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                         K_C = [[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,     K_C = (A/el) / [ a (b - A/ el)] = A /[a (bel - A)]

                        => 1/A = (1/ K_C 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/ K_C bel --------------(2)

Using (1) and (2), we can say that

        Stability constant , K_C =  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 K_C

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 K_C .

Observation and calculation :

    Temperature of the experiment:  ............

    TABLE 1 :Determination of l _max  of 5 : 5 mixture

   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 

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 K_C

From the plot of 1/A against 1/a, it is found that 

    Intercept :............

    Slope : ................

So. the stability constant ,  K_C = Intercept/Slope = ...........

Conclusion: The complex-formula ..........

                     Stability constant K_C............