Demonstration of Partition Coefficient, Chemistry tutorial

Introduction:

This series of experiments will familiarize us through the general technique of liquid-liquid extraction. Extraction is an excellent way to divide the components of a mixture depends on differential solubility in 2 immiscible solvents, generally water and an organic solvent. Separating a water-soluble compound from an organic-soluble compound is merely a matter of dissolving them in the solvent mixture and physically separating the 2 layers.

This is an especially powerful technique for chemists who have a good command of acid-base properties (and pKas) and understand how solubility depends on polarity. One can often take advantage of acidity and basicity to shift a compound from one layer to another as desired and consequence a clean division.

Experiment A. Determination of the partition coefficient for benzoic acid in CH2Cl2 and H2O

Equipment and supplies

We require a centrifuge tube fitted through a cap and a couple of Pasteur filter pipettes. Syringe pipettes will be provided for each solvent utilized in this part of the experiment. Our instructor will instruct us on how to utilize the syringe pipettes. The chemicals required for this experiment are benzoic acid, methylene chloride, water and anhydrous sodium sulfate.

Caution! Methylene chloride is a recognized carcinogen. When working through it, wear 2 sets of gloves (a blue pair over a white pair) and avoid breathing the fumes. Keep all containers and apparatus by methylene chloride in the hood at all times.

Procedure

Add 50 mg of benzoic acid followed via the addition of 1 ml of water and 1 ml of Methylene Chloride to a centrifuge tube. A syringe is supplied for each transfer (a syringe is joined to each solvent bottle). Cap the centrifuge tube and either watchfully shake the contents of the energetically for 30 seconds via hand or utilize a Vortex mixer. Remove the cap and permit the 2 layers to divide. Which solvent is the top layer? Which solvent is the bottom layer? What is a quick and easy technique/way to identify either layer?

Carefully eliminate the organic phase using a Pasteur pipette, Transfer the methylene chloride layer to a dry conical vial and add about 50 mg of anhydrous sodium sulfate. As sodium sulfate is a drying agent (absorbs water) and removes trace of moist only, you might add some extra anhydrous sodium sulphate in order to dry the organic phase entirely. Recap the vial and let the sodium sulfate dry the organic phase for 15 minutes.

Transfer the dried organic phase via a dry Pasteur pipette to a tared and dry conical vial containing a boiling chip. Rinse the sodium sulfate by about 600 ? l of methylene chloride and combine the organic extracts. Why is a rinse performed? Evaporate the organic solvent in the fume hood using a warm sand bath and  reheat the vial to eliminate the last traces of solvent (and water) until a constant weight of the solid is attained. Turn in our dried product in a properly labeled plastic bag. Determine the amount of benzoic acid recovered and compute a value for the distribution coefficient (KD).

Experiment B.  Microscale partitioning of a coloured indicator.

In this experiment, we will utilize an indicator, 2, 6- dichloroindophenol (In-OH), to see how acidity and basicity can be utilized to shift a compound between organic and aqueous layers. Think about what's happening at each step of the process. The colours will assist us keep track.  Be sure to watchfully record our observations at each step of the procedure.

988_Microscale partitioning of a coloured indicator.jpg

Fig: Microscale partitioning of a coloured indicator.

A solution of 25 mg of In-O- Na+ in 50 ml of 0.02 M NaOH will be provided. It should be BLUE; if it's not blue, don't utilize it, and don't just put it back either - hand it to your TA. Add 0.1 ml (100α l) of the solution to a 2 ml pre- made mixture of 1:1 H2O-CH2Cl2 in a 5-ml conical vial. As we know that the appearance of the mixture. Shake the vial and note any transform that take place. Next, add 100 α l of 0.05 M aq HCl and note the appearance of the mixture - what color is present, and where is it? Shake and examine. At last, add 100 α l of 0.1 M aq NaOH, then shake. How do you clarify the examined transforms? Repeat this complete experiment using diethyl ether in place of dichloromethane.

Experiment C.  Macroscale separation of an acid, a base, and a neutral compound

In this experiment, we will take benefit of acidity and basicity to divide benzoic acid, benzocaine (ethyl p-aminobenzoate), and fluorenone. This experiment will be done on a larger scale than the preceding experiments, so we'll require utilize a separator funnel ("sep funnel"). Vent  the  funnel  regularly  and  carefully  via  pointing  it  away  from  our self and  others. Diethyl ether is rather volatile, so work in the hood as much as possible to avoid filling the lab through fumes.  A few helpful hints:  (1) some of the ether will evaporate during the procedure, so you may need to add a little more - try to keep the volume from getting too low. (2) We will require eliminating one layer or another in each step of the extraction - don't throw anything away until we're done! Even though we might think there's nothing valuable in there, we could be mistaken. Chemists more experienced than we have accidentally tossed valuable - really valuable - compounds in the waste bottle. If we save all the solutions we can backtrack until we find out where we lost the goodies. (3) keep a beaker under the Sep funnel as we fill it in case the stopcock leaks (or if we forget to close it). (4) Be sure that all our flasks are labeled so we can keep track of what's what. (5) Be careful in neutralizing strongly acidic or basic solutions. Such are exothermic processes. It's good practice to have an ice bath nearby just in case things get out of hand.

Before coming to lab, we must have in our notebook a flow chart showing each step of the separation procedure, including the structures of the compounds and which compounds go where in each step. An instance is offered below.

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Fig: Mixture of the 3 compounds

A mixture of the 3 compounds (in unknown ratio) will be provided. Weigh out 300 mg of this mixture and dissolve it in about 10 ml of diethyl ether in a hood. Transfer the solution to our 30-ml separating funnel.

Watchfully add 4 ml of 3 M aqueous HCl, shake (remember to vent the funnel frequently), allow the layers to divide, and eliminate the aqueous layer. Repeat this step through another 4-ml portion of 3 M aq HCl and combine this through the other aqueous layer. (As we know that such directions can be shortened via saying: "extract the Et2O solution twice via 4-ml portions of 3 M aq HCl", or "... with 2 x 4 ml 3 M aq HCl".)

Add 6 M aq NaOH dropwise to the acidic aqueous solution until it is basic (use litmus paper). Cool the solution in ice for 10 - 15 min, isolate the solid by suction filtration, rinse it through two 2- ml portions of cold water, and permit it to air-dry. (What is this solid?!)

Extract the ether solution by 2 x 4-ml of 3 M aq NaOH. Set the joined aqueous layers aside. Wash the ether solution by 2 x 2 ml water followed by 2 ml of brine (saturated aq NaCl), and transfer it to a clean, dry Erlenmeyer flask. ('Wash' means basically the similar thing as "extract". To "extract" is to gain good stuff; to 'wash' is to eliminate dirt, for example impurities.) Add about 0.5 - 1 g of anhydrous Na2SO4, stopper the flask, and set it aside. This drying agent soaks up any residual water. Most of the sodium sulfate should be free-flowing; if it's all clumped, add a little more. Take the basic aqueous solution from above and acidify it via dropwise addition of 6 M aq HCl. Cool the solution in ice, isolate the solid through suction filtration, rinse it by two 2-ml portions of cold water, and permit it to dry.  Filter the drying agent from the ether solution and rinse it through some mls of ether. Evaporate the solvent by a stream of dry air in the hood. After all the solvent is gone, there should be a solid left.

Find out the mass, % recovery, and melting points of the 3 solids, label them, and turn them in.

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