Solvent Extraction: Procedures and Applications

Solvent extraction is a process in which a substance can be separated from its matrix. In this technique, the Salute or solutes are distributed between two immiscible liquids IE organic and aqueous layers. It is a simple non-destructive and widely used technique in the laboratory. It requires simple equipment such as separating funnel, stopper, beakers and funnel stand.

In most solvent extraction procedures the extraction of solute from an aqueous phase to an immiscible organic phase such as dichloromethane, hexane, and benzene. However, the reverse is also possible.

In analytical applications, solvent extraction may serve the following three key purposes of analyte characterization;

  • Preconcentration of trace elements
  • Elimination of matrix interference
  • Differentiation of chemical species

Importance

In analytical chemistry, it is a useful tool for extraction. It can be used to separate minute quantities of almost every metal from its ores. Later on, in 1940  this process get real importance because of its use in extracting rare earth metals. It plays a key role in Organic synthesis. such organic products which have low boiling points or are thermally unstable are preferably separated or refined by this process

Principle

The principle of solvent extraction next line works on the principle of Nernst distribution law or partition law which states that when a solute particle is distributed in the two solvents is also distributed in a fixed ratio irrespective of the solute present. Distribution equilibrium between the two liquids immiscible liquids is going by gives phase rule Hindi sex action we have peace equal to 2 because two phases are involved

Nernst distribution law

According to distribution law if x1 is a concentration of solute in Phase 1 andx2 to the concentration of salute in phase 2 the distribution Coefficient is given by x1/x2.

This law gives the best results when employed under the following conditions

  • At constant temperature
  •  Used solutions are dilute
  • The molecular state of solute is some in both phases

Partition coefficient

In common practice, the solute often exits in different molecular conditions in two immiscible phases due to polymerization ionization, and complexation, therefore partition coefficient is used to define the ratio.

To understand the distribution of solute in two solvents,  Let us consider an example of ammonia dissolved in two immiscible solvents. Both ammonia solutions with different concentrations of ammonia are placed in a separating funnel. Since solvents are immiscible they are two separate layers. The separating funnel is then shaken well, and the ammonia which is soluble in both metals ents starts traveling across the phases to establish equilibrium.

Partition coefficient

NH3(aq) NH3(org)

Kpc or KD = NH3(aq)/NH3(org)

This equilibrium constant is known as partition coefficient Kpc. That Partition coefficient relates to the concentration of solute distributed between these two layers. It can be calculated for each solute and solvents used at a particular temperature.

Extraction of acid and bases

the organic compound with acidic and basic nature can be extracted through solvent extraction. When these species are added to water they protonate and deprotonate to some extent in an aqueous medium. Hence, their extraction can be done by adjusting the pH.

The relationship between pH and distribution ratio is given by

D =KD/(1+Ka/[H+])

The above equation shows that at low pH, acid dissociation is less so it can be easily extracted in the nonpolar organic phase.

Extraction of metals

Metal ions cannot be separated in the organic phase so they are extracted through complexation. They can be separated through two main methods

1. Ion association complex formation

It is best suitable for electrically neutral metal atom extractions. The metal atom with positive charges aggregates themselves with negative charges to form neutral complexes. the can be separated in the organic layer. These metal ions are complicated first with bulky organic anionic groups. The widely used anions are oxion, triphenylphosphine, and some inorganic anions such as FeCl4-, MnO4

2. Uncharged metal Chelate complex formation

The ring structure with no charge is formed by the coordination of metals with several types of ligands depending upon the requirements and conditions. The ligands formed six-membered rings., E.g. EDTA complex are slightly acidic and contains one or more coordinating atoms.

 extraction of metals by chelate formation in solvent extraction

Extraction efficiency

Percent efficiency is calculated for every used solvent and method to get the max possible yield. It can be calculated

%E=100 x millimoles of solute extracted/ total millimoles of the aqueous phase

Batch solvent extraction

Xn=Xa [V(aq)/KD Vo + V (aq)]

where,

  • n= Number of times solute is extracted from the aqueous phase
  • Xn= Quantity of solute remained in the aqueous phase after nth extraction
  • Xa = millimoles of solute taken in the aqueous phase
  • KD =Distribution coefficient
  • V(aq) =Volume of aqueous phase taken for extraction
  • Vo =Volume of organic phase used for extraction at a time in multiple extractions

Factors affecting solvent extraction

The key requirement of solvent extraction for high efficiency is the conditions that lead to a higher distribution ratio of solute to be extracted. It depends upon the nature of the extractant, solvent, pH, and many more. Some important factors are discussed as follows;

1. Choice of solvent

It is considered the most important factor for the extraction of elements in designing a particular extraction procedure. A solvent for extraction should have the following properties;

The solvent must have the following properties

  1. high capacity of extraction: such that have high solubility of solute of entrust
  2. Sufficient density difference with the aqueous phase
  3. highly immiscible in the aqueous phase
  4. low viscosity
  5. Non-toxic
  6. Inexpensive
  7. Easy to recover

Some commonly used solvents are Trichloromethane, Methly benzene, and Diethlyether

2. Use of convenient stripping agents

Stripping is the removal of extracted solute from the organic phase for further investigations. For this purpose, to destroy the organic matter if present, acids such as Nitric acid, sulphuric acid, and aqua regia is added. These acids with some water are mixed with separated phase and shaken. thus, the metal ion is extracted in the aqueous phase. some times oxidizing and reducing agents are used to serve this purpose.

3. Masking agents

In metal extraction procedures, it is often difficult to separate the ion pairs. Masking agents are metal complexing agents, introduced here to improve the separation factor. In solvent extraction, masking agents are used to prevent unwanted metals from producing extractable complexes hence increasing the selectivity of the procedure. For example, EDTA is the most useful masking agent for anionic complex formation with several metal ions under specific conditions. Some common masking agents are 8-quinolinol for Fe3+, CN(cyanate), Tartarate, etc.

4. Salting out agents

Salting out is the method by which electrolytes are added to enhance the extractability of complexes. when the compounds are added they usually decrease the dielectric constant of an aqueous phase, which leads to the formation of ion association complexes. NaCl is widely used for this purpose.

5. Backwashing

This is a key method for the quantitative separation of elements in batch extractions. The combined organic phases from several extractions (containing extractant) are treated with a fresh aqueous layer to remove the impurities.

Separation factor for more than one solute

let’s consider a case of two solutes, A and B  present in the solution. The solubility of these solutes is different from the corresponding solvent. To extract the solute  A from the solution, extracting solvent can be used that solvent must dissolve the target solute in more quantity. in these circumstances, the separation is calculated as the partition coefficient ratio of two solutes.

β = KDA /KDB

where,

  • β is the ratio of distribution coefficients
  •  KDA =is the partition coefficient of A
  • KDB =is the partition coefficient of B

It is to be noted that ‘β’ should be very large for the extraction of solute by solvent extraction

Applications of solvent extraction

  • Reprocessing of nuclear waste
  • Water effluent treatment
  • Food industry: Cooking oil extraction from seeds
  • Extraction of rare earth metals
  • Separation of organic products in pharmaceuticals
  • Quality control labs
  • analysis of plant and animal tissues
  • purification of substances
  • preconcentration of metals

concepts Berg

How Does Solvent Extraction Work?

Its working mechanism is based upon Nernst distribution law. That distribution of a solute between two immiscible and liquids in a fix that ed ratio. This is used extraction ion and purification of the analyte.

How Does Extraction Compare To Distillation?

Solvent extraction is somehow different from distillation. Because in distillation the separation of compounds with large differences in their boiling point is separated through the heating- condensation method.

What are solvent extraction examples?

The common examples are given below:

  1. Liquid-liquid extraction: appropriate for extraction of organic compounds
  2. Acid-Base Extraction: It is suitable for the extraction of amines.
  3. Metal extraction: It is used for the extraction of precious metals such as U-235 etc

What is the use of solvent extraction in chemical engineering?

Solvent extraction has many applications in the chemical industry such as wastewater treatment, processing of Nuclear fuels, and extraction of precious metals from its ores.

What are the 2 methods of extraction?

The example of solvent extractions are; 

  1. continues extraction
  2. Batch extraction

What are the advantages of solvent extraction?

It is a simple and easily performable classical technique. It is nondestructive and best for volatile and unstable substances.

Why is methanol a good solvent for extraction?

Methanol is an organic solvent that can be used in solvent extraction It is commonly used in SX because it is cheaper than other solvents and the distribution coefficient is relatively good.

Reference links

How does solvent extraction work by temmary.com

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