Chemistry [[The A2 Practical Exam]] Redox Titrations

The two most widley used oxidising agents and their reduction equation:

Potassium Manganate (VII)
MnO4- + 8H+ + 5 e- ---> Mn2+ + 4H2O

Potassium Dichromate (VI)
Cr2O72- + 14H+ + 6e- ---> 2Cr3+ + 7H20

[[Analysis of Iron Tablets]]

[Introduction]

Iron Tablets from the pharmacist contain anhydrous iron (11) sulphate, cheap and soluble form of iron, plus unreactive binders. Iron tablets are taken to boost Fe2+ concentration in the blood of anaemic people.

Each tablet can be dissolved in dilute sulphuric acid. Assuming that all the iron in the tablet is Fe2+ and that it is all dissolved, it is possible toe estimate the percentage iron (ii) sulphate content of each tablet by titration agaisnt standardised potassium manganate VII.

[Proceedure]

1) Weigh out 8 iron t ablets and record the mass
2) Dissolve the tablets in about 100cm3 of 2M sulphuric acid
3) Outer coat will not dissolve so filter the substance
4) Pour filtrate into 250cm3 volumetric flask, add washings from conical flask and filterpaper. Make up to the mark with distilled water.
5) Pipettes 25cm3 into conical flask and add about 25cm3 of dilute sulphuric acid
6) Titrate agaisnt 0.020M potassium manganate VII. The end point is a colour change from colourless to permanant pink
7) Repeat until concordant results are obtained.

[Notes]

*Potassium Manganate (VII) in burette and acidified iron (II) solution in conical flask.
*Both oxidising agents work strongly in acidic conditions - H+ ions are needed for the reaction.
*Choice of acid is important - HCl cannot be used in manganate titrations because the Mn will oxidise the Cl- to Cl2. But HCL can be used in dichromate titrations since the dichromate ions i=s not strong enough to oxidise the Cl- ions. Weak acids (ethanoic) cannot be uised as they do not provide a high enough [H+]
*No indicator is used for managanate as the solution acts as its own indicator. The purple manganate (VII) is reduced to manganese (II) which is colourless. One drop of manganate (VII) will produce a permantly pale pink colour = endpoint.
* Potassium dichromate needs a indicator = barium N-phenolphenylamine-4-sulphonate. Endpoint = blue green to violet.
* H3PO4 is added to enhance the end point of potassium dichromate titrations.

[results]

Mass of iron tablets = 6.82g
Mean Titre Value = 24.55cm3

1) Find the moles of MnO4- = V/1000 x c = 24.55/1000 x 0.020 = 4.91 x 10-4
2) Molar ratio from equation = 1:5
3) Find moles of Fe2+ in 25cm3 = 4.91 x 10-4 x 5 = 2.45 x 10-4
4) Find moles of Fe2+ in 250cm3 = 2.45 x 10-4 x 10 = 2,45 x 10-3
5) Mr of FeS04 = 152
6) Mass of FeS04 in tablets = m = n x mr = 2.45x10-3 x 152 = 3.373g
7) find % mass =

3.373/6.8 x 100 =

54.7%