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.
What indicator is used and explain
No indicator is needed, as the manganate 7 ions are decolourised in the reaction until the end point, when a pale pink colour persists
What is in the burette and what is in the pipette
Iron (II) solution in pipette Potassium manganate (VII) in burette
Describe the appearance of the ammonium iron (II) sulfate crystals
Green crystalline material
Why do you use ammonium Iron (II) sulfate rather than just use Iron (II) sulfate?
It can be obtained in a high degree of purity
It is not affected by the air, i.e the Fe2+ ions in ammonium iron (II) sulfate are not oxidised to Fe3+ ions by oxygen in the air
Why is iron sulfate not a primary standard (NOT AMMONIUM IRON II SULFATE)
Crystals are oxidised slightly by air
Crystals lose water of crystallisation when exposed to air (efflorescence)
What colour change is observed during this titration?
Colourless - pink
Describe how you would take the reading in the burette when using KMnO4?
Take the reading from the top rather than the bottom of the mensicus
Dilute sulfuric acid is added on two occasions during the experiment, when?
1) When making up the solution of ammonium iron (II) sulfate
2) To the conical flask at the beginning of the titration
When carrying out the titration, another student observed that the first few drops of KMnO4 added to the conical flask were decolourised slowly, but subsequent drops were decolourised rapidly. Explain this observation
This is due to autocatalysis i.e the reaction is catalysed by the Mn2+ ions formed in the reaction. As soon as the Mn2+ ions are formed, the rate of the reaction increases
When performing this experiment, a student noticed a dark brown colour being formed in the conical flask. What conclusion would you draw from this observation?
The student has probably forgotten to add the dilute sulfuric acid at the beginning of the titration. This omission causes a dark brown precipitate of MnO2 to be formed
What is the oxidising and reducing agent of the reaction
Oxidising - MnO4-
Reducing - Fe2+
Describe in detail the correct procedure for preparing the burette for use in the titrations
rinse with deionised water then with the KMnO4 solution and clamp it vertically
Using a funnel, pour the KMnO4 solution into the burette. Fill to the top, open the tap and allow the KMnO4 to fill the part of the burette below the tap. remove the funnel and adjust the level of KMnO4 to the zero mark, reading from the top of the meniscus
What colour change is observed as the solution from the burette flowed into the conical flask
Pink - colourless (pink in burette, colourless in conical flask)
How is the concentration of potassium manganate solution found?
It is standardised by titration against a standard solution of ammonium iron (II) sulfate solution
Why is no indicator needed?
The manganate (VII) ions are decolourised in the reaction until the end-point, when a pale pink colour persists
What is in the burette and pipette?
Burette - Potassium Manganate (VII) solution
Pipette - Ammonium Iron (II) sulfate solution
What is present in the conical flask during the titration?
Ammonium Iron (II) sulfate solution and dilute sulfuric acid
