Volhard’s method

Introduction:

• Volhard’s method is an indirect precipitation titration used to determine halide ions (Cl⁻, Br⁻, I⁻) and thiocyanate (SCN⁻) in solution.

• It involves two steps: adding an excess of silver nitrate to precipitate the halide, followed by a back-titration of unreacted silver ions with a standardized thiocyanate solution.

• The endpoint is indicated by a red-brown ferric thiocyanate complex, using ferric ammonium sulfate as the indicator.

illustrations for Volhard’s method, showing the process of adding silver nitrate from a lab spoon into the beaker to precipitate halide ions

Principle:

• Halide ions react with silver nitrate to form a precipitate:

AgNO3 + X- → AgX(s) + NO3- (X- = Cl-, Br-, I-, or SCN-)

• The excess Ag⁺ is titrated with thiocyanate (SCN⁻), forming silver thiocyanate:

AgNO3 + SCN- → AgSCN(s) + NO3-

• The endpoint occurs when the first excess SCN⁻ forms a red-brown complex with ferric ions: 

Fe3+ + 3 SCN- → Fe(SCN)3 (red-brown)

Procedure:

1. Pipette a known volume of the halide-containing solution into a flask.

2. Add an excess of standardized AgNO₃ to precipitate all halide ions.

3. Add nitric acid (HNO₃) to maintain an acidic environment and prevent silver hydroxide formation.

4. Add ferric ammonium sulfate as the indicator (solution turns pale yellow).

5. Titrate with standardized thiocyanate (NH₄SCN) until the first permanent red-brown color appears, indicating the endpoint.

Theory:

• Volhard's method relies on the selective precipitation of silver halides and the clear color change when the ferric thiocyanate complex forms.

• The concentration of halide ions is calculated using:

C_X- = (C_AgNO3 × V_AgNO3 - C_SCN- × V_SCN-) / V_sample

Where:

• CX−​ = concentration of halide ions,

• CAgNO3​​ = concentration of AgNO₃,

• VAgNO3​​ = volume of AgNO₃ added,

• CSCN−​ = concentration of SCN⁻ titrant,

• VSCN−​ = volume of SCN⁻ used,

• V_sample​ = volume of the halide-containing sample.

This streamlined version maintains the key concepts while reducing redundancy.