8 Difference Between Volhard Method And Mohr Method

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Precipitation Titration

Titration is a process by which the concentration of an unknown substance in solution is determined by adding measured amounts of a standard solution that reacts with the unknown. Then the concentration of the unknown can be calculated using the stoichiometry of the reaction and the number of moles of standard solution needed to reach the so called end point.

Precipitation titration is a type of titration which involves the formation of precipitate during the titration technique. In precipitation titration, the titrants react with analyte and forms an insoluble substance referred to as precipitate. It continues till the last amount of analyte is consumed. When the titrant is excess, it reacts with the indicator and signals to terminate the titration process. It is used to determine chloride by using silver ions.

Precipitation titrations are based upon reactions that yield ionic compounds of limited solubility. The most important precipitating reagent is silver nitrate. Titrimetric methods based upon silver nitrate are sometimes termed argentometric methods. Potassium chromate can serve as an end point indicator for the argentometric determination of chloride, bromide and cyanide ions by reacting with silver ions to form a brick-red silver chromate precipitate in the equivalence point region.

Types of precipitation reactions include:

  • Volhard method
  • Fajan’s method
  • Mohr method

What Is Mohr’s Method?

This method was first described by Karl Friedrich Mohr, a German Chemist. It is a direct titration method. In this method, silver nitrate is used as a titrant and chloride ion solution as analyte. Potassium chromate is used as indicator. At the end point, when all chloride ions are consumed by silver ion, reddish brown colored precipitate is formed by reaction of silver ion and chromate ion.

Mohr method works in the PH range of 6-9 only. Above this PH, silver will form a precipitate with hydroxide. Below this PH, chromate converts to dichromate, a bright orange color thereby obscuring the endpoint. Calcium carbonate is often added to reduce the acidity of the solution.  

What You Need To Know About Mohr Method

  • In this method, silver nitrate is used as a titrant and chloride ion solution as analyte. Potassium chromate is used as indicator resulting in reddish brown complex.
  • It is a direct method of titration.
  • In the Mohr method, potassium chromate is used as an indicator.
  • Mohr method is used to determine the concentration of chlorides in neutral medium.
  • A red precipitate of silver chromate forms at the end point in Mohr method.
  • Neutral or alkaline conditions (pH 6.5-9.0)are required in the Mohr method because chromate ion is the conjugate base of the weak chromic acid.
  • In the Mohr method, titration of iodine and cyanate is not possible.
  • In the Mohr method, the titration is carried out at room temperature because solubility of silver chromate increases with rising temperature.

What Is Volhard’s Method?

This method was first given by German Chemist, Jacob Volhard in 1874. This method involves the titration of bromides, iodides and chlorides, in an acidic medium. The chloride in the solution is converted to silver chloride when reacted with excess silver nitrate solution. The leftover silver nitrate is estimated against potassium thiocyanate solution. When all thiocyanate consumes all the silver, the excess of thiocynate is made to react with an indicator. It gives a red color on reacting with ferric ammonium sulfate indicator and a ferrous thiocyanate complex is formed.

What You Need To Know About Volhard Method

  • In the Volhard method, chlorides are first precipitated with excess silver nitrate, then excess silver is titrated with potassium (or sodium) thiocynate, creating distinct wine red complex.
  • It is an indirect method of titration.
  • In the Volhard method, ferric ammonium sulphate is used as an indicator.
  • Volhard method is sensitive to low PH.
  • A red soluble ferric thiocyanate forms at the end point in volhard method.
  • Acidic conditions are required in the Volhard method.  
  • Thr Volhard method can be used to determine iodide, bromide and chloride.
  • In the Volhard method, titration is carried out below 20 degrees Celsius to prevent the color of ferric thiocyanate complex from fading.

Difference Between Volhard Method And Mohr Method In Tabular Form

BASIS OF COMPARISON MOHR METHOD VOLHARD  METHOD
Description  In this method, silver nitrate is used as a titrant and chloride ion solution as analyte. Potassium chromate is used as indicator resulting in reddish brown complex. In the Volhard method, chlorides are first precipitated with excess silver nitrate, then excess silver is titrated with potassium (or sodium) thiocynate, creating distinct wine red complex.  
Nature Of Titration It is a direct method of titration.   It is an indirect method of titration.  
Indicator In the Mohr method, potassium chromate is used as an indicator.   In the Volhard method, ferric ammonium sulphate is used as an indicator.  
Endpoint A red precipitate of silver chromate forms at the end point in Mohr method.   A red soluble ferric thiocyanate forms at the end point in volhard method.  
Medium Neutral or alkaline conditions (pH 6.5-9.0)are required in the Mohr method because chromate ion is the conjugate base of the weak chromic acid.   Acidic conditions are required in the Volhard method.    
Use In the Mohr method, titration of iodine and cyanate is not possible.   Thr Volhard method can be used to determine iodide, bromide and chloride.  
Temperature In the Mohr method, the titration is carried out at room temperature because solubility of silver chromate increases with rising temperature.   In the Volhard method, titration is carried out below 20 degrees Celsius to prevent the color of ferric thiocyanate complex from fading.  

Applications Of Precipitation Titration

  • It is used for the determination of halides ion in the solution.
  • It is used to measure salt content in food, beverages and water.
  • It is used for Sulphur, thiocyanate, dichromate etc.
  • Many drugs such as carbromal KCl infusion, NaCl infusion etc can be analysed by precipitation titration.
  • It can be used for the determination of anions in the analyte.

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