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Distribution Law, Its Limitations and Applications

Solubilities and distribution law, Distribution law applications, Solvent extraction, Partition chromatography, Limitations of distribution law etc.

Solubilities and distribution law

In equilibrium, both solvents are saturated with a solute when two non-miscible solvents are shaken together. Solubilities also represent concentration, therefore, the solubility distribution law can be expressed as C1/C2 = S1/S2, where S1 and S2 represent the solubilities of the solute in the two solvents, respectively. It is possible to calculate the solubility of the solute in the second solvent by knowing the meaning of the Distribution coefficient (KD) in one solvent and solubility in the first solvent.

Distribution law applications

Various applications of distribution law are found both in the laboratory and in the industry.

1. Solvent extraction - In chemicals, solvent extraction is the process of separating organic materials from aqueous solutions. Typically, organic solvents such as ether or benzene are used to shake aqueous solutions first. Due to the distribution ratio favoring ether, a great deal of the organic substance passes to the ethereal. A component of the organic substance is distilled from the ethereal layer. The organic substance remains.

2. Partition chromatography - It is applied to a bed of silica soaked in water to make a paste of this composition. The flow of hexane down the column is allowed. It is another immiscible solvent. As each component of the mixture is divided into separate stationery and mobile phases, the stationary liquid phase (water) precedes the mobile phase (hexane). Having ordered the distribution coefficients of the components, the hexane is used to extract them in an orderly manner.

3. Desilverization of lead (Parke’s process) - In the presence of molten argentiferous lead or zinc, zinc and lead form immiscible layers, distributing the silver between them. Because the zinc distribution ratio at 800° C is about 300 to 1, most silver is deposited into the zinc layer. When silver is exposed to 800° C, the distribution ratio of zinc is about 300 to 1, so most of the silver passes through the zinc layer. When zinc is sprayed over silver in a retort, the alloy AgZn is distilled. In the absence of zinc passing over, silver remains untouched. Silver is recovered by treating it with fresh zinc molten to recover as much of it as possible

4. Confirmatory test for bromine and iodide - Chlorine water is used to treat the salt solution. Consequently, bromine and iodine are released. Shake mixture with chloroform to remove the bromine or iodine. Shake mixture with chloroform to remove the bromine or iodine. Shake mixture with chloroform to remove the bromine or iodine.

5. Determination of association - In both solvents A and B, two molecules are associated (polymerized). The Distribution law defines the subsequent relationships between the molecules

n√Ca/Cb = K,

When n molecules are combining into that associated molecule

6. Determination of dissociation - In the case of substance X, it exists as single molecules in ether after being dissociated in an aqueous solution.
A distribution law can be shown to exist if x is the degree of dissociation (or ionization)

C1 / C2 (x-1) = K,

A benzene molecule containing X equals C1,
Aqueous layer concentration is C2.

It is possible to obtain the value of x from measurements of conductivity, but it is necessary to work out C1 and C2. This allows us to calculate K from the conductivity measurements. For any other concentration of X, you can determine the value of x using this value of K.

7. Determination of solubility - Assume that iodine is to be determined by its solubility in benzene. Iodine and benzene are shaken together. We find experimentally the equilibrium concentrations of iodine in benzene (Cb), and water (Cw), and calculate the distribution coefficient based on the values.

Sb/Sw = Kd,

Cb/Cw = Kd,

Sw = solubility in water, Sb = solubility in benzene.

Real-life applications
  • In addition, drugs can be predicted with regards to their solubility in a mixture of solvents and water.
  • Solvent removal
  • Drug-drug relationships can be determined by studying the Structure-Activity Relationship (SAR).
  • Separation chromatography
  • It is possible to extract one substance from a solution containing several substances.
  • the preservation of emulsions and creams;
  • Liquid chromatography is a technique used for this purpose.
  • Drugs released from quantity forms;
  • Formation of a solubilized structure.

Limitations of distribution law

  • Stable temperature - The experiment is conducted at a stable temperature.
  • Experiments at stable temperatures - The molecular state remains unchanged. The solute must maintain a stable molecular state when it is in contact with the solvent. The solute must not be dissociated or involved in the solvent.
  • Equilibrium concentrations - Following the establishment of the balance, the concentration of the solutes is noted.
  • Dilute solutions - As a result, the solute concentrations in the two solvents are low. This law does not apply to concentrations above a certain level. The solute dispersed mustn't imprudently act towards the solvent used.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of pharmaguideline.com, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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