a mixture of water and ethanol. Here the water has a BPt of 100 o while ethanol boils at 78 o C ie simple distillation is not able to isolate each of the fractions. 14

15 Fractional distillation is a useful technique for separation of soluble substances with boiling points that are close, for example water and ethanol. The method utilises a fractionating column which provides a large surface area for the separation to occur. When conducting your distillation take particular note of the temperature differences on the two thermometers. Procedure The mixture to be distilled is 100 ml of one of the following: 1:4 methanol:water mixture 1:4 trichloromethane/propanol mixture 1:4 trichloromethane/cyclohexane mixture 15

16 Part I Fractional Distillation 1. The distillation apparatus will be already set up, and its components will be explained by your teacher. Make an accurate sketch of the entire apparatus and a more detailed sketch of the operating details of the fractionating column. 2. Close the tap at the top of the column and commence heating. When the vapour reaches the top thermometer record the temperature on each thermometer. 3. When the temperature at the top of the column has remained constant for 3 minutes open the tap at the top of the column so that distillate is collected at a rate of about 1-2 drops / sec. 4. Use clean, dry sample tubes to collect a few drops of distillate at 5 ml intervals during the distillation until the second fraction has begun distilling. Record the temperature on both thermometers at each collection interval. 5. Measure the RI of the collected portion and record in the results table. EXTRACTION TECHNIQUES Purification is a general term used to describe the selective removal of components from a mixture. Typical examples would be: extraction of gold from its ore recovery of sugar from cane juices isolation of caffeine from tea/coffee Normally, extraction, is the name reserved for purification techniques which use a solvent to dissolve and recover a component of the original mixture. This can be achieved by using physical and / or chemical methods for the selection of the component of interest. Physical methods of extraction depend on: solubility differences between two immiscible solvents solubility differences between the original matrix and a solvent Typical physical methods use a specially selected solvent (or a blend of two immiscible solvents) chosen for their ability to dissolve only some components of the mixture. Hence a cup of tea is a good example of selective physical extraction since only the palatable fractions are dissolved and the nasties are supposedly left behind to be discarded with the tea leaves. An example of two immiscible solvents would be salad dressing, where different flavours are found in the different layers ie the oil and vinegar. Chemical methods of extraction depend on: selective modification of the chemical structure of the analyte subsequent change in physical properties to cause extraction to occur reversal of original chemical change to recover the desired component the most common chemical method involves change in ph Typical chemical methods utilise the solubilities at different ph of the components. Consider a mixture containing only organic chemicals which are insoluble in water eg crude oil. The two will not mix and so none of the components of the crude oil will migrate into the water. However some of the chemicals are 16

17 weak acids and can be made to react with a base such as NaOH. This reaction will cause the weak acids to become ionised and hence water soluble. ie weak acid + Na + + OH - Na + anion - + H 2 O Salting out Adding an inorganic salt (such as sodium chloride or sodium sulfate) to an aqueous solution containing an organic solute often reduces the solubility of the organic compound in the water and thus assists its separation. This salting out technique is often used in extractions and liquid-liquid separations: to maximise the transfer of an organic solute from the aqueous layer to the organic layer to separate n organic liquid from its aqueous solution to break emulsions Drying of organic solvents. Organic solutions obtained from the extraction processes will almost certainly contain a small amount of dissolved water. The presence of dissolved water is normally indicated by cloudiness of the organic liquid. The water can be removed by adding a solid drying agent, generally an anhydrous ionic salt such as magnesium or sodium sulfate or calcium chloride. These salts will absorb the water into their crystal lattice, removing it from the organic liquid. Once the cloudiness has been dispelled, the drying agent must be removed by filtration. EXTRACTION OF SOLIDS A separatory funnel is obviously unsatisfactory if the mixture is a solid, since the solid material will block up the tap at the bottom. Other than dissolution of the solid in a suitable solvent, the most common method of extraction of a solid is by the use of the Soxhlet continuous extraction apparatus shown below. The solid substance is placed in the porous thimble and the latter placed in the inner tube of the Soxhlet apparatus. This is fitted to a round-bottom flask, which contains the extraction solvent and boiling chips and attached to a reflux condenser. The solvent is gently boiled; the vapour passes up through the side tube to be cooled in the condenser. The solvent then falls back into the thimble and slowly fills the body of the Soxhlet. When the solvent reaches the top of the tube, it siphons back into the flask, and thus removes the portion of the substances it has extracted from the solid. The process is repeated automatically until complete extraction has been achieved. 17

18 AUTOMATED EXTRACTION Fat in Snack Food Fats form the major energy storage in animals, When fats are burnt in the body they produce energy. Extraction of fat from snack food can be done using an apparatus called a soxhlet extractor. PROCEDURE 1. Set up the apparatus as demonstrated. 2. Obtain and weigh a Soxhlet thimble and two evaporating basins. 3. Add to the Soxhlet about 5g of the allocated snack food and reweigh the thimble. 4. Place the thimble in the Soxhlet apparatus 5. Half fill the round bottom flask with trichloromethane or petroleum spirit. 6. Ensure the water is on to the condenser 7. Switch on the heating apparatus 8. Allow the system to cycle for at least 45 minutes. You are required to draw a schematic diagram to show the passage of solvent and fat through the system. 9. Turn off the heat and allow to cool 10. Transfer the solvent (containing the fat) to the large evaporating basin, place on a water bath in the fume hood and allow the solvent to evaporate to dryness 11. Dry the outside of the basin and reweigh 12. Place the thimble in a small evaporating basin and place in an oven at 70 o C for 1 hour 13. Reweigh the thimble 18

19 Calculations Food analysed Mass of empty thimble Mass of large basin Mass of small basin Mass of thimble + food Mass of thimble and residue in small basin Mass of large basin + fat residue Original mass of food Mass of food residue Mass of fat residue % Fat = mass of fat left in basin x 100 original mass of food Questions From the MSDS record the dangerous properties of trichloromethane What is the purpose of the cotton wool in the top of the extraction thimble? Why is trichloromethane a suitable solvent for this extraction? Soxhlet extraction is described as a continuous extraction technique. Explain what this means. 19

20 MANUAL EXTRACTION This practical is designed to separate a mixture of a neutral and acidic organic compound by extraction. Techniques involved Use of separatory funnel Neutralisation of acidic solutions Evaporation of organic solvent Safety Use of organic solvents Pressure buildup in separatory funnels Use of concentrated acids Procedure 1. Obtain the sample containing a neutral compound and an acidic compound 2. Weigh the mixture into a 150 ml beaker 3. In a fume hood, add 50 ml trichloromethane and stir with a glass stirring rod until all the solid has dissolved 4. Place the trichloromethane mixture into a 250 ml separatory funnel. 5. Add 50 ml of 1 M NaOH to the separatory funnel 6. Shake the mixture vigorously, making sure you release the pressure every 30 seconds 7. After 2 minutes shaking allow the mixture to separate (carefully remove the lid to allow this process to occur) 8. Run the bottom trichloromethane layer into a beaker. Do not leave any of this layer in the separatory funnel 9. Pour the aqueous layer out through the top of the funnel into a 250 ml beaker. 10. Return the bottom layer to the separatory funnel 11. Repeat steps 5 to 10 twice more. 12. Run the bottom layer into a clean, dry conical Flask. Avoid contamination by the top layer. If necessary, sacrifice a small volume of the bottom layer in the separatory funnel. 13. Add a spatula full of anhydrous magnesium sulfate to the flask and stopper. Leave for 10 minutes. If the liquid is not clear, add another spatula of magnesium sulfate. During this time go to step Filter the clear liquid through a cotton wool plug into a pre-weighed evaporating basin and place in the fumehood. 15. When all the trichloromethane has evaporated reweigh the evaporating basin. 16. If there is any of the bottom layer left in the separatory funnel with the top layer, run it into a waste beaker and discard in the organic residue bottle. 17. Combine all three top layers in the beaker 18. Carefully, using a pasteur pipette, add 10 M sulfuric acid unto the solution is acidic to litmus paper. 19. Cool the solution in an ice-bath 20. Filter the solid under a vacuum using a Buchner funnel and flask.. Dry the solid as much as possible. Place the moist solid and filter paper between two larger filter papers on a watchglass until completely dry. 21. Determine the melting point of the dried material. 20

21 Results Original mass of sample (g) Neutral compound Mass of empty evaporating basin (g) Acidic compound Mass of empty container (g) Mass or evaporating basin + neutral compound (g) Mass of container + acidic compound (g) Mass of neutral compound (g) Mass of acidic compound (g) Total mass of extracted material (g) Discussion Account for why the recovered material was not 100% of the original sample size. If your mixture had been of a neutral compound and a basic compound, what modifications to the procedure would you have made? How might you identify the two compounds you have separated? What reasons might there have been for the acidic solid not to have been dried in an oven? 21

22 CRYSTALLISATION METHODS There are various steps which must be followed: Dissolution in minimum volume of solvent plus 10% at boiling point Addition of charcoal with care Filtration at B.Pt without losses in the paper Addition of co-solvent if necessary until just cloudy or 50% of total Allow to cool slowly Filter, wash carefully and air dry Determine melting point Example of a procedure for recrysallisation of benzoic acid 1. Dissolve sample in a suitable solvent Water is a suitable solvent for benzoic acid Ideally a solvent should have a high solubility for the substance at its B>PT and a low solubility at room temperature. The impurities should either be insoluble in the solvent at its B>Pt (so that they are filtered out when hot solution is filtered) or they should have high solubility in the cold solvent (so they are left in the mother liquor) Therefore Place the impure material in a conical flask Dissolve the impure benzoic acid in water, using as small a volume as possible. Add some boiling chips and heat to boiling If dissolution has not occurred add 10% more water and reheat to boiling 2. Add decolourising agent Activated carbon is a decolourising agent. It has a large surface area and is able to absorb lots of impurities. Avoid excesses as loses can be incurred due to adsorption of product. Once dissolution has occurred allow the solution to cool slightly before adding the activated carbon. Disaster if you don t. 3. Filter the hot solution Use a heated, short stemmed funnel and a fluted paper. Ideally have a small portion of solvent in a conical flask and keep this heated. Sit the funnel on the top and filter the solution through as quickly as possible. Remember for benzoic acid solubility decreases dramatically as the solution cools. After filtration is complete use a small portion of hot solvent to wash the flask and filter paper. 4. Cool the filtered solution Cool in an ice water mix. If crystals fail to form there is probably too much solvent and this will need to be evaporated off (or all the sample has been lost in the filtration step). 22

23 5. Filter the crystal slurry Filter using a Buchner funnel. Pour the crystal mix onto the filter pad, use the filtrate to wash any remaining crystals onto pad if necessary. Wash the crystals with a small amount of ice cold solvent. 6. Dry the crystals Crystals can be oven dried (below the melting point) or just air dried. These practicals are designed to practice routine procedures used to grow crystals and purify solids. TECHNIQUES USED Dissolution in hot solvent Use of carbon adsorption for removal of impurities Co-solvent inducement of crystallisation Assessment of the quality of the end product SAFETY Burns from hot equipment and solutions Chemical hazards associated with unknown materials You will be provided with two impure solids, each of which will be purified by slightly different types of recrystallisation procedures. Procedure Sample A Purification initially by dissolving in a minium of hot water and then adding activated carbon to adsorb the coloured contaminant. Followed by recrystallisation using water as the solvent 1. Weigh accurately, approximately 5g of the supplied material into a labelled 250 ml conical flask. 2. Add an amount of hot water to just dissolve the solid followed by another 10% of this volume. 3. Carefully add activated carbon to your sample as shown by your teacher 4. Add boiling chips to the flask 5. Place a set of gravity filtration equipment, use a short stemmed funnel, in the drying oven 6. Gently heat your flask until the colour appears to have disappeared from the solution 7. Quickly but carefully, filter the hot solution through the hot filtration equipment 8. Wash the filter funnel with small amounts of hot water 9. Allow the crystals to grow in the filtrate ie cool the solution 10. Filter off the crystals on to a weighed filter paper 11. Leave to dry and then weigh and determine the melting point of your compound 23

24 Results Sample Mass of impure substance Mass of filter paper (step 10) Mass of recrystallised substance Melting point of substance Literature melting point for benzoic acid A Sample B Purification will involve the use of a co-solvent. The sample is les soluble in the co-solvent than it is in the original solvent and it should drop out of solution as fine crystals 1. Weigh accurately, approximately 5g of the sample into a labelled 250 ml flask.. 2. Add a small amount of hot ethanol to dissolve the sample 3. If the solution is coloured you will need to add some decolourising material such as activated carbon 4. Heat your sample and then filter to remove the carbon 5. Add small volumes of the co-solvent, water, until the solution is just cloudy 6. Allow the solution to cool 7. Filter, using a preweighed filter paper, and wash with small amounts of cold water 8. Air dry and weigh the sample 9. Determine the melting point of your sample Results Sample Mass of impure substance Mass of filter paper (step 17) Mass of recrystallised substance Melting point of substance Literature melting point substance B Calculations % recovery = mass of recovered material x 100 initial mass Sample A = Sample B = Discussion: Comment on any difficulties you had and the steps which need special attention. 24