AMMONIUM IRON(II) TITRATION.

The reagent (NH4)2S04-FeS04•6H20 (hydrated ammonium iron(Il) sulfate) was used to prepare a standard solution.

State two properties of this substance that make it suitable for use as a primary standard.

1. High molecular mass

2. Not easily oxidised.

Describe how you would prepare exactly 250 cm° of a standard solution containing 9.31 g of (NHa/2SO4•FeSO•6H20 (hydrated ammonium iron(Il) sulfate) that was supplied on a clock glass.

1. dissolve in beaker containing dilute sulfuric acid

2. add rinsings of clock glass to beaker

3. transfer to 250 cm° volumetric flask

4. transfer solution to flask using a funnel

5. add rinsings of beaker, funnel, glass rod to flask

6. add deionised (distilled, pure) water and dilute sulfuric acid // slowly (carefully) near the mark using pipette or dropper // until bottom of meniscus is on mark

7. stopper and invert (mix) several times (more than once)

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

to prevent air oxidation of Fe²+

This standard solution of Fe2+ ions was used to determine by titration the concentration of a potassium manganate(VII) solution. The balanced equation for the titration reaction is:

MnO-4 + 5Fe²+ + 8H+ → Mn²+ + 5Fe3+ + 4H20

In each titration a little dilute sulfuric acid was added to a 25.0 cm° portion of the Fe2+ ion solution in a conical flask before adding the potassium manganate(VIl) solution from a burette. On average, 22.6 cm of the KM04 solution were required for complete reaction with 25.0 cm of the solution of the Fe2+ ions in excess acid.

How was the end point of each titration detected?

faint (pale) permanent pink colour remains.

Standard solution

A solution of known concentration.