Redox Titrations

Introduction

Redox titrations are a method used to determine the concentration of an unknown substance by a redox (electron transfer) reaction.

They may be used to:

• Estimate the concentration of a transition metal ion in an unknown solution.

• Calculate the percentage composition of a metal in a solid of a compound or alloy.

These are reactions of an oxidising agent against a reducing agent.

As transition metals formed coloured compounds, most redox titrations result in a colour change at the end point so do not require an indicator.

An indicator is still sometimes added to make the end-point more distinct.

These titrations commonly involve:

• Transition metal ions (e.g.,  )

• Oxidising agents (e.g., acidified  )

• Reducing agents (e.g.,  )

as an Oxidising Agent

• In acidic solution,  is reduced:

• The  solution is purple,  is colourless, so the end-point is a permanent pink (very pale purple) colour formed.

• First image - reacting immediately with species in flask

• Second image - Species in flask fully reacted so does not react and the colour persists

a)  with  

Half equations:

Overall Reaction:

 is oxidised to ,  is reduced to .

b) (oxalate) with 

Half equations:

Overall Reaction:

1. Add a standard solution of in the burette, record the initial volume.

2. Pipette a measured volume of into a conical flask.

3. Add excess dilute sulfuric acid (to provide in overall reaction)

4. Titrate until first permanent pink colour persists, record final burette volume.

5. Repeat, adding dropwise from the burette as the endpoint is approached.

6. Repeat until concordant titres are achieved

Calculations

When in doubt… MOLES.

1. Write balanced ionic equation

2. Calculate moles of what you can (usually  from burette)

3. Use molar ratio to find moles of the other species (usually )

4. Determine unknown quantity accordingly e.g. mass, , concentration)