TITRATION INVOLVING AQUEOUS POTASSIUM PERMANGANATE (PERMANGANOMETRY)

Potassium permanganate is a powerful oxidizing agent in the presence of an acid .In acidic medium it oxidizes oxalic acid to carbondioxide; metal oxalate to metal salt and carbondioxide , ferrous salts to ferric salts and so on. In acid solution the reaction can be represented by the following equation:

MnO4- + 8H+ + 5 e- Mn2+ + 4H2O

Sulphuric acid is the most suitable acid .With HCl the reaction

2MnO4- + 10Cl- + 16H+ 2Mn2+ + 5Cl2 + 8H2O

may take place and some permanganate may be consumed in the formation of chlorine (side reaction). Therefore, HCl should not be used to acidify KMnO4

It is difficult to obtain potassium permanganate perfectly pure and completely free from manganese dioxide (MnO2). Moreover, ordinary distilled water is likely to contain reducing substances (traces of organic matter etc.) which will react with the potassium permanganate to form MnO2, which in turn, catalyses the auto decomposition of the permanganate solution on standing. Due to these reasons potassium permanganate solution is not primary standard, that is, KMnO4 solution of desired concentration can not be prepared simply by dissolving calculated amount of potassium permanganate crystals.

Therefore, potassium permanganate solution is standardized just before it is used to titrate other solutions. Potassium permanganate solution may be standardized using standard sodium oxalate or oxalic acid. In acidic medium, oxalic acid reduces MnO4 giving colourless solution.

MnO4- + 8H+ + 5e- Mn2+ + 4H2O ] 2

(COOH)2 2CO2 + 2H+ +2e- ] 5

2MnO4- + 6H+ + 5(COOH)2 2Mn2+ + 10CO2 + 8H2O

2 moles of KMnO4 5 moles of (COOH)2

No external indicator is used as the colour of KMnO4 acts as the indicator. In a flask fixed volume of oxalic acid (standard) is mixed with dil H2SO4.Warm solution (60-70C) of this mixture is titrated with KMnO4 solution .Before end point pink colour of KMnO4

disappears but at the end point faint pink colour persists for 30 seconds.

By this method the concentration of potassium permanganate solution is determined. Now the standard solution of KMnO4 can be used to titrate other reducing agents like iron (II) ion, hydrogen peroxide.

EXPERIMENT: TO ESTIMATE THE AMOUNT OF IRON IN A GIVEN SAMPLE OF MOHR'S SALT BY TITRATING IT WITH STANDARD POTASSIUM PERMANGANATE SOLUTION.

APPARATUS: Burette, pipette, Conical flask, Volumetric flask (250cm3) ,Watch glass .

THEORY:Potassium permanganate is a powerful oxidizing agent in the presence of an acid .In acidic medium it oxidizes oxalic acid to carbondioxide ,metal oxalate to metal salt and carbondioxide ,ferrous salts to ferric salts and so on .In acid solution the solution can be represented by the following equation.

MnO4- + 8H+ + 5e- Mn2+ + 4H2O

Dilute sulphuric acid is the most suitable acid for this purpose. Dilute HCl should not be used because with HCl the reaction

2MnO4- + 10Cl - + 16H + 2Mn2+ + 5Cl 2 + 8H2O

may take place and some permanganate may be used in the formation of chlorine.

Mohr's salt (ferrous ammonium sulphate) is a double salt of the formula FeSO4. (NH4)2SO4. 6H2O.Ferrrous ammonium sulphate is the most valuable of the ferrous salts for volumetric work because it does not oxidize in air rapidly as ferrous sulphate does, and it can be obtained in a state of approximating closely to perfect purity. When dissolved in water the double salt breaks up into simple salts, i.e ferrous sulphate and ammonium sulphate. Only the former (ferrous sulphate) reacts with the permanganate.

FeSO4. (NH4)2 SO4

.6H2O FeSO4 + (NH4)2 SO4 +6H2O2KMnO4 + 8H2SO4 + 10 FeSO4 5Fe2 (SO4)3 + K2SO4 + 2MnSO4 + 8H2Oor,

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

i.e., 1 mole of MnO4- 5 moles of Fe2 +

No external indicator is used in this titration. Before end point colour of permanganate disappears. At the end point last drop of permanganate produces permanent faint pink colour.From the number of moles of MnO4

- ions used in the titration, the number of moles of iron (and the amount of iron also) present in the given amount of Mohr's salt can be determined. (relative atomic mass of Fe=55.84)

PROCEDURE:

1. Weigh out accurately about 9.82g. A.R.quality Mohr's salt on a watch glass. Transfer the weighted sample in a 250cm3 measuring flask with the help of air-free distilled water (cold) .Add more air free water to fill half of the bulb. (Since distilled water contains certain amount of dissolved air and this may oxidize the ferrous ion to ferric ion the solution will have to be made in air free water. So

before starting this experiment air free water should be first prepared. To prepare air free water boil distilled water briskly for about 15 minutes in a flask fitted with a Bunsen valve).

2. Then add very slowly about 10cm3 cone. Sulphuric acid. Sulphuric acid prevents oxidation of ferrous ion by atmospheric oxygen. Shake well to dissolve the solid. Allow the flask to cool down to room temperature. Now make up to 250 cm3 with air free water. Shake well.

3. Rinse the pipette with Mohr's salt solution and then pipette out 10cm3 of the solution and transfer it into a conical flask. Add about 10cm3 bench sulphuric acid again. Run the permanganate solution from the burette until a faint pink colour persists .During titration conical flask should be constantly swirled.

(Unlike oxalic acid or oxalates, the oxidations of ferrous solutions proceeds in cold. Hence solution of ferrous salts are titrated against potassium permanganate solutions in cold otherwise oxidation of ferrous ion by atmospheric oxygen may occur).

4. Repeat procedure no. 3 to obtain at least two concurrent readings:

OBSERVATION: Concentration of potassium permanganate solution =

No. Volume of Mohr's salt solution taken

Burette readinginitial final difference concurrent reading

Titration summary: 10cm3 of Mohr's salt solution consumed_______ cm3 of supplied potassium permanganate solution.

RESULT: Amount of iron in __________ g of Mohr's salt is __________ g . Percentage Amount of iron (by weight) is _________________

SPACE FOR CALCULATION: