spectrophotometric determination of the stoichiometry of a complex

8/10/2019 Spectrophotometric Determination of the Stoichiometry of A Complex

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Chemistry 27.1, Spectrophotometric Determinationof the Stoichiometry of a Complex Page 3of 6

C 3.5 x10-6

1.4 x10-6

2.5 0.224

D 5.6 x10-6

1.4 x10-6

4 0.325

E 8.4 x10-6

1.4 x10-6

6 0.322

F 1.05 x10-5

1.4 x10-6

7.5 0.320

Table 2.2 Data of Mole-ratio method of

spectrophotometry (Mole-ratio vs absorbance)

Graph 1.1 Plot of Absorbance vs Mole-Ratio of complex

C. Slope-Ratio Method

Flask mLC12H8N2

[C12H8N2]in M

Absorbance

A 1 1.4 x 10-5

0.048B 2 2.8 x 10-5 0.092

C 3 4.2 x 10-5 0.142

D 4 5.6 x 10-5 0.195

E 5 7.0 x 10-5 0.246Table 2.3 Data when Iron (II) solution is constant

Graph 1.2 Plot absorbance vs varying C12H8N2

FlaskmLFe(II)

[Fe(II)] in M Absorbance

A 0.5 7.0 x 10-6 0.084

B 1.0 1.4 x 10-5 0.155

C 1.5 2.1 x 10-5 0.224

D 2.0 2.8 x 10-5 0.314E 2.5 3.5 x 10-5 0.384

Table 2.3 Data when phenanthroline is constant

Graph 1.3 Plot absorbance vs varying iron

VI. Discussion

The complex used in the experiment has

the iron ion as the metal, and 1,10 phenanthroline

as the ligand.

Fe2+(aq) + 3C12H8N2(aq) [Fe(C12H8N2)3]2+(aq)

The phenanthroline complex has a

theoretical stoichiometric ratio of 1:3 and has a

deep red-orange color. Specific solutions were

also added to the mixture, to make sure the

complex is form without any problems. The

acetate buffer was added in order to maintain a

pH of 2 to 9. Anything higher and/or lower than

this range, results to the ferrous ions

precipitating. The addition of 0.0007 M

hydroxlamine hydrochloride was added to ensure

that the ferrous ions (Fe2+) dont oxidize into ferric

ions (Fe3+). If ferric ion is formed, it will generate

a different colored complex with phenanthroline.

Also, 508 nm wavelength was used because this

is the optimum level of wavelength the complex

absorbs.

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 2 4 6 8

Absorbance

y = 3564.3x - 0.0051

R = 0.9989

0

0.1

0.2

0.3

0.00E+00 2.00E-05 4.00E-05 6.00E-05 8.00E-05

Absorbance

[C12H8N2] (M)

Slope-ratio method (constant

Fe(II))

y = 10843x + 0.0045

R = 0.998

0

0.1

0.2

0.3

0.4

0.5

0 0.00001 0.00002 0.00003 0.00004

Absorbance

[Iron(II)] (M)

Slope Ratio (phen constant)


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Three different methods were used: (1)

continuous variation, (2) mole-ratio method, and

(3) slope ratio method.

A. Continuous Variation Method

In the continuous variation method, the total

number of moles of the solution were keptconstant, and only mole ratio of each flask varied.

Theoretically, the right metal to ligand ratio of the

complex has the maximum aborbance. It means

that the complex has the highest concentration at

that specific ratio, and no other components

contribute significantly to the solution.

Graph 2.1 Continuous Variationmole fraction (X-axis) vs

absorbance (Y-axis)

As seen in graph 2.1, when you

extrapolate the two sections of the graph

descending and ascending, you get the correct

combining ratio of the complex. The intersection

lies on top of around 0.25 mole fraction of Iron (II)

solution. Because iron (II) solution has 0.25 mole

fraction, then phenenthroline has 0.75, and

therefore 0.25/0.75 is equal to 1/3. This indicates

that the metal to ligand ratio is 1:3.

This method is best applied to ligands

with only one complex. If more than one complex

forms, the different peaks of the graphs would be

more than one, and it would be difficult todetermine the right stoichiometric ratio.

B. Mole-Ratio Method

The mole-ratio method determines the

correct stoichiometric ratio by keeping one

reactant, usually the metal, constant while adding

the other, usually the ligand, in excess. The mole-

ratio is then plotted against absorbance.

Theoretically, same as continuous variation

method, the right stoichiometric ratio of metal to

ligand has the maximum absorbance. So when

the reaction reaches the point where the metal toligand mole ratio is right, the graph usually

plateaus. This indicates that maximum

concentration of complex is achieved, and all

other components thereafter do not significantly

contribute absorbance to the solution.

Graph 2.2 Mole-Ratio - mole ratio (X-axis) vsabsorbance (Y-axis)

As seen in graph 2.2, the graph plateaus

(almost constant) after a certain point. To find the

plateau point, find the intersection of the

extrapolated line of the increasing part of thegraph, and the flat line of the plateau. The point

of intersection is near 3, therefore the metal to

ligand ratio is 1:3.

This particular method is effective for

large complexes where the ligand can

accodomate more metal, like for example 1:3 is

more favorable than 1:1. When this happens, the

graph would look more identical to the

extrapolated lines, suggesting a clearer point of

plateau. This in turn implies that a there is a large

formation constant and the complex is stable.

C. Slope-Ratio Method

Lastly, in this method the complex was forced

into completion by adding excess amounts of

either metal or ligand. When one reactant is in

excess, the concentration of the product is limited

by the other reactant (not excess). This method

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 0.2 0.4 0.6 0.8 1 1.2

Continuous Variation

0

0.1

0.2

0.3

0.4

0.5

0 5 10 15

Absorbance


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assumes the complex follows the Beer-Lambert

law, and the reaction is complete.

After getting the plot of the excess reactant

vs absorbance for both cases, the slope of each

is then computed through linear regression. And

combined with Beers law, the ratio of the slopeswould be equal to:

/

/=

=

1

2

Where m1 is the slope of constant metal, and m2is the slope of constant ligand. And in that case,

y would be the moles of metal and x would be the

moles of ligand. It would imply a stoichiometric

ratio of y:x.

The slopes of the constant metal vs constant

ligand in graphs 1.3 and 1.4, is 3564.3 and 10843

respectively. Therefore, 10843/3546.3 is around

1:3. Thus, the stoichiometric ratio of metal to

ligand is around 1:3.

VII. Conclusion and Recommendation

Spectrophotometry can be use to

determine the stoichiometric ratio of the metal-

ligand complex. The three methods used in this

experiment each have their own disadvantages

and advatanges, and knowing the best suited onewould achieve more accurate results, and more

efficient methods. Through these methods, the

metal-ligand ratio of Fe(C12H8N2)3 was found out

to be1:3, which is the actual theoretical ratio.

It is recommended that the proper

handling of the spectrophotometer and the

cuvettes must be practiced at all times, to achieve

proper and accurate results. It is also

recommended to make sure that the complex is

formed properly, and to also prevent

inconsistencies, that may affect the results, from

happening.

VIII. References

Harvey, D. (1999). Modern Analytical Chemistry.

USA: McGraw Hill Companies.

Skoog, D., West, D., Holler, F., & Crouch, S.(2004). Fundamentals of Analytical Chemistry.

Canada: Brooks/Cole-Thomson Learning.

Wear, J.O. (1968). Mathematics of the variation

and mole ratio methods of complex

determination.Retrieved from

http://libinfo.uark.edu/aas/issues/1968v22/v22a1

7.pdf on November 27 2014.


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spectrophotometric determination of the stoichiometry of a complex

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