7. transition metal titrations

titration is used to how out

how much oxidising agent is needed to exactly react w a quantity of reducing agent

if you know the concentration of either the oxidising agent or the reducing agent

you can use the titration results to work out the concentration of the other

transition metals have variable oxidation states

they are often rpesent in either the oxidising or reducing agent

transition metal colour change also makes them useful for titration

as it is easy to spot the end point

performing the titration:

• first you measure out the quantity of the reducing agent (Fe²⁺/ C₂O₄^2-) using a pipette and put it in a conical flask

• using a measuring cylinder, add 20cm³ od dilute sulfuric acids to the flask

  • this is in excess so you dont need to be exact

• bthe acid is added to make sure there are plent of H⁺ ions to allow the oxidising agent to be reduced

• now add the oxidising agent (potassium manganate 7) to the reducing agent using a burette, swirling the conical flask as you go

• the pxidising agent you add reacts w the reducing agent, the reaction will continue untill all of the reducing agent is used up

• the very next drop you add to the flask will give the mixture the colour of the oxidising agent

  • you could use a coloured reducing agent and a colourless oxidising agent then watch for the moment the colour dissapears in the flask

• stop when the mixture in the flask just becomes tainted w the colour ox the oxidising agent (AKA end point) and record the volume of oxidising agent added

• this is the rough titration

• now you do some accurate titrations till you get results within 0.1 cm³ of eachother

titration reaction of the reducing agent Fe²⁺ against the oxidising agent Manganate 7 (MnO₄)

MnO₄^- + 8H⁺ + 5Fe²⁺ → Mn²⁺ +4H₂O + 5Fe³⁺

titration of C₂O₄^2- with MnO₄^-

2MnO₄^- + 16H⁺ + 5C₂O₄^2- → 2Mn²⁺ + 8H₂O + 10CO₂

balanced under acidic conditions

calculating the concentration of the reagent (oxidising/reducing):

1. write out a balanced equation for the redox reaction thats occured in the conical flask

2. decide what you already know and what you need to know

   • you’ll already know 2 volumes and the concentration of 1 reagent

   • you’ll need to find out the concentration for the other reagent

3. for the reagent you know both the concentration and volume for, calculate the number of moles

4. use the molar ratios in the balanced equation to find out how many moles of the other reagent were repsent in the solution

5. calculate the unknown concentration using conc = (mol × 1000)/vol (cm³)