EXPERIMENT 5

 

DETERMINATION OF THE STOICHIOMETRY OF A COMPLEX USING SPECTROMETRIC MEASEUREMENTS

 

(Adapted from the text: L. G. Harris., Analytical Chemistry: Principles & Techniques, Prentice Hall, Inc. Englewood Cliffs, New Jersey, 1988 p. 67)

 

Introduction

Several analytical techniques have been developed for the determination of  metal ions in solution.  Among these are: electrochemical oxidation reduction, ion chromatography, atomic emission or absorption and measuring the absorbance of a complex formed between the metal and a complexing agent.  In this experiment you will study the stoichiometry of complex formation for the metal ion Fe+2 and the complexing agent 1, 10 -phenanthroline (otherwise known as o-phenanthroline C12H8N2) sometimes referred to as a ligand.  The complexing agent complexes the metal by the following reaction:

 

                                    xFe  + yC12 H8N2  = Fex(C12H8N2)y +2

 

where x and y are stoichiometric terms. This  complex absorbs light strongly at 508 nm (E

= 11, 100) and thus can be studied using spectrophotometry at visible light wavelengths.  The absorbance of the complex in solution follows Beer’s law:

                       

A = ab[MxLy]

 

A = absorbance of the solution

a = molar absorptivity

b = the pathlength of the cell

[MxLy]= concentration of the complex

 

Using absorbance measurements, the values of x and y will be determined using three different experimental methods: Continuous Variation, Mole Ratio and Slope/Ratio Methods

 

Continuous-Variation Method

 

This method is performed by preparing several solutions consisting of varying amounts of the metal ion and complexing agent, however the sum of the metal ion concentration and complexing agent concentration is constant for each solution, The absorbance of each  solution is measured and plotted against the mole fraction of metal ion or mole fraction of ligand.  The mole fraction of the metal, M, is defined as

                                                mole fraction  =        Cm                                   

                                                                           (Cm   +   Cl)

where Cm = the concentration of the metal and Cl = the concentration of the ligand. 

 

A similar expression can be written for the mole fraction of the ligand.  When the absorbance is plotted against the mole fraction of M. a plot similar to the one below (Figure 1) is obtained:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Notice that there are two straight line portions of each curve (A and B) which can be extrapolate to intersection points.  The intersection point gives the mole  fraction of M in the complex.  In the case of the 1:2 complex, the mole fraction of M = 0.33.  Since the sum of the mole fraction of M and the mole fraction of L must be equal to 1, the mole fraction of L =  0.67.  Thus the ratio mole fraction M / mole fraction L = 0.33/0.67 or ½.  The stoichiometry of the complex is ML2 from the curve A

 

 

Mole Ratio Method

 

In the mole ration method, varying amounts of ligand are added to a constant amount of metal ion.  The absorbance of each solution is measured and plotted against the ration moles ligand/moles metal ions as shown in Figure 2 below:

 

 

 

 

 

 

 

 

 

 

 

 

For a given complex stoichiometry, the graph will consist of a curve having two straight line portions.  Extrapolation of each straight line portion to an intersection point gives the ratio of moles ligand/moles metal ion from the x-axis.  Notice in Figure 2 that two curves are plotted, each corresponding to a different metal complex stoichiometry.

 

Slope Ratio Method

 

In the slope ratio method, two sets of solutions are prepared.  The first contains various amounts of metal ion (M) with the same large excess of ligand (L) added to each.  The second solution contains various amounts of ligand with the same amount of metal ion added to each.  In the case of the solution containing a large excess of the ligand, the concentration of the complex formed is limited by the concentration of the metal ion added.  For this case the following expression can be written:

 

                                                [MxLy] = Cm

                                                                  x

 

Since the system we are working with conforms to Beer’s Law, the following expression holds true:

                        A = ab[MxLy] = abCm

                                                                                   x

 

The absorbance of each solution is measured and is plotted against Cm.  The plot of A versus Cm yields a straight line with a slope of ab/x.  Likewise for the solution containing a large excess of metal ion:

 

                                                [MxLy] = CL

                                                                 y

 

and

 

                                                A= ab[MxLy] = abCL

                                                                                 y

 

From this equation, a plot of A versus CL will yield a straight line with a slope of Ab/y.  The ratio of the two slopes gives the ratio for the complexation reaction:

 

                                    ab/x = y

                                                            ab/y = x

 

Reaction

xFe+2  + YC12H8N2   = Fex(C12H8N2)y+2

 

 

Equipment

10,  25-ml volumetric flasks.

Spectrophotometer

Absorption cells

 

Reagents

 

Fe(NH4)2(SO4)2 6H2O 7.00 x 10-4 M

1,10-phenanthroline 7.00 x 10-4 M

1, 10 phenanthroline 2.10 x 10-3 M

CH3CO2H/CH3CO2Na bufffer, pH 4.0 total acetate 0.10 M

Hydroxylamine hydrochloride, 0.7 M

 

Procedure

 

Continuous Variation Method

1)     Pipet 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10 ml of the iron solution into separate 25-ml

volumetric flasks.  Add 5 ml of the acetate buffer and 1 ml of the hydroxylamine hydrochloride solution to each flask.

 

2)     Into each flask, respectively , pipet 10, 9, 8, 7, 6, 5, 4, 3, 2, 1 and 0 ml of the 7.00 a

10-4 M of phenanthroline solution, Dilute to the mark with distilled water and mix thoroughly.

 

3)     After 10 minutes measure the absorbance of each solution at 508 nm using the

distilled water as a reference.

 

4)     Plot absorbance versus mole fraction of iron (II).  Extrapolate the linear portions of

the plot until they intersect and compute the indicated stoichiometry. Please list the figures in your notebook following the example of Figure 1 (including data points and the extrapolation lines.)

 

Mole Ratio Method

1)     Pipet 2 mL of the the standard iron solution into ten, separate 25-ml volumetric flasks.

Add 5 ml of the acetate buffer, 1 ml of the hydroxylamine hydrochloride solution and mix. 

 

2)     Pipet 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12 and 15 ml of the 7.00 x 10-4 M phenanthroline

solution  into the flasks.  Dilute to the mark with distilled water and mix thoroughly. 

 

3)     After 10 minutes, measure the absorbance of each solution at 508 nm using distilled

water as a reference.

 

4)     Plot absorbance versus the ratio of the moles of phenanthroline to the moles of iron. 

Extrapolate the linear portions of the plot until they intersect and report the indicated Stoichiometry. Please list the figures in your notebook following the example of Figure 2 (including data points and the extrapolation lines.)

 

 

Slope Ratio Method

1)     Pipet 5 mL of the iron solution into each of five 25-mL volumetric flasks.  Add 5 ml of

the acetate buffer and 1 ml of the hydroxylamine solution and mix.

 

2)     Pipet 1, 2, 3, 4 and 5 ml of the 7.00 x 10-4 M phenanthroline solution into the flasks.

Dilute to the mark with distilled water and mix.

 

3)     After 10 minutes measure the absorbance of each at 508 nm using distilled water as

A reference.

 

4)     Pipet 5 ml of the 2.10 x 10-3 M phenanthroline solution into each of five 25-ml

volumetric flasks. Add 5 ml of the acetate buffer and 1 ml of the hydroxylamine

solution and mix.

 

5)     Pipet 0.5, .1.0, 1.5, 2.0 and 2.5 mL of the iron solution. Dilute to the mark with

distilled water and mix.

 

6)     Plot absorbance versus concentration of iron and absorbance versus concentration of

phemnanntroline.  From the slopes of these two graphs, calculate the Stoichiometry.

 

7) Perform a literature search and determine the documented value for the stoichiometry of this complex.  How does your value compare?  Be sure to include the reference.
DATA SHEET                                                                                       

 

COMPARISON OF SPECTROPHOTOMETRIC METHODS FOR DETERMINING THE STOICHIOMETRY OF A COMPLEX

 

A. Continuous Variation Method

(23 point)

                       

Mole Fraction of Iron (II)                                         

 

            Stoichiometry (Fe[II]; Phenanthroline)                                

 

 

 

B.     Mole-Ratio Method

(23 points)

 

# mol of phenanthroline/ # mol of Iron (II)              

 

Stoichiometry (Fe[II]; Phenanthroline)                                

 

 

C.     Slope- Ratio Method

(23 points)

 

Slope of Graph of Absorbance versus [Fe (II) ]                              

 

Slope of Graph of Absorbance versus [phenanthroline]                             

 

Stoichiometry (Fe[II]; Phenanthroline)                                

 

 

Overview

(11 points)

           

Average Stoichiometry (Fe{II]; Phenanthroline)                            

 

 

 

 

 

 

 

Notebook Grade:                                                                   Final Grade: