Acid-Base Extraction Lab: Procedure, Theory, and Recovery Results Report

Background and Theory

In chemistry, extraction is a key method for separating chemicals from mixtures. It depends on a substance’s partitioning between two immiscible phases, often an aqueous solution and an organic solvent. Liquid-liquid extraction is frequently used throughout the “work-up” process. It is invaluable for separating products from a reaction mixture. Based on a solute’s solubility properties, extraction aims to distribute the component of interest across the two phases in a targeted manner.

The partition or distribution coefficient K = ASaq/ASorganic can be used to explain the equilibrium between the solute in the aqueous solvent (Saq) and the solute in the organic solvent (Sorganic). The value of K depends on the solubility of the compound in both solvents (Gilbert & Martin, 2015, p. 156). If K > 1, the solute will predominantly reside in the aqueous phase, while if K < 1, the solute will mainly be found in the organic phase, assuming equal volumes of Saq and Sorganic are used.

Acid-base extraction is very helpful for separating chemicals according to their acidic or basic characteristics. This approach uses the polarity variations that arise from chemically changing the polarity of a molecule’s acidic or basic functional groups (Gilbert & Martin, 2015, p. 160). By extracting carboxylic acids with an aqueous base, carboxylic acids can be changed into water-soluble carboxylate salts.

Conversely, aqueous acids may extract organic bases, protonate them, and transform them into water-soluble forms. In the lab exercise, the combination of benzoic acid (an acid), p-nitroaniline (a base), and anthracene (a neutral molecule) must be separated (Nichols, 2022). The combination is extracted using sodium hydroxide to obtain the basic extract, followed by hydrochloric acid to obtain the acid extract during the acid-base extraction method. The neutral portion is the organic solution that is still present.

The neutral portion is treated with anhydrous sodium sulfate to remove water; then, the organic solvent is evaporated to produce solid anthracene to separate the various components. Hydrochloric acid neutralizes the extract, causing the acid component to precipitate and be recovered by vacuum filtering. The organic base is then extracted using the appropriate techniques after the basic extract has been neutralized with sodium hydroxide (Gilbert & Martin, 2015, p. 162). Therefore, for efficient separation and purification of organic molecules in the laboratory, knowing the principles of acid-base extraction is crucial.

Procedure

The setup of the extraction equipment was followed by an array of extractions and isolations for the acid-base extraction technique. It was done with a solution that had 40 mL of dichloromethane and 0.5 g of benzoic acid, p-nitroaniline, and anthracene. Following two extractions of the organic solution using 25 mL parts of 6M HCl, the organic phase was retained, and the aqueous extracts were combined to form the “Acid Extract” (Gilbert & Martin, 2015, p. 164). The leftover organic solution was subsequently put through two extractions using 25 mL portions of 6M NaOH, producing the “Basic Extract.”

The leftover organic solution, the Neutral Fraction, was poured into a different flask. By including anhydrous Na2SO4 and decanting the solution after it had stood, anthracene was separated from the Neutral Fraction, and the acquired solid’s weight was noted. 56 mL of HCl was used to neutralize the Basic Extract, and the resultant solution was then chilled to cause precipitation (Gilbert & Martin, 2015, p. 166).

The residue was extracted, cleaned, dried, and weighed through vacuum filtration. The acidic extract was neutralized with 50 mL of NaOH, and the resulting residue was handled similarly. The solid weights of each extraction step were measured and recorded.

Results

Table 1. Results of Acid-Base Extraction

Flow Chart

1. START
2. Prepare a mixture of compounds in a dichloromethane solution.
3. Perform acid-base extraction using 6M HCl:
   • Vent a separatory funnel;
   • Allow layers to separate;
   • Retain the organic phase (bottom layer);
   • Combine aqueous extracts as “Acid Extract.”
4. Perform acid-base extraction using 6M NaOH:
   • Vent a separatory funnel;
   • Allow layers to separate;
   • Retain the organic phase (bottom layer);
   • Combine aqueous extracts as “Basic Extract.”
5. Transfer the remaining organic solution to a separate flask as a “Neutral Fraction.”
6. Add anhydrous Na2SO4 to the “Neutral Fraction” and let it stand.
7. Decant the “Neutral Fraction” into a pre-weighed beaker:
   • Remove dichloromethane using a steam bath.
8. Record the weight of the solid obtained (Anthracene).
9. Neutralize the “Basic Extract” by adding 56 mL of HCl:
   • Check pH periodically;
   • Place the flask in an ice bath.
10. Isolate precipitate by vacuum filtration:
    • Wash the solid with cold distilled water;
    • Dry and record weight (Base compound).
11. Neutralize the “Acid Extract” by adding 50 mL NaOH:
    • Check pH periodically;
    • Place the flask in an ice bath.
12. Isolate precipitate by vacuum filtration:
    • Wash the solid with cold distilled water;
    • Dry and record weight (Acid compound).
13. END

Discussion

The “acidic extract” produced during the experiment and other compounds extracted during the acid-base extraction procedure contain the neutral chemical anthracene. On the other hand, the “basic extract” and any other chemicals extracted with it are predicted to contain the basic component p-nitroaniline (Gilbert & Martin, 2015, p. 157). Moreover, to compute the % recovery for each component, the initial quantity of each compound added to the mixture and the final amount collected after the extraction and isolation procedures must be known. The percent recovery is computed by dividing the final amount by the beginning amount and multiplying it by 100%.

Percent Recovery = (Final Amount / Initial Amount) × 100%

1. For Benzoic acid. Percent Recovery = (0.240 g / 0.5 g) × 100% = 48%.
2. For p-nitroaniline. Percent Recovery = (0.003 g / 0.5 g) × 100% = 0.6%.
3. For Anthracene. Percent Recovery = (1.132 g / 0.5 g) × 100% = 226%.

Their immiscibility separates the aqueous and organic phases in the separatory funnel. The organic phase, made up of an organic solvent like dichloromethane, creates a separate layer on top of the aqueous phase, which is commonly water-based (Gilbert & Martin, 2015, p. 158). The two phases’ different densities help to separate them, with the less dense phase floating on top and the denser phase sinking to the bottom. This experiment has several possible causes of inaccuracy.

Cross-contamination can occur during the extraction if the organic and aqueous phases are not entirely separated, reducing the purity of the extracted chemicals. Calculations for recovery might be off if chemicals are lost during the transfer or decanting stages. Compounds may not fully extract if there is insufficient mixing or contact time during extraction.

Moreover, a reagent or compound measurement that is not precise can impact the stoichiometry of reactions and the following recovery estimates. Inaccurate weight measurements might result from incomplete drying of the separated chemicals. The purity of the derived compounds may be impacted by contamination from impurities or other chemicals during the extraction or isolation process. Finally, modifications to the experimental setup or mistakes made when carrying out the steps manually might add other sources of uncertainty (Nichols, 2022). Therefore, to guarantee accurate results and draw valid conclusions from the experiment, it is essential to properly regulate and account for various potential sources of error.

References

Gilbert, J. C., & Martin, S. F. (2015). Extraction. In Gilbert, J. C., & Martin, S. F (Eds) Experimental Organic Chemistry: A Miniscale & Microscale Approach (6^th ed., pp. 155–177). Cengage Learning.

Nichols, L. (2022). 4.8: Acid-base extraction. Chemistry LibreTexts. Web.