Redox Titrations Flashcards
Possible redox titrations
H+/MnO4- with Fe2+ ions, H2O2 or with ethanedioate ions (C2O4 2-)
Iodine and thiosulfate ions
What is a redox titration?
One that is self indicating because when one solution is added to another, a redox reaction occurs which causes a change in colour
Titrating MnO4- with Fe 2+ ions
MnO4- acts as an oxidising agent to oxidise Fe 2+ to Fe 3+
So Mn 7+ is reduced to Mn2+ by reducing agent Fe 2+
Colour change MnO4- to Mn2+ if we add the manganate from the burette into the conical flask
Colourless in flask to purple
A solution of manganate is usually…
The standard solution containing a known concentration
Used to find concentration and number of moles of Fe2+ ions/H2O2/Ethanedioate ions in a conical flask
Thus scale up to whole solution
In a manganate titration, what do we use as the acid?
Dilute sulfuric acid always
Why can’t we use hydrochloric acid for a manganate titration?
Because then MnO4- would act as the OXIDISING AGENT for Cl- ions and oxidise Cl- to Cl2
So a greater volume of MnO4- would be needed to see the colour change: not accurate reading of amount needed to oxidise contents of the conical flask (AND Cl2 IS POISONOUS)
Why does MnO4- act as the reducing agent to Cl-?
Because the electrode potential for MnO4-/Mn2+ is more positive than for Cl-/Cl2
So MnO4- is REDUCED and oxidised Cl-
What happens if not enough dilute sulfuric acid is used in a manganate titration?
Then not enough acidic conditions (only 4H+ per mangante when w need 8H+)
Sooooo a brown precipitate of MnO2 is produced not colourless Mn2+
What happens if a brown precipitate of MnO2 is produced instead of Mn2+?
Masks colour change of when all Fe2+/H2O2/ethanedioate ions have reacted with the MnO4- to reduce it to Mn2+ (colourless)
So we don’t know when the conical flask will remain pale pink instead of turning colourless (cant see end point) so larger INACCURATE VOLUMES of MnO4- are added
Why can we not use nitric acid in a manganate titration?
Nitric acid is also an oxidising agent so will also oxidise the Fe2+/H2O2/ ethanedioate so the colour change will happen quicker, leading to an inaccurate smaller volume of MnO4- needed because the acid already oxidised conical flask contents
Ethanedioate ions + manganate titrations needs:
To be heated to 60°c because the reaction is slow to begin
Why is the reaction between manganate and ethanedioate slow?
Because between 2 negative ions so less likely to successfully collide as the charges repel each other
Titrations manganate with iron 2+ ethanedioate
Both Fe 2+ and ethanedioate reacts with MnO4-
Ratio of MnO4- : Fe2+ethanedioate ions 2-
3:5
Worked out by writing both the ionic equations for manganate Fe2+ and ethanedioate ions separately then combining by multiplying by LCM of number of electrons
