Laboratory‎ > ‎Liquid – Liquid Extraction

INTRODUCTION

        When a solution of a solute in one solvent is shaken with a second solvent, which is immiscible with the first, the solute distributes itself between the two solvents in proportion to its solubility in the two pure solvents. Thus a constant ratio is set up between the concentrations of the solute in the two phases or immiscible solvent layers. This ratio or equilibrium can be represented by the equation:

             C1

K  =  ———

            C2     

                K is called the distribution or partition coefficient. C1 and C2 are the concentrations of the solute in the two solvents. These concentrations are expressed in grams/mL.

K is approximately constant for all concentrations of C1 and C2. However, K will vary with temperature as solubility is temperature dependent.

Properties of a Good Extracting Solvent

Properties desirable for a good extracting solvent are:

  • It should be immiscible, or very sparingly soluble in the liquid from which the solute is to be extracted.

  • It should readily dissolve the substance to be extracted.

  • It should be capable of being easily separated from the solute after the extraction. This is usually by distillation.

  • It should ideally be cheap and non-flammable.

Choice of Solvents

The choice of immiscible solvent pair can be generalized as follows. One solvent should be polar and the other solvent should be considerably less polar. The polar solvent is usually water and the low polarity solvent is diethyl ether or a volatile hydrocarbon.

A useful generalization in predicting solubility and hence partitioning between solvents is the following:

Salts and other compounds having an ionic structure are generally soluble in water and insoluble in ether and hydrocarbons. Conversely, organic compounds, including the unionized forms of organic acids and bases are generally insoluble in water and soluble in ether and organic solvents. However it must be remembered that most
compounds have some solubility in both types of solvents.

Solid-Liquid Extraction

When the analyte needs to be extracted from a solid material rather than a liquid, it is refrred to as a solid-liquid extraction. In this case, a solid sample is placed in the same container as the liquid and the analyte is separated from the solid because it dissolves in the liquid while the other sample components do not.

The extraction process can be carried out in two ways:

  • The sample and the liquid are shaken together in the same container. The resultant mixture is filtered, and the filtrate, which then contains the analyte, is collected.

  • The fresh extracting solvent is continously cycled over a period of hours through the solid sample via a continuous evaporation-condensation process. This maximizes the transfer of the analyte to the liquid phase and the filtrate is collected. This technique is known as a Soxhlet extraction. The advantages of this technique are:

    • fresh solvent is continuously in contact with the sample without having to introduce more solvent, which could dilute the extract.

    1. the process is automated so that the extraction can be conveniently set to occur overnight if desired.

MATERIALS

Separatory funnel, acidic,basic and neutral organic compound,ether,erlenmeyer flask,beaker and pipet.

PROCEDURE

1 g of  each  acidic, basic and neutral organic compound  were weighed.These compounds were dissolved in 30 mL of ether.Solution was poured into the separatory funnel.25 mL  % 5  HCl was added and the organic base was extracted from mixture by using seperatory funnel using method of shake and vent.Then the lower aqueous layer was poured into one erlenmeyer flask.The organic acid was extracted from the mixture using 25 mL of NaOH and 10 mL of water.Again all the lower aqueous layer was poured into one erlenmeyer flask.Finally in the remaining ether layer neutral compound existed.

DISCUSSION

Liquid-liquid extraction, also known as solvent extraction and partitioning, is a method to separate compounds based on their solutionpreferences for two different immiscible liquids, usually water and an organic solvent. It is an extraction of a substance from one liquid phase into another liquid phase. Liquid-liquid extraction is a basic technique in chemical laboratories, where it is done in separating funnels, as well as a common process in chemical industry and ore processing.

In the practical use, usually one phase is a water or water-based (aqueous) solution and the other an organic solvent which is immisciblewith water. Solvent extraction is used in nuclear reprocessing, ore processing, the production of fine organic compounds, the processing of perfumesand other industries. It is interesting to note that liquid-liquid extraction is possible in non aqueous systems, for instance in a system consisting of amolten metal in contact with molten salt, metals can be extracted from one phase to the other. This is related to a mercury electrode where a metal can be reduced, the metal will often then dissolve in the mercury to form an amalgam which modifies the electrochemistry greatly. For example it is possible for sodium cations to be reduced at a mercury cathode to form sodium amalgam, while at an inert electrode (such as platinium the sodium cations will not be reduced, instead water is reduced to hydrogen).

It is important to note that if a detergent or fine solid can stablise an emulsion which in the solvent extraction community is known as a third phase.

EXPERIMENT 3

PRELAB QUESTIONS

1- Extraction is a method for moving a compound from one medium to another. For example, if you make coffee from coffee beans, you are extracting some flavorful components of the bean and some caffeine into the water. The remainder of the beans (grounds) are left behind and discarded. This is called a solid-liquid extraction. If you are trying to move a compound from one liquid phase (solvent 1) into another liquid phase (solvent 2), this is liquid-liquid extraction but the two solvents must be immiscible or insoluble to the extent that they form two distinct layers.

By measuring the concentrations of compound in the two solvents (c1 and c2) we obtain a distribution coefficient, K, which is a constant for a given compound and given solvents at a given temperature, irrespective of the amounts of solvent present. A simple equation shows the relationship K=c2/c1.

Separatory Funnel: This glass equipment is very cleverly designed to carry out the task of separating two immiscible liquids (which form two distinct layers). Work with this equipment in a proper fashion and it will perform remarkably well

2-It is not always possible to quantitative remove the solute using a single extraction.Your options typically are to increase the volume of the extracting solvent(not usually good idea) and use multiple extractions.Multiple extractions can effectively remove a single species or a group of related species at the same time.So only uses small amount of solvent is the advantage.

3-  When an organic solvent has been exposed to aqueous solutions it will contain a small amount of water, the amount depending on the solubility of water in the solvent. To prepare a pure product, it is necessary to dry the solution using an appropriate drying agent. A drying agent is usually an anhydrous inorganic salt which reacts with the water present to form a hydrate. Anhydrous MgSO4, for example, reacts with water to form the heptahydrate MgSO4 7H2O.  Drying agents are distinguished by their capacity (the amount of water they can absorb), the rate at which they absorb the water, and their intensity (or completeness), which is the amount of water left behind in the solvent at equilibrium.

We can use drying agent in the extraction process.The product can be isolated from the organic solvent with drying agent. The most commonly drying agents are magnesium sulfate,sodium sulfate and calcium chloride.

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