Titration - Standardisation Flashcards
Why must potassium manganate solutions be standardised?
To find concentration
Why was it necessary to standardise the potassium manganate solution immediately before use in the titration?
Unstable and affected by light
Decomposes in presence of heat
What reagent is used to standardise potassium manganate immediately before use?
Ammonium iron II sulfate
Why is additional dilute sulfuric acid added to the titration flask before each titration is carried out?
Ensure complete reduction of MnO4- to Mn2+ (supplies H+ ions
Explain why it is necessary to use dilute sulfuric acid when preparing the standard solution from hydrated ammonium iron II sulfate
To prevent air oxidation of Fe II to Fe III
Describe fully the procedure used during titrations to ensure the accuracy of the end point
Swirl flask while adding manganate solution from burette
Rise walls of conical flask with deionised water at intervals
Add manganate dropwise slowly as endpoint approaches
Read burette at eye level at top of meniscus
White tile
What observation indicates that the correct end point has been reached?
Pink colour remains
Describe fully the procedure used during titrations to ensure the accuracy of the end point
Swirl flask while added manganate to ensure homogenous solution
Rinse walls of conical flask with deionised water at intervals
Add manganate dropwise slowly as end point approaches
Read burette at eye level, top of meniscus
Repeat and find two/three titres
White tile
Why is ammonium iron (II) sulfate suitable as a primary standard?
Water soluble, stable and available in pure form, can make solutions of known concentration
Could hydrochloric acid or nitric acid be used instead of sulfuric acid? Explain
No
Nitric acid - very powerful oxidising agent
Hydrochloric acid - would react with KMNO4 to give off chlorine gas
In preparing for the titration, explain why the pipette and burette were rinsed with deionised water followed by solutions they were to contain
Deionised - wash away residue
Solution to contain - remove any excess water i.e avoid dilution
During the titration, the sides of the conical flask were washed down with deionised water from the wash bottle. Explain why this procedure is necessary and why it can be carried out without affecting the result of the titration
Washing process - to ensure all the Mn7+ solution added from burette reacts with Iron II solution
Does not affect - only deionised water was added, no extra reactants were introduced into the flask. - Adding water to the solution doesn’t change the amount of the solution as both solutions were measured before they entered the flask and so it doesn’t affect the overall titration.
One of the products of this reaction acts as a catalyst for the reaction. Which product is this? How could you demonstrate what substance is acting as the catalyst?
The reaction is catalysed by Mn2+ ions. This can be shown by taking a clean conical flask, pipetting the Fe2+ solution into it, acidifying it and then before starting to titrate adding some MnSO4 solution (a convenient source of Mn2+). Now the first droplet of MnO4- added decolourises immediately as there is Mn2+ in place to act as catalyst.
Why is the conical flask only rinsed with deionised water?
Deionised water washes out any residual solution in the conical flask. If it were then washed out with the solution it was to contain, traces of it would remain, and there would not be a precisely known amount of the solution in the flask.
In what conditions is Iron(II) very susceptible to air oxidation and how may it be inhibited
under neutral or alkaline conditions
This oxidation is inhibited in the presence of acids. The ammonium iron(II) sulfate solution is made up in dilute sulfuric acid solution to make it stable towards air oxidation.
