Wednesday, June 2, 2010

Extraction and maceration

Extraction:
Extraction is the process of separation of medicinally active substances of plant or animal from a mixture by a mechanical or chemical action such as by distillation or pressure.
This separation is done with the help of dissolving one or more of the substances in a solvent in which it is easily soluble and are separated on the basis of their physical or chemical properties.

Forms of Extraction:

Liquid-liquid Extraction:
It is also referred to as Solvent extraction or Partitionaing. Often the solute is miscible in one solvent and immiscible in the other solvent and this thing is used in the process of Solvent extraction.
If the solute is present in an aquous solvent than it can be separated by pouring another solvent in this solution which is immiscible with the previous solvent. The solute will also be dissolved in the other solvent and in this way the solute will be easily separated by separating one of the solvents.

In solvent extraction, two immiscible liquids are shaken together. The more polar solutes dissolve preferentially in the more polar solvent, and the less polar solutes in the less polar solvent based on the famous statement "like dissolves like".
With the help of liquid-liquid extraction, the standard of a chemical is improved after a chemical reaction.

Instrument used:
Separating funnel is used in laboratories.

Uses of Solvent Extraction:
Solvent extraction is used in
1. Ore processing,
2. Nuclear reprocessing, 
3. The processing of perfumes,
4. The production of fine organic compounds,
5. The production of vegetable oils and biodiesel, and other industries.

Measures of effectiveness

Distribution ratio (D):
Distribution ratio is also referred to as partition co-efficient. This ratio gives indication of the quality of extraction. Distribution ratio is the ratio of concentration of a solute in organic phase and the concentration of the same solute in aquous phase.

D = Conc. of solute in organic phase / conc. of solute in aquous phase

Distribution ratio is dependent on the temperature, concentration of the chemical entities in the system and some other parameters in and around the system.

Separation Factor:
The ratio of two distribution ratios is the separation factor.

Separation factor = Distribution ratio for one solute / Distribution ratio for the other solute

With the help of this ratio, it is easy to assess the ability of the system to separate two solutes.

Decontamination factor:
This factor is used to represent the ability of a process to remove an impurity or contaminant from a product.
Techniques

Batchwise single stage extractions

This is commonly used on the small scale in chemical labs. It is normal to use a separating funnel. For instance, if a chemist were to extract anisole from a mixture of water and 5% acetic acid using ether, then the anisole will enter the organic phase. The two phases would then be separated.
The acetic acid can then be scrubbed (removed) from the organic phase by shaking the organic extract with sodium bicarbonate. The acetic acid reacts with the sodium bicarbonate to form sodium acetate, carbon dioxide, and water.

Multistage countercurrent continuous processes


These are commonly used in industry for the processing of metals such as the lanthanides; because the separation factors between the lanthanides are so small many extraction stages are needed. In the multistage processes, the aqueous raffinate from one extraction unit is fed to the next unit as the aqueous feed, while the organic phase is moved in the opposite direction. Hence, in this way, even if the separation between two metals in each stage is small, the overall system can have a higher decontamination factor.
Multistage countercurrent arrays have been used for the separation of lanthanides. For the design of a good process, the distribution ratio should be not too high (>100) or too low (<0.1) in the extraction portion of the process. It is often the case that the process will have a section for scrubbing unwanted metals from the organic phase, and finally a stripping section to obtain the metal back from the organic phase.
Multistage Podbielniak contactor centrifuges produce three to five stages of theoretical extraction in a single countercurrent pass, and are used in fermentation-based pharmaceutical and food additive production facilities.

Extraction without chemical change

Some solutes such as noble gases can be extracted from one phase to another without the need for a chemical reaction. This is the simplest type of solvent extraction. When a solvent is extracted, two immiscible liquids are shaken together. The more polar solutes dissolve preferentially in the more polar solvent, and the less polar solutes in the less polar solvent. Some solutes that do not at first sight appear to undergo a reaction during the extraction process do not have distribution ratio that is independent of concentration. A classic example is the extraction of carboxylic acids (HA) into nonpolar media such as benzene. Here, it is often the case that the carboxylic acid will form a dimer in the organic layer so the distribution ratio will change as a function of the acid concentration (measured in either phase).
For this case, the extraction constant k is described by k = [[HAorganic]]2/[[HAaqueous]]

Solvation mechanism

Using solvent extraction it is possible to extract uranium, plutornium, or thorium from acid solutions. One solvent used for this purpose is the organophosphate tri-n-butyl phosphate. The PUREX process that is commonly used in nuclear reprocessing uses a mixture of tri-n-butyl phosphate and an inert hydrocarbon (kerosene), the uranium(VI) are extracted from strong nitric acid and are back-extracted (stripped) using weak nitric acid. An organic soluble uranium complex [UO2(TBP)2(NO3)2] is formed, then the organic layer bearing the uranium is brought into contact with a dilute nitric acid solution; the equilibrium is shifted away from the organic soluble uranium complex and towards the free TBP and uranyl nitrate in dilute nitric acid. The plutonium(IV) forms a similar complex to the uranium(VI), but it is possible to strip the plutonium in more than one way; a reducing agent that converts the plutonium to the trivalent oxidation state can be added. This oxidation state does not form a stable complex with TBP and nitrate unless the nitrate concentration is very high (circa 10 mol/L nitrate is required in the aqueous phase). Another method is to simply use dilute nitric acid as a stripping agent for the plutonium. This PUREX chemistry is a classic example of a solvation extraction.
Here in this case DU = k TBP2[[NO3]]2

Ion exchange mechanism

Another extraction mechanism is known as the ion exchange mechanism. Here, when an ion is transferred from the aqueous phase to the organic phase, another ion is transferred in the other direction to maintain the charge balance. This additional ion is often a hydrogen ion; for ion exchange mechanisms, the distribution ratio is often a function of pH. An example of an ion exchange extraction would be the extraction of americium by a combination of terpyridine and a carboxylic acid in tert-butyl benzene. In this case
DAm = k terpyridine1carboxylic acid3H+−3
Another example is the extraction of zinc, cadmium, or lead by a dialkyl phosphinic acid (R2PO2H) into a nonpolar diluent such as an alkane. A non-polar diluent favours the formation of uncharged non-polar metal complexes.
Some extraction systems are able to extract metals by both the solvation and ion exchange mechanisms; an example of such a system is the americium (and lanthanide) extraction from nitric acid by a combination of 6,6'-bis-(5,6-dipentyl-1,2,4-triazin-3-yl)-2,2'-bipyridine and 2-bromohexanoic acid in tert-butyl benzene. At both high- and low-nitric acid concentrations, the metal distribution ratio is higher than it is for an intermidate nitric acid concentration.

Ion pair extraction

It is possible by careful choice of counterion to extract a metal. For instance, if the nitrate concentration is high, it is possible to extract americium as an anionic nitrate complex if the mixture contains a lipophilic quaternary ammonium salt.
An example that is more likely to be encountered by the 'average' chemist is the use of a phase transfer catalyst. This is a charged species that transfers another ion to the organic phase. The ion reacts and then forms another ion, which is then transferred back to the aqueous phase.
For instance, the 31.1 kJ mol−1 is required to transfer an acetate anion into nitrobenzene, while the energy required to transfer a chloride anion from an aqueous phase to nitrobenzene is 43.8 kJ mol−1. Hence, if the aqueous phase in a reaction is a solution of sodium acetate while the organic phase is a nitrobenzene solution of benzyl chloride, then, when a phase transfer catalyst, the acetate anions can be transferred from the aqueous layer where they react with the benzyl chloride to form benzyl acetate and a chloride anion. The chloride anion is then transferred to the aqueous phase. The transfer energies of the anions contribute to that given out by the reaction.
A 43.8 to 31.1 kJ mol−1 = 12.7 kJ mol−1 of additional energy is given out by the reaction when compared with energy if the reaction had been done in nitrobenzene using one equivalent weight of a tetraalkylammonium acetate.

Kinetics of extraction

It is important to investigate the rate at which the solute is transferred between the two phases, in some cases by an alteration of the contact time it is possible to alter the selectivity of the extraction. For instance, the extraction of palladium or nickel can be very slow because the rate of ligand exchange at these metal centers is much lower than the rates for iron or silver complexes.

Aqueous complexing agents

If a complexing agent is present in the aqueous phase then it can lower the distribution ratio. For instance, in the case of iodine being distributed between water and an inert organic solvent such as carbon tetrachloride then the presence of iodide in the aqueous phase can alter the extraction chemistry.
Instead of D_{\mathrm{I}^{+2}} being a constant it becomes D_{\mathrm{I}^{+2}} = k[[I2.Organic]]/[I2.Aqueous] [[I-.Aqueous]]
This is because the iodine reacts with the iodide to form I3-. The I3- anion is an example of a polyhalide anion that is quite common.

Industrial process design

In a typical scenario, an industrial process will use an extraction step in which solutes are transferred from the aqueous phase to the organic phase; this is often followed by a scrubbing stage in which unwanted solutes are removed from the organic phase, then a stripping stage in which the wanted solutes are removed from the organic phase. The organic phase may then be treated to make it ready for use again.
After use, the organic phase may be subjected to a cleaning step to remove any degradation products; for instance, in PUREX plants, the used organic phase is washed with sodium carbonate solution to remove any dibutyl hydrogen phosphate or butyl dihydrogen phosphate that might be present.

Equipment

Two layers separating during a liquid-liquid extraction.  An organic MTBE solution is extracted with aqueous sodium bicarbonate solution. This base removes benzoic acid as benzoate but leaves non-acidic benzil
(yellow) behind in the upper organic phase.
While solvent extraction is often done on a small scale by synthetic lab chemists using a separatory funnel or Craig apparatus, it is normally done on the industrial scale using machines that bring the two liquid phases into contact with each other. Such machines include centrifugal contactors, thin layer extractors, spray columns, pulsed columns, and mixer-settlers.

Solid-Phase Extraction:
It is one of the most important form of extraction for the purification and separation of a large number of chemicals on small scale like in laboratory.
In this process, the substance is dissolved in a solvent and is passed throught a bed or layer of adsorbent having uniform particle sizes. The substance is separated either by adsorption of the solvent or the substance itself into the adsorbent.

Types of Extraction techniques:
1. Continuous Extractions
2. Batch Extractions
3. Extractions involving partition between two immiscible liquids

Questions and Answers:
Q: What is extraction?

Ans: Extraction is withdrawal of dissolved constituents from crude drugs through the use of selective solvent in which the desired constituents are soluble.

Q: What are the processes of extraction?
Ans: The principle methods of drug extractions are,

• Maceration

• Percolation

Frequently a combination of percolation and maceration is used in extraction.

Q: What is maceration?
Ans: The word maceration comes from a latin word “macerare” meaning “to soak”.

Q: What is the principle of maceration?
Ans: It is a process in which the properly comminuted drug is permitted to soak in menstruum until the cellular structure is softened and penetrated by the menstruum and the soluble constituents are dissolved.

Q: How soluble contents are settled down in Maceration?
Ans: As the soluble constituents dissolved in the menstrum, they tend to settle to the bottom as a result of an increase in the specific gravity of the liquid due to its added weights.

Q: In which type of drugs Maceration is used?
Ans: For drugs containing little or no cellular material such as benzoin, aloe, and tolu, which dissolve almost completely in the menstruum, maceration is the most efficient method of extraction

Q: On which temperature Maceration is performed?
Ans: Maceration is usually conducted at a temperature of 15-20 C for 3-5 days or until the soluble matter is dissolved.

Q: Why dipping is essential in Maceration?
Ans: Occasional dipping of the drug bag may facilitate the speed of extraction.

Q: What do you mean by menstruum?
Ans: Menstruum refers to the solvent, which is used to extract ingredients from plant or animal origin.

Further Reading:
Remington: The Science and Practice of Pharmacy (Remington the Science and Practice of Pharmacy)

British Pharmacopoeia 2010

Textbook of Pharmaceutics
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