ESTIMATION OF FERROUS IRON (Redox Titrations)

Aim: To estimate the amount of Ferrous iron in the whole of the given solution using a standard solution of Potassium Dichromate.

Apparatus:             100 ml standard flask,
                             Burette,
                             250 ml Conical Flask,
                             20 ml Pipette,
                             Funnel & Simple balance with Fractional weights.

Chemicals Required:   Potassium dichromate (K2Cr2O7)
                                Sulphuric acid (H2SO4)
                                Syrupy phosphoric acid (H3PO4)
                                Diphenylamine
                                Ferrous iron solution & distilled water.

Principle: Ferrous Iron is oxidized to Ferric iron by Potassium Dichromate in acid solution. The completion of oxidation reaction is marked by the appearance of Blue violet color of Diphenylamine, which is used as an internal indicator.

Chemical reactions:

K2Cr2O7 + 4H2SO4 -> K2SO4 + Cr2(SO4)3 + 4H2 + 3(O)
2FeSO4 + H2SO4 + (O) -> Fe2(SO4)3 + H2O
K2Cr2O7 = 3(O) = 6Fe

Procedure:

1. Preparation of standard potassium dichromate: Weigh out accurately the given pure crystalline sample of potassium dichromate and transfer into 100 ml standard (volumetric) flask provided with a funnel. Dissolve the dichromate in a small quantity of distilled water, and make upto the mark. The contents in the flask are shaken well for uniform concentration. Calculate the normality of potassium dichromate.

2. Estimation of Iron: Make up given solution upto the mark with distilled water and shake the flask for uniform concentration. Rinse the pipette with the ferrous solution and pipette out 20ml into a clean conical flask add 20ml of the acid mixture (sulphuric acid and phosphoric acid), and four to five drop of diphenylamine indicator. Fill the burette with the prepared potassium dichromate solution after rinsing it, with the same. Titrate the solution in the conical flask against the standard potassium dichromate from the burette till the color changes to blue violet. Repeat the titrations for concurrent titre values. 

Preparation of Standard (K2Cr2O7) solution:
 
W1 = Weight of bottle + substance(K2Cr2O7) = ____________ gms
W2 = Weight of bottle = ____________ gms
Weight of substance (K2Cr2O7) = (W1-W2) = ____________ gms.

Normality of the K2Cr2O7solution = ((W1-W2) X 10)/Equivalent Weight
N = ((W1-W2) X 10)/49 =

Calculation:

N1 = Normality of K2Cr2O7 solution =
N2 = Normality of Ferrous iron solution = 

V1 = Volume of K2Cr2O7 solution =
V2 = Volume of Ferrous iron solution = 20 ml

N1 V1= N2 V2
N2 = (N1 V1)/V2

Normality of Ferrous iron solution = N2 = (N1 V1)/V2

Amount of Ferrous iron present in the whole of the given solution (100 ml) = (N2 X 55.85)/10 =

Result: Amount of Ferrous iron present in the whole of the given solution (100 ml) = _______ gm.