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In this experiment, we are required to determine the composition of complex ion by continuous variation method. Complex ions refer to a metal cation bonded to either one or more than one molecules. It is a stepwise process to produce the complex ion because every process has its own equilibrium constant (Gretchen E. Potts,2001). However, continuous variation method involves the continue changing of the different concentration of the solution.
Besides that, spectrophotometric method can be used to quantitatively identify the unknown composition or concentration of a sample solution with compare to a sample with known concentration. Then, spectrophotometer refers to an instrument which we can use it to measure the light intensity. As we know, most of the UV-Vis spectrophotometers have a range of the wavelength between 190nm-1100nm. This range lies between UV and Visible light region. (Frans F,2004)
Figure 1 : UV-visible spectrophotometer working mechanisms
From : bouman.chem.georgetown.edu
In this instrument, it has 2 light sources. D2 and W lamp are used for UV and Visible light respectively. It will ensure that only monochromatic light will flow through the slit. Reference cuvette is used as a blank. At the end we can get the reading from the detector.
Furthermore, Beer - Lambert law states that when a monochromatic radiation passes through a transparent solution, the intensity of the absorbed radiation is directly proportional to the concentration (c) and the optical pathlength, (l) of the solution. So, Absorbance, A = ε x l x c , where ε is the molar absorptivity.
The materials we used during the experiment are Ammonium iron (III) sulphate and Salicylic acid. Therefore, we used the following method to determine the concentration of the iron (III) - salicylic acid complexes.
Materials and Methods :
500 cm3 of Ammonium iron 500 cm3 of salicylic acid solution
(III) sulphate solution was prepared. was prepared.
Transferred into burette
A series of mixture was prepared in each test tube and the volume of the solution added was in the table below.
Fe3+ solution (ml)
Salicylic acid (ml)
Table 1 The Mole fraction of each component
Optical density of the other samples were determined using the same instrument and wavelength.
The most suitable wavelength for measuring was determined.
These mixture was then measured using spectrometer over the range of 350-700nm.
From the spectrum, the maximum peak has a wavelength of 531.5 nm.
With the same wavelength, we obtained the following Absorbance for different samples and the data was recorded in the following table.
Salicylic acid (cm3)
A graph of Absorbance against mole fraction was plotted.
From the graph, X = 0.495
To get y ,
Y = ax / a(1-x)
Y = 0.495 / ( 1- 0.495)
=0.98 ≡ 1
So the molecular ratio of the Fe3+- salicylic acid complex is 1 : 1
A graph of Abs against mole fraction of L was drawn.
From the results above, the molecular ratio of the Fe3+- salicylic acid complex by using Job's method was 1:1. The complex is formed by which the salicylic acid donated its lone pair electrons from hydroxyl oxygen atom to iron metal. This is because, salicylic acid is a bidentate to the metal ion. Therefore, it needed three salicylic acid to donate its lone pair electrons to the iron(III) ion. So, the complex formed is trisalicylateferrate(III). The structure will be drawn as follow :
Figure 1- trisalicylateferrate(III) complex
<adapted from http://www2.volstate.edu/chem/1110/Purity_of_Aspirin.htm>
In this experiment, we measured the absorbance of our samples from UV-Vis spectrometer. The sample was poured into a cuvette. The sample was not destroy which means that with the using of this spectrometer, the sample was non- destructive. The UV- Vis light pass through the cuvette and the light was absorbed by the sample. Therefore, the sample must be in liquid form.
Besides that, before mixing of Fe3+ and salicylic acid, the two solutions were colourless. However, after mixing the two solutions, the sample had a violet in colour. So, they can absorb visible light and because of the electrons from d orbital was excited from one to another electronic state. The complex gave violet in colour is because it absorbed green light and let the other light passed through. Green light is a complementary colour to the violet light, so the complex gives the maximum absorbance at wavelength 531.5 nm.
In addition, we normally will select the maximum absorbance is because its sensitivity to the concentration of the solution is the greatest at the maximum wavelength. If the λmax is not the analytical wavelength, strong radiation may be strongly absorbed and caused deviation from the Beer Lambert Law. However, in this experiment, if let say we did not use theλmax, the result we obtained from graph will not be affected much. We will also need to round off it to integer so that we can get the ratio.
Furthermore, stoichiometry formula of a complex ion can be obtained through continuous variation method. Besides that, this method is actually help in the maintaining the number of moles of the sample throughout a series of different mixture in the sample. However, only different is the mole fraction of the different samples.
The maximum peak for iron(III)-salicylic acid complex was 531.5 nm.
The molecular ratio of the Fe3+- salicylic acid complex is 1 : 1