Jobs Method is also known as the Method of Continuous Variation. It has been used for the study of complexes in solution. The purpose of this method is to determine the chemical reaction stoichiometry. Besides, it is also used to determine the chemical formula of a precipitate and the oxidation state of an ion in the solution. For example, the amount of metal ion and ligand used can be determined colorimetrically using Method of Continuous Variation.
In order to determine the ratio of metal complexes, a series of solutions is prepared and each of them containing the same number of moles of metal ion and ligand but in different ratio in which the concentration of metal ion and the ligand used are kept constant. The absorbance of the metal complex is measured and the maximum wavelength which is the λmax is determined. The measurement at the maximum optical density indicates that the ratio of ligand to metal is at the same proportions.
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Maximum change occurs at the point at which both reactants which is the metal ion and the ligand are limiting reactants. As a result, when a graph of absorbance against the mole fraction of ligand is plotted, there will be a region where the amount of metal ion is in excess and increases slowly (positive slope) until the maximum point and then causes the amount of ligand decreases with a negative slope. The maximum point is the point at which the ratio of ligand to metal is equal to the complex.
Therefore, the metal ion to ligand ratio can be obtained by calculating the x value and substitute into the equation of
The success of a Job's method experiment depends upon the extent to which Beer's law is followed. Beer's law is one of the most widely-applied relationships in chemistry, and is usually cast in terms of the absorbance A of a solution, defined by
A = log (1) 10 Io/I
in which Io is the intensity of light incident upon a sample and I the intensity of the transmitted light. The absorbance is related to concentration of the solution, c, through Beer's law, which is
A = ε c l
In this equation ε is the molar extinction coefficient for a species and l is the optical path length.
A series of mixture of 2x10-3M of ammonium iron(III) sulphate solution and 2x10-3M of salicylic acid solution (each in M/500 hydrochloric acid) were prepared differing by an increments of 0.1 in the mole fraction of each component, as indicated below.
Ammonium iron(III) sulphate solution (cm3)
Salicylic acid solution (cm3)
1 2 3 4 5 6 7 8 9
Salicylic acid solution
Ammonium iron(III) sulphate solution
A spectrophotometer was used to measure the spectrum of the solution with the darkest colour over the range of 350-700nm and hence the maximum wavelength for measuring the optical density was chosen.
The optical density of each sample tube was measured at the selected λmax. A graph of optical density (D) against mole fraction of L (x) was plotted and the approximate value of x at the maximum D was obtained.
Mole fraction of L, x
Absorbance at λmax= 528.5nm
Average absorbance at λmax= 528.5nm
From the graph,
Y1 = 0.4027x
Y2 = -0.3648x + 0.365
When two graph intersect, Y1 = Y2,
M : L
From the equation,
Mn+ + yL [MLy]n+ where M is Fe3+ and L is salicylic acid
The initial colour of ammonium iron(III) sulphate and salicylic acid are colourless. However, the when these two solution are mixed together, a purplish colour solution is formed. The colour change indicates the formation of complexes.
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In this experiment, Job's method is used to find the formula of the complexes formed between ammonium iron(III) sulphate and salicylic acid. A series of solution with different concentration were prepared and the maximum wavelength was determined to find out the absorbance of each solution. The maximum wavelength is the wavelength at which the complex absorbs most strongly. In this experiment,
[ salicylic acid ] + [ Fe3+ ] = constant
When the concentration of Fe3+ increases from zero, the amount of complex also increases and this causes an increase in the absorbance. Therefore, we can conclude that the higher the concentration of Fe3+, the higher the absorbance. The absorbance increases linearly and follows the Beer Lambert Law until it reaches a maximum absorbance. The maximum absorbance is a point in which the metal ion and ligand are in the same proportions as in the complex. This solution is therefore containing the highest concentration of the complex. Further additions of metal ion cause the solutions that contain insufficient salicylic acid to complex with all the metal, so absorbance due to the complex falls.
As the Beer Lambert Law is obeyed, a graph of absorbance against mole fraction of L will give rise to two straight lines. The intersection between the two lines gives the formula of the complex directly, since, where the two lines cross, ligand and metal are in the correct proportion to give maximum complex formation.
From the calculation, the salicylic acid and Fe3+ are at 1:1 ratio (ML) in which the value of y = 1. Salicylic acid is a bidendate ligand thus, forming two bonds with Fe3+.
Ultraviolet and Visible Absorption Spectroscopy (UV-Vis)
UV-Vis spectroscopy is used to measure the wavelength and the intensity of adsorption by a sample. Ultraviolet and visible light have the right energy to cause an electronic transition in which the outer electrons will be promoted to higher energy levels. If a molecule absorbs ultraviolet light, a UV spectrum id obtained, if it absorbs lower-energy visible light, a visible spectrum will be obtained.
The light source is usually a hydrogen or deuterium lamp for UV measurements whereas tungsten lamp is responsible for the visible measurement. A wavelength separator is used to selects the wavelength of the continuous light source. The example of wavelength separator includes a prism or the grating monochromator. Spectra can be obtained by scanning the wavelength separator. The quantitative measurements can be made from a spectrum or at a single wavelength.
Schematic diagram for single beam UV-Vis spectrophotometer
The complex formed from the reaction between salicylic acid and Fe3+ is ML