Jove Lab Chem
Identification of Unknown Aldehydes and Ketones — Lab Prep
Source: Lara Al Hariri at the University of Massachusetts Amherst, MA, USA
- Preparation of Test Solutions
Here, we show the laboratory preparation for 10 students working in pairs, with some excess. Please adjust quantities as needed.
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Before you start, put on a lab coat, splash-proof safety glasses, and nitrile gloves. Note: DNPH is a shock explosive, so handle it gently with low friction tools. Phosphoric acid is also toxic and corrosive.
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Prepare 100 mL of a 0.1 M solution of DNPH in a 50/50 mixture of 85% phosphoric acid and ethanol. Add a medium stir bar to a 250-mL beaker. In a fume hood, measure 3 g of DNPH and gently pour it into the beaker.
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Measure 50 mL of 85% by weight aqueous phosphoric acid and slowly add it to the beaker. Place the mixture on a stir plate and gently stir it until the DNPH dissolves.
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Measure 50 mL of ethanol and pour it into the beaker. Once the solution looks homogeneous, turn off the stir motor and retrieve the stir bar.
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Vacuum filter the solution to remove residual solids before transferring it to a 100-mL container. Cap the container, label it ‘DNPH solution’, and place it in a hood for the test reagents.
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Prepare 200 mL of iodoform reagent. Measure 50 g of potassium iodide into a clean 250-mL beaker. Add 200 mL of deionized water and stir the mixture on a stir plate until the potassium iodide has completely dissolved.
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Measure 25 g of iodine and add it to the stirring solution. Once the iodine has dissolved, turn off the stir motor. Transfer the solution to a labeled 200-mL container and cap it. Place the iodoform reagent in the test solution hood.
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Make 22 mL of Tollens’ reagent. Measure 0.6115 g of silver nitrate and pour it into a 50-mL beaker. Note: Prepare this solution right before the lab because explosive silver nitride will start to form as the reagent decomposes.
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Add 12 mL of deionized water and stir until the silver nitrate has dissolved completely to form a 0.3 M silver nitrate solution.
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Measure 6 mL of 3 M NaOH and add it to the stirring silver nitrate solution. Brown silver oxide will precipitate from the solution.
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Measure 4 mL of 2 M aqueous ammonia. Use a Pasteur pipette to slowly add it to the stirring solution.
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Once you have added all 4 mL of ammonia, remove the stir bar and vacuum filter the solution. Pour the filtrate into a 25-mL brown glass bottle.
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Wrap the bottle in foil to shield it from light and label it ‘Tollens' Reagent’. Put the wrapped bottle with the other test solutions.
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- Preparation of the Laboratory
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Make sure that the sinks have paper towels and test tube brushes.
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Set up appropriate waste containers in a waste collection hood and place glass waste nearby.
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Fill a wash bottle with acetone and place it in the waste hood so that students can rinse strong-smelling chemicals out of their Pasteur pipettes before disposing of them.
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In a fume hood, pour about 5 mL of butanone in a labeled vial for each lab group.
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Prepare and label vials containing 5 mL of benzaldehyde in the same way. These are the known compounds that every student group will need.
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The unknown compounds are acetophenone, butanol, 3-methyl-2-butanone, pentanol, 2-pentanone, and 3-pentanone. For each compound, prepare two vials containing 5 mL of that compound and label them with a code name. Place the known and unknown vials in a central area.
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Set out the following glassware and equipment at each student lab station (we suggest that students work in pairs):
1 Lab stand 1 Small 3-prong clamp 1 Hotplate 1 Vacuum line 1 Test tube rack 1 Test tube tongs 1 600-mL beaker 4 250-mL beakers 1 150-mL beaker 5 50-mL beakers 3 10-mL graduated cylinders 5 5-mL graduated cylinders 1 250-mL filter flask 1 Büchner funnel 1 Filter adapter 4 Small watch glasses 1 Glass stirring rod 1 Thermometer with beaker clip 3 Pipette bulbs 1 Package of pH paper 1 Roll of lab tape and marking pen 1 250-mL bottle of DI water -
Put Pasteur pipettes, filter papers, and small test tubes in a central area.
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Place 3 M NaOH and 1,2-dimethoxyethane in a hood reserved for solvents.
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Set up the melting point analyzer and ensure that there are enough capillaries.
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At the end of the lab, promptly dilute any unused Tollens' reagent with at least 20 mL of water and flush it down the drain with copious tap water. Note: If necessary, use concentrated hydrochloric acid to precipitate the silver from the Tollens' reagent waste first.
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Clean silver-coated test tubes with concentrated nitric acid in a fume hood. Neutralize and dispose of the silver waste according to your lab's standard procedures.
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