Overview
This lecture covers the isolation and purification steps of acid-base extraction, focusing on the recovery of benzoic acid, para-nitroaniline, and naphthalene through neutralization, precipitation, drying, and filtration.
Neutralization and Precipitation
- Sodium hydroxide and hydrochloric acid extracts are cooled in an ice bath before neutralization.
- HCl is added dropwise to the NaOH extract until solid benzoic acid forms and the solution turns strongly acidic.
- NaOH is added to the HCl extract until the pH becomes strongly basic, allowing para-nitroaniline to precipitate.
- Benzoic acid precipitates when sodium benzoate reacts with HCl, as benzoic acid is less soluble in water.
- Para-nitroaniline precipitates when its ammonium salt reacts with NaOH, becoming less polar and less soluble.
Organic Layer Isolation
- The organic extract (containing naphthalene) is treated with anhydrous magnesium sulfate to remove water.
- The mixture is filtered using gravity filtration to remove magnesium sulfate.
- The filtrate is transferred to a round-bottom flask and washed with ethyl acetate.
Benzoic Acid Isolation and Purification
- The NaOH extract containing benzoic acid is filtered using vacuum filtration.
- Filter paper is wetted with cold water and crystals are washed to maximize recovery.
- Benzoic acid crystals are weighed and transferred to another flask for recrystallization.
- Small portions of hot water are used to dissolve benzoic acid, then the solution is filtered while hot.
- Both the funnel and flask should be pre-heated to prevent premature crystallization.
- The hot solution is cooled to room temperature, then cooled further in an ice bath to maximize crystal formation.
- Final purification is achieved by suction filtration of the recrystallized benzoic acid.
Key Terms & Definitions
- Neutralization — Chemical reaction between acid and base, forming water and a salt.
- Precipitate — Solid formed from a solution during a chemical reaction.
- Gravity Filtration — Technique to separate solids from liquids using filter paper and gravity.
- Vacuum (Suction) Filtration — Filtration method using reduced pressure to speed up solid-liquid separation.
- Recrystallization — Purification process where solids are dissolved and then crystallized out as pure compounds.
- Drying Agent — Substance (e.g., anhydrous magnesium sulfate) used to remove water from organic solvents.
Action Items / Next Steps
- Perform suction filtration to collect and dry the recrystallized benzoic acid.
- Review the process for isolating and purifying each compound (benzoic acid, para-nitroaniline, naphthalene).