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The determination of the stoichiometry of a metal complex by UV-visible spectrophotometry

| December 14, 2012

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AIM: to determine the stoichiometry of the complex formed between iron(III) abd 2-hydroxybenzoic acid(salicylic acid) in aqueous solution by visible using job’s method of continuous variation.


The principle involved in UV/Visible spectrophotometry is that when a molecule is bombarded with electromagnetic radiations the electrons in the last energy shells get excited by absorbing that radiation and there is a transition between different energy levels. As the amount of light absorbed by the solution is directly proportional to the amount of the solute present and the concentration of the solution can be determined.

UV/visible spectrophotometry depend on the ability of the substance to absorb light. In these spectrophotometry methods the solution of unknown concentration is compared with a set of solutions of known concentrations and this gives the concentration of the unknown solution.

This method usually involves the use of metal complexing agents such as di-sodium EDTA, iron (III) for various purposes (such as masking agents). 2-hydroxybenzoic acid combines with iron (III) forming a coloured complex. This complex is then using a UV/visible spectroscopy. The method employed in this experiment is the Job’s method where different proportions of metal and reagent are mixed with a fixed molarity. The absorbance readings are taken for the series of solutions prepared and a graph is plotted between the absorbance and the mole fractions of the metal and reagents. This UV/Visible method can only be used if the complex formed is coloured. In this experiment Fe(III) and salicylic acid combine to form a blue complex which can be detected spectroscopically at 525nm with distilled or de-ionised water as an indicator.


All the glassware was thoroughly cleaned using distilled water in order to avoid contamination. Avoid the contamination of salicylic acid solution by fe(III).


CHEMICALS: Iron nitrate,2-hydroxy benzoic acid(salicylic acid),Distilled water

APPARATUS: Two 5cm3 pipettes, ten 10cm3 volumetric flasks

INSTRUMENT: Dual-beam visible spectrometer 1-cm path length glass cells (cuvettes)


Prepare solutions of iron (III) ammoniun nitrate  and 2-hydroxy benzoic acid with distilled water and each having a concentration of 2×103 mol dm-3(slightly heat salicylic acid) using appropriate amount of each in 75cm3 of water in a beaker. Transfer each solution to a 100cm3 flask and dilute to the mark with distilled water.

Nine solutions of the iron (III) complex of 2-hydroxy benzoic acid were prepared by mixing the acid solution and iron (III) complex in 10cm3 volumetric flask using different burettes as per the following scheme.

Volume of acid/cm3 Colume of fe(iii) cm3 Mole fraction of acid,x
1 9 0.1
2 8 0.2
3 7 0.3
4 6 0.4
5 5 0.5
6 4 0.6
7 3 0.7
8 2 0.8
9 1 0.9


Absorbance of each of the above solutions and the original Fe(III) solution was measured at 525nm.


  1. Calculation of  the amount of Iron (III) nitrate required:

Concentration of solution prepared = 2×103

Molecular weight of Iron (III) nitrate  = 404 gm/ mol

Amount of Iron (III) nitrate required for 100 ml = No. of moles × Mol.wt × 100


= 2 × 10-3 × 404 × 100


= 0.0808 gm


  1. To calculate the amount of 2-Hydroxy benzoic acid (Salicylic acid ) required to prepare s soluton of conc of 2×103:


Molecular weight of Salicylic acid = 138.12 gm /mol


Amount of Salicylic acid required for 100ml = No. of moles × Mol.wt × 100


= 2 × 10-3 × 138.12 × 100


=0.02762 gm

  1. Absorbances of prepared solutions when measured at 525nm under a UV spectrophotometer:


Volume of acid /cm3

Volume of Fe (III) / cm-3

Mole Fraction of acid , X

Absorbance at

525 nm

Corrected absorbance Ac

1 9


0.3441 0.304
2 8


0.5726 0.5325
3 7


0.9220 0.8819
4 6


1.1955 1.1554
5 5


1.4464 1.4063
6 4


1.3720 1.3319
7 3


1.0028 0.9627
8 2


0.7129 0.6728
9 2


0.3861 0.346
Original Fe(III) 0.0401



Calculation of molar absorbtivity of Fe(III) using beer-lambert’s law:





A = absorbance,

Fe. = molar absorbitivity of Fe(III)

b = path length,

C = concentration of the solution.

è  €Fe = 0.0401/1×0.002

=   20.05


We have,

Ac = A – [Є Fe (1-x).b.c]

Where, A is uncorrected absorbance for each of the solutions.

Є Fe = 20.05

X= Mole fraction

b = Path length (1 cm)

c = Molar concentration (2 × 10-3 mol dm-3)



From the above graph the value of x from the intersected portion of the graph was found to be

Stoichiometry of the Fe(III) / 2-hydroxy benzoic acid complex, Fe(HB)n by evaluation of n by following expression:

X= n/1+n

ð     0.5 = n/(1+n)

= 0.5(1+n) = n,          è  0.5= 0.5*n    è n=1


  1. Structural formula of Fe(III)/ 2-hydroxybenzoic acid.



In the above structure the Fe+3 molecule reacts with the lone pair of electrons present on the oxygen molecule.


  1. It is necessary to correct the absorbance readings as the absorbance’s obtained also include the radiation absorbed by the Fe+3 molecule and the concentration obtained from uncorrected absorbance would be inaccurate.

Also these corrections bring about significant difference in the final value as we can see the difference between both the values is 0.05(approx).


  1.  The point of intersection gives us the volume of reactant mixture required to obtain the maximum absorbance. While the graph plotted against mole fraction and absorbance initially gives a straight line it reaches it shows a curvature at the end as it reaches saturation point and after this point there is a decline in the absorbance by the solution. Hence the graph of mole fraction against absorbance gives a curvature between linear portions.
  2. Alternative procedures for establishing the stoichiometry of a metal complex are as follows:

Fluorescence spectroscopy

Gas-Liquid chromatography

Slope ratio method.


  1. -. (2010). Manufacturer Opens up about Japanese Honeysuckle Extract . Available: Last accessed 25/03/2011.
  2. Bauer Christian O’Reilly (2008). instumental analysis. -: Allyn and Bacon,Inc.. 154.
  3. skoog west holler (1996). analytical chemistry. 7th ed. kentucky,USA: Saunders College Publishing. 557.

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