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Principle:
Potassium dichromate oxidizes ferrous ion to ferric ion in the presence of acid as per the
reaction:
5Fe
→ 5Fe
MnO 4
**5Fe
- + 8H + → 5Fe + + Mn 2+ + 4H 2 O
The electrode potential (oxidation potential) in the titration depends upon the concentration of
Fe 2+ , Fe 3+ and hydrogen ions. To avoid the effect of the change in the hydrogen ion
concentration, the titration is usually carried out in large excess of an acid. The oxidation
potential of this redox system is given by
ln( ) 2
3
(^0)
Fe
Fe
nF
RT
E E
Connecting the redox electrode (Platinum) with a saturated calomel electrode (SCE)
completes the necessary cell as indicated below:
Hg Hg 2 Cl 2 (s), Saturated KCl Fe 3+ , Fe 2+ Pt
(SCE)
When potassium dichromate is added Fe 2+ is oxidised to Fe 3+ whose concentration increases
with progressive addition of potassium permanganate. The observed EMF gradually
increases. At the end point, there will be a sharp increase due to sudden removal of all Fe
2+
ions. Plot between EMF measured (E) Vs. Volume of potassium permanganate added and
another plot between E /V vs. Average volume of potassium permanganate are made. The
end point of the titration is measured graphically.
Requirements:
Reagents and solutions: 100 mL of K 2 Cr 2 O 7 solution, 100 mL of steel solution, 2.0 N
H 2 SO4.
Apparatus: Calomel electrode, Platinum electrode, Potentiometer, Volumetric flasks,
Burette, Pipette, Beakers.
Experiment Analysis of Iron in steel
Problem definition Mechanical properties of steel depend on its composition. Hence its
analysis is important for its use in industry.
Methodology Potentiometric method using K 2 Cr 2 O 7 as oxidizing agent to oxidize
Fe(II) in steel to Fe(III) facilitating its estimation.
Solution Estimation of percentage of iron in different steel samples.
Student learning
outcomes
Students will learn to perform
a)Potentiometric method
b)Composition of iron in different grades of steel
Procedure:
Calibration of Potentiometer: Switch on the potentiometer and connect the standard cell
terminals to either channel A (move channel switch to position A) or channel B (move the
channel switch to position B). The meter should read 1.018 V. In case it is not 1.018 V, adjust
the std. knob to obtain reference value.
Estimation of Fe(II) in steel: Transfer the given unknown steel [containing Fe(II)] solution
into a clean 100 mL standard flask and make the solution up to the mark with distilled water
and mix well. Pipette out 20 mL made up steel sample solution into a clean 100 mL beaker.
Add one test tube dilute sulphuric acid (2.0 N). Place Pt electrode in the beaker and connect
to the +ve terminal of the potentiometer. In another beaker, place 50 mL of saturated KCl
solution and dip the SCE in the solution and connect to the - ve terminal of the potentiometer.
Place a salt bridge to complete the cell. Read the EMF of the cell and note down the value.
Add 1 mL of K 2 Cr 2 O 7 solution from a burette to the beaker containing steel sample solution.
Stir the solution carefully and measure the EMF. Continue the addition of K 2 Cr 2 O 7 solution
and record the EMF for every 1 mL addition as per procedure till the potential shows a
tendency to increase rapidly. After the abrupt change in cell EMF is observed, continue the
titration to take 5 more reading by adding 1 mL burette solution every time. Plot
EMF(ordinate) vs. volume of K 2 Cr 2 O 7 added (abscissa) to get S-shaped curve which will
indicate the volumes between whish the end point lies.
Carry out another titration in similar way but by adding 1 mL aliquots of (^) KMnO4 initially and
then 0.1 mL aliquots between the two volumes where the end point is detected. Continue the
titration beyond the end point as done above. The exact end point is determined by
differential method i.e. by plotting ΔE/ΔV vs. average volume of KMnO 4 added. Calculate
the normality strength of the Fe(II) in the given solution.
EMF vs volume of KMnO 4 added (mL)
0
100
200
300
400
0 5 10
Potentiometric Titration-II
Steel solution containing Fe(II) 20 mL + 20 mL (one test tube) 2.0 N H 2 SO 4 in beaker,
K 2 Cr 2 O 7 in burette
Sl. No.
Vol. of
K2Cr2O
( mL)
EMF (Volt)
Therefore, Fe(II) present in the 20 mL steel solution = V x 1.7669 mg = X mg
This Fe(II) is present in 20 mL of steel solution given which contains Y mg/mLof steel.
Therefore, % Fe(II) in the steel sample = X x 100
Y
Result
Percentage of Fe (II) in the given steel sample =
