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Separation of Acidic, Basic and Neutral Substances by Extraction

By mar_tin_s Oct 02, 2013 1819 Words
Separation of Acidic, Basic and Neutral Substances by Extraction

KEYWORDS
Solubility, density, organic layer, aqueous layer, drying agent, protonation, recrystallization, neutralization, extraction

ABSTRACT: The purpose of this experiment is to separate the substances in the acidic mixture and identify each constituent in the mixture that were individually separated using extraction. Three reaction tubes were used in the experiment. An acidic mixture containing benzoic acid, naphthalene, and phenol was weighed and placed in Tube 1 and the mixture was allowed to separate after adding saturated solution of sodium bicarbonate. The aqueous layer was then transferred to Tube 2. Contents of Tube 2 were allowed to separate and the aqueous layer was then transferred to Tube 3. To Tubes 2 and 3, concentrated HCl was added, and their pH was tested prior to heating for recrystallization. Results showed that the benzoic acid and phenol were separated and recovered. However, there were impurities that were collected together with the substances that caused discrepancies in the results.

INTRODUCTION. In organic chemistry, extraction is a separation technique between two immiscible solvents, often an aqueous or alcoholic phase and a nonpolar organic phase.

Benzoic acid, C7H6O2, is a white crystalline solid and a simple aromatic carboxylic acid with a faint, pleasant odor. Its molecular weight is 122.12 g/mol. It is very slightly soluble in cold water, soluble to acetone and benzene, and very soluble to ether and ethanol. Its solubility in water is 1.9 g/L at 0°C and 68 g/L at 95°C. It has a density of 1.2659 g/cm3 at 15 torr and has a melting point of 122.4°C. It has a boiling point of 249.2°C.

Figure 1. Structure of Benzoic Acid

Naphthalene is a white solid substance with a strong smell. It has a molecular weight of 128.19 g/mol. It is partially soluble in methanol and n-octanol, very slightly soluble in cold water and hot water. It has a density of 1.14 g/cm3, a melting point of 80.2°C, and a boiling point of 218°C.

Figure 2. Structure of Naphthalene

Phenol, C6H6O, is a white crystalline solid that has a molecular weight of 94.11 g/mol. It is an acid that is very soluble in alcohol and chloroform and soluble to water and acetone. It has a density of 1.07 g/cm3, a melting point of 40.89°C, and a boiling point of 182°C.

Figure 3. Structure of Phenol

This experiment aims to separate a mixture containing an acidic substance and a neutral substance using the principle of solubility and the method of extraction, to identify the substances in the mixture, to test the acidity and basicity of the substances and to calculate the percent recovery of the substances at the end of the procedure.

MATERIALS AND METHODS. The goals of this experiment are to separate the substances in the acidic mixture and identify each constituent in the mixture that were individually separated. In this experiment, the materials used were three 20-mL test tubes, which were used to contain the mixtures, 10-mL pipets, which were used to transfer substances accurately into test tubes, Pasteur pipettes, which were used to mix the aqueous layer and the organic layer, as well as to transfer one layer to another test tube, microspatulas, which were used to obtain pure substances from their respective containers, litmus paper, which was used to determine the pH level of the mixtures, a water bath, which served as the heating instrument for the mixture that will be essential for purifying the end product through recrystallization, and a beaker, which was used to hold the test tubes while heated in the water bath.

At the beginning of the experiment, the density of the mixture, which was 0.3 g/mL, was given. Therefore, the weight of the mixture can be derived from it and using an analytical balance to weigh is not needed anymore. The analytical balance was only used when weighing the end products obtained at the end of the experiment when each substance were individually separated already. The substances to be weighed were put in an evaporating dish.

Separation of Substances. The acidic mixture was initially prepared by the professors. 0.5 grams of this mixture should be put in one 20-mL test tube. This test tube can be referred to as Test Tube 1. A saturated sodium bicarbonate solution of 3 mL was added to Test Tube 1 using a 10-mL pipette. Then, a Pasteur pipette was used to mix the solutions. This was done by obtaining the lower layer then pushing it back into the test tube at the top of the above layer. This way the constituents of each layer can be mixed thoroughly while having enough contact to each layer, in which polar substances will remain in the aqueous solution while the nonpolar substances were dissolved in the nonpolar solvent. After mixing the layers in Test Tube 1 for three minutes, the layers were given enough time to separate themselves again. After that, the lower layer was transferred to another 20-mL test tube, which we can refer to as Test Tube 2, by using a Pasteur pipette.

Then, Test Tube 2 was backwashed with 0.5mL of ether. This is done by mixing them together and discarding the ether layer. This was done to ensure that all of the organic material that might contaminate the contents of Test Tube 2 will be removed.

After that, 3.0 mL of a 3M aqueous sodium hydroxide was added to Test Tube 1, and mixed together using a Pasteur pipette by getting the lower layer then forcefully pushing it back into the tube on the upper layer. The layers were allowed enough time to separate from each other, then the resulting lower layer was drawn using a Pasteur pipette then transferred to another test tube. This test tube was referred to as Test Tube 3. Test Tube 3 was backwashed using the same method as with Test Tube 2.

A saturated sodium chloride was added to Test Tube 1, then mixed using the same method mentioned above, and then removed using a Pasteur pipette. Ether anhydrous pellets were added to Test Tube 1 until they no longer clump together. Ether was added, removed and then discarded. The ether anhydrous pellets were also discarded.

The addition of concentrated HCl until the end of the experiment was not performed due to the lack of time.

However, these are the results that the students have obtained. As for the data of pH and percentage yield, the theoretical data will be shown since the procedures for these data were not successfully accomplished.

RESULTS AND DISCUSSION

Test Tube #
Constituent
Test Tube 1
Naphthalene
Test Tube 2
Sodium benzoate
Test Tube 3
Sodium phenolate
Table 1. Contents of Test Tubes

The original acidic mixture which was put in Test Tube 1 consisted of benzoic acid (C7H6O2), phenol (C6H6O), and naphthalene (C10H8). These three constituents were separated from each other based on their different solubilities and the application of protonation, which refers to the addition of a proton to an atom, molecule, or ion, and deprotonation, which is the removal of a hydrogen ion from a substance. In this experiment, the benzoic acid was deprotonated through a reaction with saturated sodium bicarbonate solution, which is a weak base. This reaction formed sodium benzoate in which the sodium from sodium bicarbonate bonded with benzoic acid. Sodium benzoate was found in Test Tube 2. On the other hand, in order to deprotonate phenol, a stronger base must be used. Sodium hydroxide served to be the base in this experiment. This reaction formed sodium phenolate, which was found in Test Tube 3. In order to reprotonate sodium benzoate and sodium phenolate, concentrated HCl was used.

Substance
Theoretical weight (in grams)
Observed weight (in grams)
Acidic Mixture
0.50
0.03
Table 2. Weight of Mixture

Material
Observed (in grams)
Theoretical (in grams)
Evaporating dish
N/A
N/A
Dish w/ C7H6O2
N/A
N/A
C7H6O2 crystals
N/A
0.167
Evaporating dish
N/A
N/A
Dish w/ C6H6O
N/A
N/A
C6H6O crystals
N/A
0.167
Table 3. Weight of Crystals

The students were not able to record the weights of the evaporating dish and the C7H6O2 crystals and C6H6O crystals.

CONCLUSION AND RECOMMENDATIONS. The pH level of the substances as well as the percent recovery we're not successfully recorded since the students we're not able to finish the experiment. In order to successfully accomplish the experiment, a carefully prepared schematic diagram must be planned and there should be proper coordination and cooperation from the students. The students must be able to efficiently divide the work in order to work in less time. Furthermore, the materials which will be used must work properly and be careful when using the equipments in a way that the substances won't be contaminated to obtain data as accurate as possible.

ASSOCIATED CONTENT
The process of transferring a solute from one solvent to another is called extraction. Solubility plays a major role in affecting extraction. For instance, to extract an organic acid, one must put a base, like NaOH for instance, because it deprotonates the acid for it to become a salt. To extract an organic base, on the other hand, one should make use of an acid, like HCl, because it leads to its protonation.

Density is also a factor in extraction. This is needed in order to form layers during the extraction process and to know where the compound to isolate is. Some compounds, like the non polar ones, are soluble in the organic layer or the "oil" layer, while polar compounds are soluble in the aqueous layer or the "water" layer. Usually, a separatory funnel is used in extraction, but test tubes may suffice, especially when the extraction of different compounds is needed. The addition of diethyl ether or backwashing to a solution removes any organic material in the solution, while the addition of a drying agent removes the water component in the solution. After extraction, the backwashing with ether is used to recover the solid form or the acid or base.

ABBREVIATIONS
˚C: degree Celsius
g: grams
g/mol: grams per mol
mL: millilitre
%: per cent

ACKNOWLEDGEMENT
We would like to express our deepest gratitude to our Pharm Chem 125.1 professors and lab technicians to have guided us all throughout in this experiment.

REFERENCES

Boston College. Experiment 3: Extraction: Separation of an Acidic, a Basic and a Neutral Substance; http://www.bc.edu/schools/cas/chemistry/undergrad/org/fall/Extract.pdf(accessed July 20, 2013) Eckerd College. Separating Acids and NeutralCompounds by Solvent Extraction; http://academics.eckerd.edu/instructor/grove/gorgchem/Extraction%20Lab.pdf (accessed July 20, 2013) Fox, M.; Whitesell, J. Organic Chemistry 3rd Edition. [Online] 2013. http://physicalscience.jbpub.com/orgo/interactive_glossary_showterm.cfm?term=Extraction (accessed July 20, 2013)

Kiessling, A., Organic Chemistry Laboratory #3 Extraction. [Online]2004. http://faculty.mansfield.edu/akiessli/Organic%20Chem%201/Lab%20handouts/Extraction.pdf (accessed July 21, 2013)

Lide, D., Physical Constants of Organic Compounds. In CRC Handbook of Chemistry and Physics; CRC Press, Boca Raton, FL, 2005; pp. 3.1-3.572

Masters, K., Macroscale and Microscale Organic Experiments 6th Edition, Cengage Learning, 2011; pp 132-149

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