Experiment Ic

Separation of Solutions: Liquid Extractions

Reading (in Zubrick): Ch 15 & 16 (all)

Outline: Separate a solid-phase mixture of 2-methoxynaphthalene, m-nitroaniline, and benzoic acid by initially dissolving the entire mixture in a suitable organic solvent, and then selectively extracting each solute from the original organic solution into other solvents, and finally isolating, characterizing and quantifying the benzoic acid and m-nitroaniline.

Benzoic Acid	m-nitroaniline1	2-methoxynaphthalene
1para or meta-nitroaniline may be used for this experiment

Background

In Experiments Ia and Ib you purified a solid by selecting a solvent that a particular chemical is soluble in, but which another remains insoluble (e.g., water dissolved NaCl but not aspirin, or ether dissolved aspirin, but not NaCl) and by using the method of recrystallization to isolate a pure form of benzoic acid, from contaminants.  However, there are many times when neither of these methods will be appropriate, such as when you need to isolate two or more solids from the same mixture and both solids are organic in nature, with similar solubility properties, or when the liquids you are trying to separate boil at nearly the same temperature (distillation is a technique we will use in a later experiment).  Solvent extraction is a procedure that will work well in many of these cases. During an extraction, one solvent is used to dissolve (or partition) one or more of the components of the mixture, leaving other components behind (in the solid phase, the liquid phase, or in another solvent). In this experiment we will be working with "liquid-liquid" extractions, but the general principles will be the same for "solid-liquid" extractions as well.

Zubrick makes the important distinction between "extraction" and "washing". While the same series of experimental steps are generally used to carry out both of these techniques, and they use the same theory, the main difference is that in an extraction you are moving a substance you want to keep into a new solvent, while in washing you are moving a substance you will throw away into a new substance. In future experiments you may be instructed to "extract the product mixture with 3, 10-mL portions, of ether," or "wash the ether extracts with 10% sodium bicarbonate, and then with water." These instructions assume you understand the difference between extraction and washing.

In this experiment you will be given a mixture containing 2-methoxynaphthalene, m-nitroaniline and benzoic acid. The first step in the separation will be to dissolve the entire mixture in one organic solvent (diethyl ether). You will use the chemical and physical properties of these compounds to determine how to separate them in pure form using extractions (and washings, where necessary), and isolate the m-nitroaniline and benzoic acid in solid form. Follow the guidelines given in chapter 15 of Zubrick in determining the solvents you will use, the order in which you perform the extractions, and the methods you will use to get each compound back into the solid form.

• Day 1
  • Do separatory funnel extractions of several organic chemicals
  • Isolate pure benzoic acid
  • Isolate the nitro aniline compound
• Day 2
  • Quantitate recovered chemicals
  • Determine melt points for recovered chemicals

Protocol

Safety:

• 2-methoxynaphthalene is an irritant — wear gloves while handling it.
• m-nitroaniline (or p-nitroaniline) is an irritant, and is highly toxic — wear gloves while handling it; take special care not to ingest any of it or to inhale its vapors or dust.
• Benzoic acid is an irritant — wear gloves while handling it.
• Hydrochloric acid solution is corrosive and toxic — wear gloves while handling it.
• Sodium hydroxide solution is corrosive and toxic — wear gloves while handling it.  
• Diethyl ether (most often just referred to as ether) is highly volatile with a boiling point of about 35^oC, and extremely flammable — handle with care and pour from the bottle in the hood.

To do the experiment, first obtain about 3 grams (2.5-3.0 grams acceptable) of the solid mixture from the reagent cart. Dissolve your solid mixture in about 20-30 mL of diethyl ether (this can be accomplished since all three organic compounds are readily soluble in ether). It is best to add the dry chemicals to the empty separatory funnel and then add the ether directly to the separatory funnel. Add about 10 mL of DI water also to the separatory funnel.

Caution: Keep the lid on the ether bottle when not in use.  

Dissolve the solids in the separatory funnel by adding the stopper and mixing thoroughly. You can do your separatory funnel extractions in the lab (outside of the hood), but any open containers with ether must be handled in the hood (you can remove the stopper to the separatory funnel in the lab without problem).

Using known chemical and physical properties for these compounds develop an extraction scheme in order to separate the three solids, and to isolate each compound in solid form.  For example, what are the physical properties of these compounds, what are their solubilities or their lack of solubility in certain solvents (e.g., in ether or aqueous solvents). The extraction and washing solvents available are: water; 10% sodium bicarbonate solution; 10% sodium hydroxide solution; 10% hydrochloric acid solution.

Once you have successfully extracted either the m-nitroaniline or benzoic acid into one of the aqueous extraction solvents, you will need to isolate that chemical in pure solid form.  All of the organic chemicals will be initially soluble in the organic solvent (diethyl ether).  For example, how can you force the benzoic acid to be partitioned (extracted) into one of the extraction or wash solutions for purification?  Once it is in the appropriate solution as a dissolved solute, how can you convert it into an insoluble solid to isolate and characterize?

After you have isolated both the m-nitroaniline and benzoic acid in pure forms, you can discard the remaining organic liquid, containing the 2-methoxynaphthalene in the organic liquid waste container.  

How much m-nitroaniline did you recover? How much benzoic acid did you recover?  What are the physical properties of these compounds that you observed (color, crystal shape, etc.)?  What is the melting point for each these compounds? How do these values compare to the literature values? Do you think that you isolated these compounds in pure form, based on the observed melting points?

Guidelines for doing separatory funnel extractions:

Acidic solutions	Uses of acid solution	Basic Solutions	Uses of basic solutions
6 M HCl	Use this solution only to neutralize a basic solution never for extractions	6 M NaOH	Use this solution only to neutralize an acidic solution never for extractions
10% HCl (1.2 M)	Use this solution for washes and extractions (e.g., in a separatory funnel) never for neutralizations	10% NaOH (2.5 M)	Use this solution for washes and extractions (e.g., in a separatory funnel) never for neutralizations
10% NaHCO3 (sodium bicarbonate solution) can be used to neutralize an acidic solution in a separatory funnel, if you want to end up with a solution near neutral. To get rid of the NaHCO3, simply use 2-3 DI water washes, effectively neutralizing the ether layer after the bicarbonate washes.

Checking pH (acidity or basicity) of a solution.

Litmus paper is an effective and inexpensive method to determine if a solution is either basic or acidic. Use litmus paper in this experiment to determine if you have produced an acidic or basic solution.

• Red litmus paper is used to monitor for basic solutions (red paper turns blue in base)
• Blue litmus paper is used to monitor for acidic solutions (blue paper turns red in acid)

Chemicals or equipment used during this experiment:

6 M HCl 6 M NaOH 10% HCl 10% NaOH 10% NaHCO3

Diethyl ether Benzoic acid m-nitroaniline 2-methoxynaphthlene separatory funnel

Go To Experiment: VSEPR 1a 1b 1c 2 3 4 5 6 7 8 9 10
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Copyright © Donald L. Robertson (Modified: 09/11/2006)