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Separation of Acid, Base and Neutral Compounds from a Solid Mixture by Liquid-Liquid Extraction

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Experiment #1: Separation of Acid, Base, and Neutral Compounds from a Solid Mixture by Liquid-Liquid Extraction

Introduction

Liquid-liquid extraction is a technique used to separate chemical substances in order to purify or identify the various components of a mixture. Flavors, spices, perfumes, and medicines are just some of the everyday things that are extracted from plants and other natural sources [1]. The basic principle used to carry out this separation is the mixing of two liquids that are immiscible with each other. This creates layers of liquid, which can be separated and then isolated to help to identify compounds. Another important concept to remember in this experiment is that ionic salts are polar and therefore water soluble, and neutral molecules are non-polar and will usually not dissolve in water.

The student is given an unknown containing either an acid and a neutral compound or a base and a neutral compound, but since the components are unknown, the student conducts the experiment as if all 3 compounds are present. In this particular experiment, the compounds are benzocaine, salicylic acid, and naphthalene. An appropriate organic solvent is used to create the ether phase. Diethyl ether is used because it has low solubility in water but will dissolve the components of the mixture (creating liquid layers), is not acidic or basic, is less dense than water, and can be easily removed by evaporation. To begin the separation, the student needs to produce a salt that is water-soluble, so that the ether phase can separate from an aqueous phase. To accomplish this, dilute HCl is added to the solution. The HCl protonates the base (benzocaine) in the solution to create an organic polar salt, and it therefore dissolves in water. This aqueous phase is drained from the ether phase and then deprotonated again with NaOH to reproduce the original neutral benzocaine.

The same concept is used to isolate the salicylic acid. Sodium hydroxide is added to...

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