Chapter 7, Problem 7.35P

Interpretation Introduction

Interpretation:

The silver nitrate precipitate is not formed and the reason behind it has to be explained.

Concept introduction:

• Volhard method is a titration method used to identify the anions that forms precipitate while reacting with silver ion.
• Sodium iodide (analyte) is mixed with more silver nitrate.  Formation of iodide precipitate confirms the presence of sodium iodide.

${\text{I}}^{\text{-}}\text{+}{\text{Ag}}^{\text{+}}\stackrel{}{\to }\text{AgI}\text{+}{\text{Ag}}^{\text{+}}\text{(excess)}$

• Thiocyanate ion with known concentration is titrated with excess silver ions.  $\text{Fe(III)}$ is used as indicator

  ${\text{Ag}}^{\text{+}}\text{+}{\text{SCN}}^{\text{-}}\stackrel{}{\to }\text{AgSCN}$

• Increase in concentration of thiocyanate ion leads to formation of red colour.

  ${\text{Fe}}^{\text{3+}}\text{+}{\text{SCN}}^{\text{-}}\stackrel{}{\to }\text{FeSCN}$