The Estimation Of Iron(II) And Iron(III) In A Mixture Containing Both

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Concentration of MnO4- required for Second Titration:

• 1/5 concentration of Iron solution:                0.1075

(Using mole ratio from equation, pg3)             5

                                =        0.0215 mol dm-3

From rough calculations I know that a 30 – 70% percentage composition, where 70% is Iron(II) will give rise to a concentration of Potassium Manganate which is similar to the concentration calculated for the second titration.

Concentration of MnO4- required for First Titration:

• 70% of total Iron solution concentration:        0.1075 x 0.7

=        0.07525

• Concentration of MnO4:                        0.07525

      5

                                        =        0.0151 mol dm-3

Therefore Potassium Manganate with concentration 0.02M can be used for both titrations.

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Titration of Iron(II) with Potassium Manganate

MnO4-    +    8H+    +    5Fe2+                         Mn2-    +    5Fe3+    +    4H2O

Apparatus

1 Burette (white markings)

1 25cm3 pipette

1 Clamp stand

1 Conical flask

1 Funnel

1 White tile

1 30cm3 Measuring cylinder

Chemicals

100cm3 of solution containing Iron(II) and Iron(III) ions

100cm3 of Potassium Manganate with concentration 0.02M

80cm3 of Sulphuric acid with concentration 1M

Method

Wash out the burette using a small amount of the Potassium Manganate.  Clamp the burette carefully making sure that the tap is closed and fill using a funnel with the Potassium Manganate.  Open the tap and let some of the Potassium Manganate run out into a beaker to fill the jet of the burette making sure that there are no air bubbles.  Record the initial volume of Potassium Manganate in the burette, reading from the bottom of the meniscus and record to two decimal places.

Carefully place the pipette into the pipette filler and place it in the Iron solution.  Turn the thumb dial to suck up the solution.  Suck up more solution than what is needed, remove the pipette filler and cover the top of the pipette with your index finger and apply pressure.  By decreasing the pressure the level of the solution will fall.  Carefully allow some of the solution out so that the bottom of the meniscus is in level with the line marked on the pipette.  Transfer the solution into the conical flask.  Touch the end of the pipette against the inside of the conical flask to release the remaining solution from the pipette.  Add about 20cm3 of dilute sulphuric acid to the solution in the conical flask.  

Place the conical flask on the white tile underneath the jet of the burette. Open the tap of the burette and allow about 2 cm3 of the Potassium Manganate out into the conical flask, then swirl the solution.  Repeat this until the solution in the conical flask is almost pink.  Slow the flow of Potassium Manganate so that it is coming out drop by drop, swirling the solution all the time.  As soon as the solution turns pink it has reached the end point.  Close the tap and record the volume of Potassium Manganate used.  This is the rough titration.  

Refill the burette as before recording the initial volume and wash out the conical flask using distilled water and fill as before and repeat the titration.  However when within 2cm3 of the volume recorded for the rough titration, slow the flow of Potassium Manganate so that it is coming out drop at a time to make sure that the end point is not crossed.  This will ensure the highest level of accuracy.  Repeat this process until you obtain results that are within 0.1cm3 of each other.  

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Titration of Iron(III) with Potassium Manganate

2Fe3+    +    Zn                 2Fe2+    +    Zn2+

MnO4-    +    8H+    +    5Fe2+                         Mn2-    +    5Fe3+    +    4H2O

Apparatus

1 Burette (white markings)

1 25cm3 pipette

1 Clamp stand

2 Funnels

2 Conical Flasks

1 White tile

1 Glass rod

Filter paper

1 Spotting Tile

1 Bunsen burner

1 30cm3 Measuring cylinder

1 10cm3 Measuring cylinder

Chemicals

100cm3 of solution containing Iron(II) and Iron(III) ions

100cm3 of Potassium Manganate with concentration 0.02M

100 cm3 of sulphuric acid with concentration 1M

10 cm3 of Copper(II) sulphate solution with  concentration 1M

9 nuggets of zinc

Potassium thiocyanate solution

Method

Pipette 25cm3 of the iron containing solution into a conical flask.  Add about 25cm3 of sulphuric acid and about 1cm3 of copper(II) sulphate, also add 1 to 3 nuggets of zinc.  Heat over a gentle Bunsen flame until effervescence has almost ceased, this should take about 10 to 15 minutes.  When a drop of the solution drawn out on a glass rod no longer gives a red precipitate with potassium thiocyanate solution Iron(III) is no longer present.

Fe3+    +    SCN-                      (FeSCN)2+

Filter the iron solution and rinse the remaining zinc, adding the rinsings to the filtrate.  Titrate the iron solution with Potassium Manganate and record the results using the same techniques as in the previous titration.  

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Results of Titration of Iron(II)

Average Titre:

Results of Titration of Iron(III)

Average Titre:

Calculations

Titration of 1(Iron(II) only)

• Moles of MnO4- used:                        Volume x Concentration

             1000

                                        =        Volume x 0.02

                                                        1000

                                        =        A

• Moles of Iron(II) in 25cm3:                A x 5

(using mole ratio)

                                =        B

• Moles of Iron(II) in 200 cm3:                B x 8

=        C

• Mass of Iron(II) in solution:                Moles x Mr

=        C x 55.8

=        D

Titration 2 (Iron(II) and Iron(III)):

• Moles of MnO4- used:                        Volume x Concentration

             1000

                                        =        Volume x 0.02

                                                        1000

                                        =        E

• Moles of Iron(II) in 25cm3:                A x 5

(using mole ratio)

                                =        F

• Moles of Iron(II) in 200 cm3:                B x 8

=        G

• Mass of Iron(II) in solution:                Moles x Mr

=        C x 55.8

=        H

• Mass of Iron(III) present                 H- D

=        X

The Percentage composition by mass for each ion is therefore:

• Iron(II):        _D_     x 100

  H

• Iron(III):        _X_     x 100

  H

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Health And Safety*

Potassium Manganate:        May act as an irritant

Harmful by ingestion

Sulphuric Acid:                Corrosive; will burn

                                Ingestion may be fatal

                                Toxic

Copper(II) Sulphate:                Harmful by ingestion

Potassium Thiocyanate:        Harmful by ingestion

                                Irritant; skin contact may lead to ulceration

Zinc:                                Harmful by ingestion

                                May act as an irritant

Due to the health risks listed above safety glasses must be warn at all time when working with the chemicals.  Lab coats should also be worn to protect clothing and skin should any chemicals be spilt.  Care should be taken when heating the solution as chemicals may spit.  Tongs should be used at all times when handling the zinc to avoid skin contact.  A safety filler should also be used to fill pipettes.  If any chemicals come into contact with the skin was effected area immediately with warm water and if necessary seek medical advice

Due to the amount of glass apparatus being used, care should be taken when moving around the class.  All breakages should be reported immediately.  Any cuts should be covered before continuing with practical work.  

All practical work should be carried out in a well-ventilated area.

References

# Transition Elements, Cambridge Advanced Sciences.

* Physical and Theoretical Chemistry Lab, Oxford University, Chemical and other safety information                         web site  

±As Practical Work