Redox: Potassium Manganate / Ammonium Iron (II) Sulfate

Describe how you would prepare exactly 250cm³ of a standard solution containing 9.31g of (NH₄)₂SO₄.FeSO₄.6H₂O (hydrated ammonium iron(ii) sulfate) that was supplied on a clock glass

Dissolve in beaker containing dilute sufluric acid //

add rinsings of clock glass to beaker //

transfer to 250cm³ volumetric flask //

transfer solution to flask using a funnel //

add rinsings of beaker, funnel, glass rod to flask //

add deionised water and dilute sulfuric acid //

slowly near the mark using dropper //

until bottom of meniscus is on mark, reading at eye level //

stopper and invert several times

Why was dilute sulfuric acid added during the preparation of this standard solution?

To prevent oxidation of Fe²⁺

What is the indicator for this titration?

Potassium Manganate acts as its won indicator

Describe fully the procedure used during the titrations to ensure the accuracy of the end point

Swirl flask //

Rinse down inside walls of flask with deionised water //

Add manganate dropwise as end point is approached //

read burette at eye-level //

keep burette vertical //

read top of meniscus [solⁿ is very dark in colour - bottom of meniscus is not visible] //

Carry out a rough titration //

repeat to find two or more titres that agree within 0.1cm³ //

use a white tile to see colour changes in flask clearly

How was the end point of each titration detected?

Permanent pale pink colour remains

If insufficient acid was added to the conical flask, what would have been observed as the KMnO₄ solution was added from the burette to the Fe²⁺ ion solution? Explain

(Why was excess dilute sulfuric acid added to each portion of the standard solution?)

Brown precipitate

MnO₂ formed {Mn⁴⁺}

Explain the term standard solution

A solution whose concentration is accurately known

Give two essential properties of a primary standard for volumetric analysis

Pure // water soluble // solid // high molecular mass // stable air // anhydrous