Extractions–Determination of Distribution Coefficient, Chemistry tutorial

Extractions-Determination of Distribution Coefficient:

PURPOSE:

a)  To decontaminate examples of organic compounds those are solids at room temperature 

b)  To separate the impure example in the smallest amount of an appropriate hot solvent 

Equipment / materials:

Discussion: 

Crystallization, decontamination, and isolation (might only be restricted to a solid) are unsatisfactory ways to divide mixtures of compounds. Extraction is the recovery of a material from a mixture via bringing it into contact through a solvent that dissolves the wanted material. Partitioning is the division between 2 distinct phases (immiscible liquids) and as well termed fractional division.

Similar to recrystallisation and distillation, extraction is a division method frequently utilized in the laboratory to isolate one or more components from a mixture. Different recrystallisation and distillation, it doesn't give up a pure product; therefore, the former methods might be needed to purify a product isolated via extraction. In the technological sense extraction is depend on the principle of the stability distribution of a material (solute) between 2 immiscible phases, one of which is usually a solvent. The solvent need not be a pure liquid but may be a mixture of several solvents or a solution of several chemical reagents which will react through one or more components of the mixture being extracted to form a new material soluble in the solution. The substance being extracted might be a liquid, a solid, or a mixture of such. Extraction is an extremely common, extremely versatile method that is of huge value not only in the laboratory but as well in everyday life. Extraction is a suitable technique for separating an organic material from a mixture, these as an aqueous reaction mixture or a steam distillate. The extraction solvent is generally a volatile organic liquid that can be eliminated via evaporation after the needed component has been extracted.

The extraction method is depending on the fact that if a material is insoluble to several extents in 2 immiscible liquids, it can be moved from 1 liquid to the other via shaking it jointly through the 2 liquids. For instance, acetanilide is partly soluble in both water and ethyl ether. If a solution of acetanilide in water is shaken through a portion of ethyl ether (that is immiscible through water), several of the acetanilide will be moved to the ether layer. The ether layer, being less dense than water, separates out above the water layer and can be removed and replaced by another portion of ether. Whenever this in turn is shaken through the aqueous solution, more acetanilide passes into the new ether layer. This new layer can be removed and combined through the first. By repeating this process sufficient time, virtually all of the acetanilide can be moved from the water to the ether.

As we stated above, the material being extracted might be a solid. Extractions of this kind will not be conducted here, but they are almost certainly previously part of our own experience. The brewing of tea from tea leaves (or the tea bag that combines extraction and filtration) and of coffee from the ground bean are brilliant instances of the extraction of a solid mixture through a hot solvent (water).

In the laboratory one of the more significant applications of the extraction procedure has been it's utilized to eliminate an organic compound from a solution whenever distillation isn't possible. Extraction is accomplished by shaking the solution in a separator funnel through a 2nd solvent that is immiscible by the one in which the compound is dissolved, but dissolves the compound more readily. Two liquid layers are appearance, and the layer that has most of the desired product in it can be divided from the other. Sometimes not the whole product is removed in a single operation and the procedure must be repeated once or twice more to assure a clean division. It has been found that whenever 2 immiscible solvents are shaken jointly, the solute distributes itself between them in a ratio approximately proportional to its solubility in each. The ratio of the concentration of the solute in each solvent at equilibrium is a steady termed the distribution ratio (d) or partition coefficient (Kd).

The superior the value of Kd, the more solute will be shifted to the ether through each extraction, and the fewer portions of ether will be needed for essentially whole elimination of the solute.

1052_kd.jpg

Where o and aq refer to the organic (ether) and aqueous layers, respectively; and mo and maq  are the masses in grams of material dissolved in each particular layer.

Co = concentration of organic solution and Caq  = concentration of aqueous solution

Extraction of Solvents:

If a solvent is to be utilized to extract an organic compound from aqueous mixture or solution, it must be virtually insoluble in water, and it should contain a low boiling point so that the solvent can be more soluble in the extraction solvent than in water, because otherwise as well many extraction steps will be needed to eliminate all of the solute.

Ethyl ether is the most general extraction solvent. It has an extremely low boiling point (34.5oC) and can dissolve a huge number of organic compounds, both polar and non-polar. Though, ethyl ether must be utilized through great care, because it is tremendously flammable and tends to form explosive peroxides on standing.

Methylene Chloride (dichloromethane) has most of the benefits of ethyl ether; in calculation, it is non-flammable and denser than water. Though, it has a tendency to form emulsions, which can build it hard to divide the layers cleanly. Other helpful solvents and their properties are cataloged in the Table diverse grades of petroleum ether (a blend of low boiling hydrocarbons) can be utilized in place of pentane.

From the foregoing discussions some of the desirable properties of an organic extraction solvent become apparent: 

i. It must readily dissolve the material being extracted but must not dissolve to any noticeable extent in the solvent from which desired material is being extracted. 

ii. It should extract neither the impurities nor other materials present in the original mixture. 

iii. It should not react by the material being extracted.

iv. It should be readily divided from the desired solute after extraction. 

Few solvents will meet all such criteria, and in several cases an entirely satisfactory solvent can't be found. Hence, the scientist must select a solvent system that most virtually approaches the ideal. Several of the solvents usually utilized for extracting aqueous solutions or mixtures contain diethyl ether, methylene chloride, chloroform, carbon tetrachloride, benzene, n-pentane, n-hexane, and different mixtures of saturated hydrocarbons from petroleum (petroleum ether, ligroin, etc.). Each of such has a moderately low boiling point so that it might be fairly simply divided from the solute via evaporation or distillation. Methanol and ethanol are not good solvents for extracting aqueous solutions or mixtures since of their solubility in water; though, if an aqueous solution can be saturated through potassium carbonate with no affecting the 'solute, ethanol can be utilized to extract polar solutes from the solution.

Table: Useful Solvents and their Properties

Solvent

Boiling Points (oC)

D (g/ml)

Comments

Ethyl acetate

77

  0.90 

Absorbs much of

water good general

solvent

Ethyl ether

34.5

0.90

absorb some water,

easy to remove;

very flammable;

vapour should not

be inhaled

Methylene dichloride

40 

1.34

Good general solvent; easy to dry and remove; suspected

carcinogen

Chloroform

62

1.48

Can form emulsion

easy to dry and

remove; health

hazard; suspected

carcinogen

1,1,2-

trichlorotrifluroethane

(Freon TF)

48

1.58

Maybe substituted

for carbon

tetrachloride

Pentane

36

0.63

Easy to dry &

remove; very

flammable

Hexane

69

0.66

Easily dried

Criteria for selecting an extracting solvent

i. It should be insoluble or a little soluble through the solvent of the solution being extracted. 

ii. It should contain a favorable distribution coefficient for the substance being extracted and an unfavorable distribution. 

iii. It should be capable to be simply eliminated from the extracted material after the extraction. 

iv. Because the elimination is often via distillation, the solvent should hence have a logically low boiling point. 

v. It should be chemically inert to the extracted stuff, other components in the mixture, and the solvent to the solution being removed. 

vi. It should be rationally safe to work through and moderately inexpensive. 

Use of the Separatory Funnel:

The process in this experiment involves the utilize of the separatory funnel. It is significant that we learn how to utilize this piece of equipment correctly, for an efficient division and security. It is made of thin glass and is effortlessly broken unless handled watchfully. Unfortunately, in a variety of student manuals we will discover descriptions of ways of holding the separatory funnel. Probably for us there are several best techniques, depending on the size of our hands, the strength of our fingers, our manual dexterity, and the size and shape of the funnel. The following are vital laws to examine.

1.  Hold the funnel resolutely but gently in both hands so that it can be twisted from the vertical to horizontal direction and back again simply and can be shaken forcefully whilst observing (2) and (3). 

2.  Remain the stopper strongly seated by one hand at all times, using the forefinger of that hand, the base of the forefinger, or the palm of the hand. 

3.  Keep the stopcock forcefully seated through the fingers of the other hand in these ways that the fingers can release and seal the stopcock rapidly to liberate the pressure that might be built up from solvent vapour or evolved gases. The utilize of the separatory funnel is an ability and is best studied via practice through a vacant funnel whilst watching our instructor express the technique. Two vaguely diverse process of handling the separatory funnel are following in the Figures and in another figure. In the first process, the stem of the funnel schemes between the thumb and 1st finger of the left hand (for a right-handed person). The stopcock is held in place and activated through the thumb and 1st finger. The stopper is reserved in place through pressure alongside the base of the 1st finger of the right hand.

In the 2nd technique, the stem of the funnel projects between the 1st and 2nd fingers of the left hand. The stopcock is held in place through the pressure from such fingers and is operated in conjunction through the thumb. The stopper is held in place via pressure alongside the middle of the palm of the right hand. (Figure).

1688_Methods for Holding and Shaking.jpg

Fig: Methods for Holding and Shaking the Separatory Funnel      

635_Support and Use of the Separatory Funnel.jpg

Fig: Support and Use of the Separatory Funnel 

Support the separatory funnel in a ring on ring stand. Close the stopcock and add the liquids to the funnel to be separated. Insert the stopper, and instantly invert the funnel. Point the barrel away from our face and that of our neighbors. Open the stopcock to liberate the pressure that might have accumulated inside the funnel (volatile solvents such as ether develop considerable pressure).

Close the stopcock and, hold the funnel horizontally; shake the funnel 2-3 times. Invert the funnel and liberate the pressure as before. Does again this procedure until opening the stopcock causes no additional pressure liberate. Close the stopcock and shake the funnel 15-20 times. Replace the funnel in the holder (ring on ring stand) and eliminate the stopper (Figure). Permit the liquids to stand until the layers contain entirely separated. Draw the lower layer into a flask or beaker of proper size (Figure).

Never draw the liquid during the stopcock as well quickly. Slow the flow cautiously as the boundary between the 2 layers approaches the stopcock. Stop the flow of liquid entirely just as the upper layer go through the hole in the stopcock. Pour the upper layer throughout the neck of the funnel into a second flask. Do not remove either layer until we are absolutely sure that the proper layer to keep is. Generally, one layer will be an aqueous layer or solution, and the other will be an organic liquid. The one of superior density will be at the bottom.

To make sure the identity of a layer, should we be in suspicion, extract a few milliliters of the lower layer into a test tube enclosing an equal volume of water. If the lower layer in the separatory funnel is water or an aqueous solution it will be homogeneous (only one layer). If the layer being examinational is the organic layer, the model withdrawn will fall to the bottom of our test tube and as well form 2 liquid layers. In either event, return the test mixture to the separatory funnel.

EXPERIMENT: Extraction, Determination of Distribution Coefficient

Experimental procedures:

Part 1: Standardisation of NaOH Solution

Use a 10 mL graduated cylinder, measure 10.0 ml of the acid solution and move the solution to a 125 mL Erlenmeyer flask. Add 2-3 drops of phenolphthalein and titrate to the end point (light pink) through a standardized (≈ 0.1M or 0.02 M) NaOH solution. Verification in report form the number of milliliters of base needed to neutralize this volume of acid solution. Compute the molarity of the NaOH.

Discard the neutralized acid solution and rinse our flasks.  Do again (2 times).

Part 2: single extraction

Use a 50 mL regulated cylinder to calculate out a second 50.0 ml volume of acid solution and move it to our separatory funnel. Add 10 ml of methylene dichloride, CH2Cl2, to the funnel and extract according to the process outlined in the part 1 of this testing. Divide the bottom  layer (organic phase) in a 100 mL beaker and collect the top layer (aqueous) into a 125 mL Erlenmeyer flask and add 2-3 drops of indicator. Confirmation the volume of the sodium hydroxide solution in the burette and titrate to the phenolphthalein end point (light pink). Once more, record the volume of base needed and calculates a - g below. Remove the neutralized acid solution and the methylene dichloride layer into the large bottle marked "Organic Waste"

NOTE: This laboratory has been modified in these that methylene dichloride is now utilized in place of ether as the organic phase. This avoids the difficulty of ether fumes and explosions. Though, the extraction by methylene dichloride is not as clean since methylene dichloride is more miscible in water than ether. Consequently, we will discover that our aqueous layer is cloudy after extraction. We can still titrate the aqueous layer to a light pink endpoint.

Part 3: multiple extractions

Do again the process from Step 2, but this instance; extract 50 ml of fresh acid solution by two 5 ml portions of methylene dichloride. Divide the aqueous layer into a flask and order of the organic layer. Move the aqueous layer back into the vacant, fresh, separatory funnel and extract it by a second 5 ml portion of fresh methylene dichloride. Divide the extracted aqueous layer, add indicator as before and titrate to the end point. Record the volume of standard base needed and calculates a - f below. Organize of the organic layer extracts as directed and clean our separatory funnel.

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