Mohr’s method, Volhard’s method, Modified Volhard’s method, Fajan’s method of Precipitation Titration : Pharmaceutical Guidelines -->

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Mohr’s method, Volhard’s method, Modified Volhard’s method, Fajan’s method of Precipitation Titration

Mohr’s method, Volhard's method, Modified Volhard’s method, Fagan's method of precipitation titration.
Daily, we undergo several chemical reactions. The reactions that taken place during the cooking and digestion of food are examples of these, Among chemical reactions, precipitation reactions fall into this category. A soluble salt (which is in an aqueous solution) is combined with another salt (which is in an aqueous solution) to form two products. A few of these precipitate out of solution because they are insoluble in the solution (and are therefore called precipitates).

A precipitation reaction is the formation of insoluble salts from the combination of two ionic bonds in an aqueous solution. Precipitates are salts that are insoluble in water but form during precipitation reactions. Reactions involving precipitation result in solid residues called precipitates due to the double displacement involved. As well, insoluble salts may form as the result of the precipitation of salts when two or more solutions with different salts are mixed.

Mohr’s method
A German chemist named Karl Friedrich Mohr devised this method. Thus, the name Mohr's method was given to this method. The method involves direct titration. Silver nitrate is used to prepare titrants and chloride ion solution is used as an analyte in this procedure. As an indicator, potassium chromate is utilized. The reaction between silver ion and chromate ion produces a reddish-brown colored precipitate at the endpoint of the reaction when all chloride ions have been consumed by silver ion.

Here are the steps involved in the reaction -

AgNO3      +         Cl-          🡪      AgCl       +          NO3-
(In the solution of NaCl)            (White ppt if formed)

At the end point –

2Ag+ + CrO4-2 🡪 Ag2CrO4
(Reddish Brown ppt)

Volhard's method
1874 is the year German chemist Jacob Volhard published this method. Using silver ions, this method determines halide ions (F, Cl, Br, I) and anions such as phosphates and chromates in acidic media. Iron ion precipitation as hydrated oxide occurs if this titration is performed in an acidic medium. The method of Volhard uses iron ions as indicators. An oxidizing agent such as AgNO3 (or any anionic solution) is used to titrate the acidic first analyte (halide ion solution).

A chloride anion reaction occurs when analytes contain chloride anions. Here is what will happen:

Cl- + Ag+ 🡪 AgCl + Ag+ (in excess)

Titration is carried out with KSCN standard solution using iron ion (Fe+3) as an indicator. The iron ion gives a red color at the end of the procedure.

An example of a reaction is provided here -

Ag++ SCN- 🡪 AgSCN

Due to the presence of an excess of thiocyanate ion in the titration mixture, red color occurs as a result of the formation of the FeSCN(II) compound.

An example of the reaction involves the following:

Fe+3 + SCN- 🡪 FeSCN+2
(Red colored compound)

Precipitation by this method is indirect.

Modified Volhard’s method
Chloride analysis requires a slight modification to the Volhard method. As a result of Titration, the solution comes into contact with two precipitation stimuli simultaneously, silver chloride and ammonium thiocyanate, both of which are inherently insoluble. Therefore, thiocyanate ions react with AgCl after the titration, resulting in a gradual disappearance of the red color of the iron thiocyanate complex. As more SCN is added, the red color returns which introduce a huge error in analysis. To avoid this error, a solution of a few milliliters of an organic liquid that will not mix with water must be added before titration with thiocyanate. Upon contact with this compound, the surface of the PowerPoint will be moistened, making the surface insoluble in water and thereby isolating it from the solution. PPT and solution cannot exchange ions, so this prevents ion exchange.

Fagan's method
A chemist in the United States named Kazimierz Fajan presented this technique. Fagan's method is called that because of this reason. Silver chloride is also known as indicator adsorption because chloride ions are adsorbed on its surface. A dichlorofluorescein indicator is used in this method. By turning pink from green, the suspension (of AgCl and indicator) indicates the endpoint has been reached.

To illustrate how a reaction involves a reaction, you can write the following.

Ag+ + Cl- 🡪 AgCl

Ag+ + AgCl + Indicator 🡪 AgCl-Ag+ Indicator

During this process, precipitation takes place directly.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, 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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