Understanding Precipitation Titrations: Mohr's and Volhard's Methods

Expert reviewed • 23 November 2024 • 4 minute read

Introduction

Precipitation titration is a fundamental analytical technique in chemistry where the reaction between analyte and titrant produces an insoluble precipitate. This quantitative method allows chemists to determine analyte concentrations by measuring the volume of titrant required to complete the reaction.

Mohr's Method

Mohr's method is a direct titration technique used to determine chloride ion concentration in solutions. This method employs silver nitrate (AgNO₃) as the titrant and potassium chromate (K₂CrO₄) as the indicator.

Principle and Process

The method relies on two key precipitation reactions:

  • Primary Reaction (White Precipitate): AgNO3(aq)+Cl−(aq)→AgCl(s)+NO3−(aq)\text{AgNO}_3(\text{aq}) + \text{Cl}^-(\text{aq}) \rightarrow \text{AgCl}(\text{s}) + \text{NO}_3^-(\text{aq})

  • Endpoint Reaction (Reddish-Brown Precipitate): 2Ag+(aq)+CrO42−(aq)→Ag2CrO4(s)2\text{Ag}^+(\text{aq}) + \text{CrO}_4^{2-}(\text{aq}) \rightarrow \text{Ag}_2\text{CrO}_4(\text{s})

The selective precipitation occurs due to differences in solubility product constants:

  • AgCl: Ksp=1.8×10−10K_\text{sp} = 1.8 \times 10^{-10}
  • Ag₂CrO₄: Ksp=1.2×10−12K_\text{sp} = 1.2 \times 10^{-12}

Limitations

Mohr's method operates effectively only within pH 7-10:

  • Below pH 7: Chromate converts to chromic acid
  • Above pH 10: Silver hydroxide may precipitate

Volhard's Method

Volhard's method is a back-titration technique particularly useful in acidic conditions. It employs multiple reagents:

  • Silver nitrate (AgNO₃)
  • Ammonium thiocyanate (NH₄SCN)
  • Iron(III) nitrate (Fe(NO₃)₃) as indicator

Process and Reactions

  • Initial Reaction: AgNO3(aq)+Cl−(aq)→AgCl(s)+NO3−(aq)\text{AgNO}_3(\text{aq}) + \text{Cl}^-(\text{aq}) \rightarrow \text{AgCl}(\text{s}) + \text{NO}_3^-(\text{aq})

  • Back Titration: Ag+(aq)+SCN−(aq)→AgSCN(s)\text{Ag}^+(\text{aq}) + \text{SCN}^-(\text{aq}) \rightarrow \text{AgSCN}(\text{s})

  • Endpoint Reaction (Blood-red complex): Fe3+(aq)+SCN−(aq)→FeSCN2+(aq)\text{Fe}^{3+}(\text{aq}) + \text{SCN}^-(\text{aq}) \rightarrow \text{FeSCN}^{2+}(\text{aq})

Calculation

The chloride concentration is determined by: moles of Cl−=initial moles of AgNO3−unreacted moles of AgNO3\text{moles of Cl}^- = \text{initial moles of AgNO}_3 - \text{unreacted moles of AgNO}_3

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