complexation reactions

2. Complexation
Determination of stability constant of iodine-
potassium Iodide complex
KI+I2 KI3
K=[KI3] / [I2] [KI]
[KI3] = conc. of complex.
[I2] = conc. Of free iodine.
[KI] = conc. Of free pot. iodine.

3. This determination can be done using the
distribution method.
It depends on distribution of iodine between
2 phases (organic phase & aqueous phase).

4. Distribution coefficient (partition coefficient)
when iodine dissolved in an organic phase
immiscible with H2O and allowed to distribute
from this organic solvent into water,
the P.C. can be calculated from the conc. of I2
in both organic & aqueous phase.
P.C. = Co/Cw
P.C. = partition coefficient.
Co = conc. in organic phase.
Cw = conc. in water (aqueous Phase).

5. Then when distribution of iodine occures
between the organic solvent & KI solution in
water as the aqueous phase,
Then aq.phase will contain :
the complex KI3, free I2 & free KI.
Organic layer will contain free I2 only and the
stability constant can be calculated by
calculating these concentrations.

6. Definition of stability constant:
It’s the product of the molar concentration
of products divided by the product of the
molar concentration of the reactants each
raised to a power equal to the number of
moles.

7. Procedure
1) 10 mL I2 soln. + 10 mL H2O
Shake for 15 min.
Organic ( pink )
5 mL
Titrate with N / 200
Na2S2O3
Colorless
Aqueous ( colorless )
5 mL
+2 drops Starch indicator
Blue color
Titrate with N / 200
Na2S2O3

8. 2) 10 mL I2 soln. + 10 mL KI soln.
Shake for 15 min.
Organic ( pink )
5 mL
Titrate with N / 200
Na2S2O3
Colorless
Aqueous ( brown )
5 mL
+2 drops Starch indicator
Blue color
Titrate with N / 200
Na2S2O3
Straw yellow

9. Calculations
1) I2 solution & H2O:
determination of partition coefficient.
 P.C. = Co / Cw
= C in organic layer (CCl4) / C in water.
 Concentration of iodine in organic layer
= E .P × eq. × 1000
5
= …………….. g / L.

10.  Concentration of iodine in aqueous layer
= E.P × eq. × 1000
5
= …………….. g / L.
 1 mL of N / 200 Na2S2O3 = 0.0006345 g I2
 Equivalent = N x M wt. = 1/ 200 x 127
1000 1000

11. 2) I2 solution & KI solution:
 Concentration of iodine in organic layer
= E.P × eq. × 1000
5 x 127
= …………….. Mole / L.
 Molecular weight of I2 = 127.

12.  Concentration of free iodine in aq. layer
= conc. of I2 in organic layer
P.C.
=…………….. Mole / L
 Concentration of total iodine in aq. layer
= E.P × eq. × 1000
5 x 127
= …………….. Mole / L

13. 3) Concentration of complexed I2
= conc. of total I2 – conc. of free I2
= …..................mole / L.
4) KI+I2 KI3
Stoichiometric ratio = 1:1
Then complexed I2 = complexed KI.

14. 5) Total KI:
KI 10% w/v soln.
10 gm 100 mL.
or 100 gm 1000 mL.
Then total KI = 100 / 166 = 0.60241 mole / L.
Molecular weight of KI = 166
6) Conc. of free KI
= total KI – complexed KI
= …………..mole / L.

15. 7) K = [KI3]
[I2]Free [KI] Free