Determination of amount of ferrous ions by titrating against standard potassium dichromate solution, potentiometrically

1. SHREE MALLIKARJUN COLLEGE CLASS:SYBSC
Dr. Mithil Fal Desai
Aim: Determination of amount of ferrous ions by titrating against standard potassium
dichromate solution, potentiometrically.
Chemicals: Ferrous salt, sulphuric acid, potassium dichromate
Apparatus: Beakers, standard flask and glass rod.
Instrument: Potentiometer, platinum electrode and SCE.
Theory: In a potentiometric titration, a change in potential is recorded against the addition of a
standard or standardized solution. A sharp change in potential indicates the endpoint of the
titration. Thus, the use of any internal indicator is not required. Titration of an unknown amount
of Fe2+
with known strength of oxidizing an agent like potassium dichromate in the acidic
medium can be performed by recording the change in potential with the additional standard
potassium dichromate solution. A standard calomel electrode coupled with a Pt electrode
(indicator electrode) can be used for the determination of the potential of the solution. The initial
electrochemical cell can be represented as
(-)
SCE // Fe2+
, (SO4)2-
, H+
/ Pt(+)
The addition of dichromate shall change the potential of the system gradually as chromate ions
will oxidize Fe2+
ions generating Cr3+
ions. The electrochemical cell at the final stage of titration
could be represented as
(-)
SCE // Fe3+
, Cr3+
,(Cr2O7)2-
,K+
, (SO4)2-
, H+
/ Pt(+)
During the course of titration the potential of a reaction system is dependent on the amount of
Fe2+
, Fe3+
, Cr3+
, (Cr2O7)2-
and H+
ions majorly.
Procedure:
i) Calibration of potentiometer
Refer to the instrument manual.
ii) Titration of ferrous ions vs. dichromate.
Connect reference electrode to the negative terminal (usually black port) and Pt electrode to the
positive terminal of the potentiometer (usually red). Take 10 mL of unknown ferrous ion solution
in a 250 mL beaker. Place the platinum and SCE in it and 40 mL of 2N H2SO4. Add water if
necessary so that the electrode system shows the change in potential with the addition of
K2Cr2O7. Record the initial potential. Rinse and fill the burette with standard K2Cr2O7 solution.
Add 0.5 mL of standard K2Cr2O7 in the beaker containing Fe2+
ions. Continue the titration by
adding 0.5 mL dichromate solution each time until distinct change initial potential is observed,
which changes marginally with the further addition of oxidizing agent. Record the observations
in Table 1. Simultaneously plot a graph of potential v/s. volume of standard K2Cr2O7. Also, plot
the graph of ΔE/ΔV vs. volume of standard K2Cr2O7.

2. SHREE MALLIKARJUN COLLEGE CLASS:SYBSC
Dr. Mithil Fal Desai
Observations:
Room temperature =____ ◦C.
Table 1
Volume of
standard
K2Cr2O7 ‘V’ (mL)
Potential ‘E’
(mV)
ΔE=|E2-E1|
(mV)
ΔV=V2-V1
(mL)
ΔE/ΔV
(mV mL-1
)
V1=
V2=
Result: 1) Strength of Fe2+
solution= _____ N.
2) Amount of iron in 500 mL solution = _____ g.
Further Reading
1) Experiments in Applied Chemistry, S. Rattan, S. K Kataria and Sons.
2) Vogel’s textbook of quantitative chemical analysis, J. Mendham, R. C. Denney, J. D. Barns,
M. Thomas, B. Sivasankar, Pearson India Education, 6th
Edition.
Questions
1) What are the advantages of potentiometric titration offer over the conventional titration
method?
2) Write the balanced redox reaction for the above titration.
3) What are the various parameters which are responsible for observed potential?
4) How does the temperature affects the observed EMF?