EXPERIMENT 7A Chemical Separation by Filtration and Recrystallization INTRODUCTION The solubilities of solid substances in different kinds of liquid solvents vary widely. Substances that we call salts (ionic compounds) often have very appreciable solubilities in water. Substances we call nonmetallic networks (such as sand or diamond) are completely insoluble in water. The separation of soluble and insoluble substances can be carried out by filtration. When one substance, substance A, is soluble in water, and a second substance, substance B, is insoluble in water, the two can be separated by filtration. By adding water to the mixture, substance A will dissolve, while substance B will not. The liquid that is in contact with the solid, called the supernatant, will contain substance A. By passing the supernatant and solid through filter paper in a funnel, the solid (substance B) will be trapped on the filter paper, while the solution that has passed through the filter paper, called the filtrate, will contain substance A. When a vacuum is applied during filtration, the technique is called vacuum filtration, and this speeds up the filtration process. After the filtration, the small amount of solution that remains in contact with the solid substance B contains dissolved substance A. This is a source of contamination. To remove these contaminants, the solid substance B is washed with deionized water. The water is added directly to the funnel with the solid substance B, and is removed along with the contaminants by vacuum filtration. The solid substance B can then be dried to remove moisture before it is weighed. However, filtration will not work as a chemical separation technique when the substances to be separated are both soluble in water. In this case, a highly effective way of separating these two soluble substances is called recrystallization. To obtain one of the substances from a mixture in pure form, two conditions must apply. First, the substance to be collected (or purified), substance C, must be more soluble in water at high temperatures than at low temperatures. Second, the amount of the other solid, substance D, which is also soluble in water, must be rather small. For this reason, the substance D is sometimes referred to as the impurity. When the impurity D consists of such a small amount that it is completely soluble in the water, either cold or hot, recrystallization can be used to separate it from substance C. The technique consists of dissolving the mixture in the minimum amount of water possible at or near its boiling point, then cooling the hot, saturated solution. As the solution cools, substance C becomes less soluble, and crystallization of pure substance C occurs, with the impurity D remaining dissolved in the cold water. In this experiment you will be given a sample containing relatively large amounts of silicon carbide and potassium nitrate, and a small amount of copper (II) sulfate pentahydrate. Silicon carbide, or carborundum, is an insoluble macromolecular substance. Potassium nitrate and copper (II) sulfate pentahydrate are both water soluble ionic compounds. By adding water to the mixture, the potassium nitrate and the copper (II) sulfate pentahydrate will dissolve, while the silicon carbide will not. By passing this mixture through filter paper in a funnel, the silicon carbide will be trapped on the filter paper, while the solution of potassium nitrate and copper (II) sulfate will pass through by applying a vacuum. After the filtration, the small amount of solution that remains in contact with the silicon carbide contains dissolved potassium nitrate and copper (II) sulfate pentahydrate. To remove these contaminants, the silicon carbide is washed with deionized water. The water is added directly to the funnel with the silicon carbide, and is removed along with the contaminants by vacuum filtration. 63
At this point in your experiment, you have roughly a large amount of potassium nitrate and a small amount of copper (II) sulfate pentahydrate dissolved in water. These two remaining components of the mixture, potassium nitrate and copper (II) sulfate pentahydrate, are both soluble in water. The solubility curves for these two compounds, as well as for silicon carbide, are shown below. They indicate the number of grams of each compound that will dissolve in 100 grams of water, at temperatures ranging from 0 C to 100 C. To begin the separation of the two components by recrystallization, the solution is boiled to reduce the volume of water to the minimum amount needed for both components to remain dissolved. The solution is then cooled to 0 C, lowering the solubilty of both componenets, as seen on the solubilty chart above. At 0 C the potassium nitrate is not very soluble (10 grams per 100 grams of water), and most of it crystallizes out of solution. Notice, reducing the volume of water to the minimum amount needed for both components to remain dissolved, to say to 50 grams, would reduce the solubilty of the potassium nitrate to only 5 grams, so more of it crystallizes out of solution. Since copper (II) sulfate is not present in a large amount (hopefully less than 20 grams per 100 grams of water), its solubility limit is not exceeded and all of it remains in solution. The solid potassium nitrate is then separated from the solution by vacuum filtration. PROCEDURE 1. Students will work individually for this experiment. Except for the laboratory handout, remove all books, purses, and such items from the laboratory bench top, and placed them in the storage area by the front door. For laboratory experiments you should be wearing closed-toe shoes. Tie back long hair, and do not wear long, dangling jewelry or clothes with loose and baggy sleeves. Open you lab locker. Put on your safety goggles, your lab coat, and gloves. 2. Record the number of your unknown mixture in the space provided in the Data Table. Take a clean, dry 150-mL beaker and your unknown to an analytical balance. Before weighing anything on the balance, press the on/tare button and make sure the display reads "0.0000 g". Carefully open the side sliding door of the balance. Watch for debris in the track of the sliding door and clean it out if it is present because the sliding door is one of the most fragile parts of the balance. Place the 150-mL beaker on the balance pan and close the door. When the display shows a mass with four places past the decimal point followed by a "g" for grams, press the on/tare button to zero-out the mass of the beaker. 64
3. Remove the beaker from the balance and add all of the unknown mixture to the beaker. Replace the beaker on the balance pan and close the sliding door. Record the mass of the unknown mixture in your Data Table. NOTE: If any crystals are spilled on the balance or on the lab bench, clean them up immediately, and dispose of them in the solid waste container in the fume hood. If there are any crystals left on the balance or the lab bench at the end of the lab period, the instructor will deduct one point from everyone s lab score as a charge for cleaning up after you. At your lab station, add enough deionized water to the beaker so the total volume reached 50 ml, which will be enough to dissolve the soluble compounds. SEPARATION OF SILICON CARBIDE 4. Obtain a Büchner funnel with gray conical funnel-support from drawer 040, and a filter flask from drawer 044 or the drying rack by the sink to the left of the white board. CAUTION: Immediately clamp your filter flask to a ring stand with a universal clamp. If you do not, the weight of the rubber tubing will knock over the filter flask and it may break. Clamp the filter flask such that it is sitting on the base of the ring stand. Assemble the Büchner funnel, funnel-support, and filter flask, attach it to your ring stand as shown below, and attach the rubber tubing to the vacuum line. 5. Obtain a piece of Whatman Grade No. 2 filter paper (medium porosity, medium flow rate, 8 μm particle retention) from the back of the lab room, and determine its mass on your analytical balance. One side of the filter paper has a microlip that you can feel when you run your fingertip along its circular edge. With the microlip side of the filter paper face down, place it in the Büchner funnel. Turn on the vacuum and spray the filter paper with deionized water from your wash bottle, then turn off the vacuum. 6. Place the 150-mL beaker with its solution on a hot plate, and adjust the hot plate to a setting of 40ºC. Heat the mixture in the 150-mL beaker to 40ºC, while stirring the mixture with a glass stirring rod. Monitor the temperature with your thermometer. CAUTION: Do not stir with the thermometer. If the solution is not heating up, increase the setting on the hot plate. Do not let any of the solid or liquid leave the solution on your stirring rod or thermometer! Rinse the stirring rod and the thermometer off with a very small amount of deionized water before you remove them from the solution. 65
7. Once the solution has reached 40ºC, insure that the blue and white solids are completely dissolved, turn on the vacuum and pour the solution and solid into the Büchner funnel. Transfer all of the liquid and as much of the solid as possible to the Büchner funnel without using any water from your deionized wash bottle as shown in the left diagram below. For now, leave the large amount of solid in the 150-mL beaker. 8. Turn off the vacuum, and carefully remove the Büchner funnel and funnel-support from the filter flask, setting the Büchner funnel on an unused beaker for balance. Transfer the blue filtrate from the filter flask to another clean 150-mL beaker. Add 15 drops of 6 M HNO 3 to the blue filtrate, which will help ensure that the copper (II) sulfate remains in solution in later steps. Save this solution for step 15. 9. Reattach the Büchner funnel and funnel-support to the filter flask and turn on the vacuum. Transfer the remaining amount of solid to the funnel using a stream of deionized water projected from your wash bottle as shown in the right diagram below. All of the solid should be transferred to the Büchner funnel. You are trying to determine the amount of solid silicon dioxide in your unknown, so do not leave any of the solid in the 150-mL beaker! Use as much water as necessary. 10. Wash the solid in the funnel thoroughly with deionized water to remove any potassium nitrate-copper (II) sulfate impurities. You should wash the crystals until the wash liquid that passes through the Büchner funnel into the filter flask is no longer blue. Continue to apply suction for about 5 minutes to dry the solid. Test the solid to see if it is dry by moving it around with a clean spatula. If it is not dry, continue applying suction, testing with a spatula. Once it is dry, gradually turn off the vacuum. 66
11. Label a 250-mL beaker with your name. Determine its mass on your analytical balance, and record its mass in the Data Table. With the vacuum off, use your microspatula to lift the filter paper and the silicon carbide crystals from the funnel, and place the filter paper in the 250-mL beaker. Remove any of the silicon carbide crystals that are adhering to the Büchner funnel with your rubber policeman, and wipe the crystals onto the inside of the 250-mL beaker. All of the crystals should be transferred to the 250-mL beaker, do not leave any of the crystals in the Büchner funnel. NOTE: If any of the silicon carbide passes through the Büchner funnel into the filter flask, obtain another piece of filter paper, record its mass, place it in the Büchner funnel, and refilter the solution from the filter flask. Add this piece of filter paper and the collected silicon carbide crystals to the 250- ml beaker. If your instructor wants you to dry the crystals in the drying oven, proceed to step 12. If your instructor wants you to dry the crystals in your lab lock, proceed to step 13. 12. Cover the 250-mL beaker with a watchglass, and carefully place it in the drying oven for at least one hour. The washings in the filter flask may be discarded in the liquid waste container in the Fume Hood A. Rinse out the filter flask with deionized water and reclamp it to a ring stand with a universal clamp. Wipe clean the Büchner funnel so that all the black crystals are removed, and set it on a paper towel to dry. While the crystals are drying, proceed to step 14. 13. Cover the 250-mL beaker with a watchglass, and carefully place it in your laboratory locker to dry until the next laboratory period. The washings in the filter flask may be discarded in the liquid waste container in the fume hood. Rinse out the filter flask with deionized water and reclamp it to a ring stand with a universal clamp. Wipe clean the Büchner funnel so that all the black crystals are removed, and set it on a paper towel to dry. PURIFICATION OF POTASSIUM NITRATE 14. Half-fill a plastic container with ice from the back of the lab room, and prepare some ice-cold deionized water by placing a wash bottle at your table into the ice bath. 15. Place the 150-mL beaker with the blue filtrate on the hot plate, adjusted to a setting above water s boiling point. Place your stirring rod in the beaker, heat the mixture to the boiling point, and boil the solution gently and constantly. If the solution is not heating up, increase the setting on the hot plate. Once the boiling point is reached, the solution should be clear and blue. CAUTION: The hot liquid will have a tendency to bump, so do not heat it too strongly. Turn down the hot plate setting if necessary. When the volume has been reduced to about 25 ml, pick up the beaker using beaker tongs (not the crucible tongs), and carefully swirl the contents to wash as many of the crystals on the inside of the beaker back into the solution. Wash any remaining crystals on the inside of the beaker back into the solution using a stream of deionized water from a wash bottle with room-temperature deionized water, trying to use as little water as possible so that the total volume of solution is no more than 30 ml. If the solution is clear blue with no solid present, proceed to step 13. If the solution becomes cloudy, heat again on the hot plate, and while still heating, mix with a glass stirring rod to produce a clear blue solution with no solid present. If the solution will not clear, add a few milliliters of water and mix until a clear solution is produced. Turn off the heat and proceed to step 16. 67
16. Using beaker tongs remove the 150-mL beaker containing your solution from the hot plate, and set it onto a hot pad on your lab bench for ten minutes. After cooling on the hot pad, transfer the 150-mL beaker to the ice bath (the plastic container) and let your solution cool undisturbed for ten more minutes. White crystals of potassium nitrate will come out of solution. 17. Reassemble the Büchner funnel, funnel-support, and filter flask. Obtain another piece of circular filter paper from the back of the lab room and place it in the Büchner funnel. Turn on the vacuum and spray the filter paper with deionized water from your wash bottle, then turn off the vacuum. Chill the Büchner funnel by adding 25 ml of ice-cold deionized water to the funnel and allowing the water to sit for one minute. After one minute, turn on the vacuum, and draw the water through the funnel with suction. 18. Filter the potassium nitrate slurry through the cold Büchner funnel with suction. Use your microspatula to transfer the crystals to the funnel if necessary. Wash the crystals in the funnel with the ice-cold deionized water to remove any of the blue copper (II) sulfate impurities, trying to use as little water as possible. Wash the crystals until the wash liquid that passes through the Büchner funnel into the filter flask is no longer blue. Continue to apply suction for about 5 minutes to dry the solid. Gradually turn off the vacuum and remove the filter flask from the Büchner funnel. The solution may be discarded in the liquid waste container in the fume hood. 19. With the vacuum off, use your microspatula to lift the filter paper and the white crystals from the funnel, and place the filter paper and crystals in a clean, 250-mL beaker. Cover the 250-mL beaker with a watchglass, and carefully place it in your laboratory locker to dry until the next laboratory period. 20. To clean the Büchner funnel, reconnect it to the filter flask, and then attach the rubber tubing from the filter flask to the vacuum line. With the vacuum off, wash the funnel with deionized water, and turn on the vacuum to draw the water through the funnel. Turn off the vacuum, remove the Büchner funnel from the filter flask, and dry it. Return the Büchner funnel, funnel-support, and unknown container to the back of the lab room. To clean the filter flask, rinse it three times with deionized water, emptying the washings into the sink. Hang the filter flask on the drying rack by the sink to the left of the white board, with the hose suspended over the sink. If your black silicon carbide crystals are in the drying oven, proceed to step 21. If your black silicon carbide crystals are in your lab lock, proceed to step 22. SILICON CARBIDE CRYSTALS 21. Remove the beaker with the black silicon carbide crystals from the drying oven and allow them to cool to room temperature on your lab bench. Once cool, use your analytical balance to determine the mass of the beaker, filter paper and silicon carbide crystals. Place your filter paper and silicon carbide crystals in the wide-mouth waste bottle in Fume Hood A. Proceed to step 23. 22. In Boxes 1 and 2, set up all of the calculations necessary to complete the experiment. At the beginning of the next laboratory period, you will (1) remove the beaker from your locker, (2) use your analytical balance to determine the mass of the beaker, filter paper and silicon carbide, and (3) complete your calculations in Boxes 1 and 2. 23. Clean and wipe dry your laboratory work area and all apparatus. When you have completed your lab report have the instructor inspect your working area. Once your working area has been checked your lab report can then be turned in to the instructor. 68
EXPERIMENT 7A LAB REPORT Name: Student Lab Score: Date/Lab Start Time: Lab Station Number: DATA TABLE BEFORE DRYING Unknown Code Number Mass of Unknown Sample. g Mass of Filter Paper. g Mass of 250-mL Beaker. g AFTER DRYING Mass of 250-mL Beaker, Filter Paper, Silicon Carbide. g 1 Mass of Silicon Carbide. g 2 Percentage of Silicon Carbide in Unknown Sample. % CALCULATIONS 1. 2. 69
QUESTIONS 1. From the solubility curve on page 64, what is the solubility of KNO 3 in 100 grams of water at (a) 0 C? grams (b) 100 C? grams 2. From the solubility curve on page 64, what is the solubility of CuSO 4. 5H2 O in 100 grams of water at (a) 0 C? grams (b) 100 C? grams 3. What is the solubility of KNO 3 in 10 grams of water at (a) 0 C? grams (b) 100 C? grams 4. What is the solubility of CuSO 4. 5H2 O in 10 grams of water at (a) 0 C? grams (b) 100 C? grams 5. A mixture of 9 grams of KNO 3 and 1 gram of CuSO 4. 5H2 O is mixed with 10 grams of water. (a) Does the entire mixture dissolve at 100 C? (b) When cooled to 0 C and crystals form, will the crystals be 100% KNO 3? (c) To 3 significant figures, what percent of the KNO 3 from the mixture is collected in the crystals? 70