Aim of Experiment: To determine the composition of iron-thiocyanate complex spectrophotometrically and to find out the stability constant of the complex.
Theory: The complex ion formation in solution by mixing Fe (III) and thiocyanate solution takes place as
Fe3+ + nCNS- = Fe (CNS)n(3-n)+
However generally the 1:1 complex with n = 1 is formed, with the complex formulae
The stoichiometry of the equation can be determined by
Job's method. When equimolar solution of the reactants are mixed in varying
proportion, the maximum amount of complex at equilibrium is formed when the
proportion of the complex employed correspond to the empirical formula of the
complex .Thus if X and 1-X are the respective parts of the equimolar solution of
Fe (III) and thiocyanate in the mixture correspond to the maximum amount of the
complex formed, then
n = (1-X)/X
Since the complex is colored, its equilibrium conc. can
be followed by measuring the absorbance of the solution. (X can be found from
graph).
The stability constant or the equilibrium constant can for the formation of the complex is given by KC = [[Fe(CNS)]2+] / [Fe3+][CNS-]
From Beers Law, it is found that
A = [[Fe(CNS)]2+] e l or [ [Fe(CNS)]2+] = A /
( el )
Where e = Molar extinction coefficient
l = Path length
At equilibrium the conc. of CNS- and Fe3+ will be (b-A/el ) and a respectively (Fe3+ was in large excess)
Here, a = [Fe3+]0 b = [CNS-]0
There fore, KC = (A/el) / [ a (b - A/ el)] = A /[a (bel - A)]
=> 1/A = (1/ KC bel) (1/a) + 1 / bel
Thus a plot of 1/A as ordinate ,against 1/a at constant conc. of CNS- gives a straight line the intercept of which is:
Intercept = 1/ bel ------------ (1) &nnbsp; and Slope = 1/ KC bel --------------(2)
Using (1) and (2), we can say that
Stability constant , KC = Intercept / Slope
Requirements: 1) Spectrophotometer
2) 0.1 M Fe3+ solution
3) 0.002 M Thiocyanate solution
4) 0.5 M HCl solution
Procedure: A) Determination of "n"
1)0.002 M, each of Ferric ammonium sulphate and ammonium thiocyanate is prepared by dissolving the required amount in 0.02 M HCl .
2)The two solution are mixed in 5:5 proportion and l max is found out .
3)At the wavelength of maximum absorption ( l max ) absorbance of different mixture ( 1:9, 2:8, 3:7,........8:2, 9:1 ) is measured.
4)The plot of "A" against the mole fraction of thiocyanate helps to determine the value of "X" correspond to the maximum and hence "n " can be determined .
B) Determination of stability constant KC
1) 0.1 M Fe3+ solution and 0.02 M solution of thiocyanate are prepared by dissolving the required amount of both in 0.5 M HCl .
2) Now varying conc. of Fe3+ solution is prepared and mixed with constant conc. of thiocyanate solution and the absorbance of each conc. are determined .
3)A graph of 1/A against 1/a is plotted which gives the KC .
Observation and calculation :
Temperature of the experiment: ............
TABLE 1 :Determination of l max of 5 : 5 mixture
Wavelength (nm) | Optical Density |
420 | |
440 | |
460 | |
480 | |
500 | |
520 | |
540 |
A graph is plotted between Absorbance (Ordinate) Vs Wavelength
From the graph it is observed that l max =.........(Hint: 465 nm)
TABLE 2 : Determination of absorbance of the mixture of varying proportion of Fe3+ and thiocyanate at l max
Fe 3+: CNS- | Molar ratio of CNS - mixture (molar fraction) | Optical Density |
1 : 9 | ............ | ............... |
2 : 8 | ............ | ............... |
3 : 7 | ............ | ............... |
4 : 6 | ............ | ............... |
5 : 7 | ............ | ............... |
6 : 4 | ............ | ............... |
7 : 3 | ............ | ................ |
8 : 2 | ............ | ................ |
9 : 1 | ............ | ................ |
A graph is plotted between Absorbance (Ordinate) Vs Mole fraction .
From the plot it is found that
X =........
So, n = 1-X/X =..........=.........
Hence the equation becomes Fe3+ + n CNS- === Fe (CNS)n (3-n)+ (after putting value of "n" )
TABLE 3:Determination of KC
Volume of
Fe 3+ |
Volume of
CNS - |
Volume of
HCl |
Absorbance
(A) |
1/A
|
a (10 -3)
|
1/a (10 3)
|
2.0 | 1.0 | 22.0 | ............. | ............. | ............... | ........ |
1.6 | 1.0 | 22.4 | ............. | ............. | ............... | ....... |
1.2 | 1.0 | 22.8 | ............. | ............ | ............... | ....... |
0.9 | 1.0 | 23.1 | .............. | ............ | ............... | ...... |
0.6 | 1.0 | 23.4 | .............. | ............ | .............. | ...... |
0.4 | 1.0 | 23.6 | ............... | ............ | .............. | ...... |
0.2 | 1.0 | 23.8 | .............. | ........... | ............. | ...... |
From the plot of 1/A against 1/a, it is found that
Intercept :............
Slope : ................
So. the stability constant , KC = Intercept/Slope = ...........
Conclusion: The complex-formula ..........
Stability constant KC............