Absorbance of light by a transition metal complex investigation

Stephanie Chan 12HT

Chemistry HL – Mr. Fryer

Introduction

Commonly known as transition metals, d block elements have partially filled d sublevels in one or more of their oxidation states. It is in the first row of transition elements that the 3d sub-level is incomplete. These d block elements show certain characteristic properties such as multiple oxidation states, ability to form complex ions, coloured compounds and good catalytic properties. In terms of variable oxidation states, d block elements usually have a +2 oxidation number which corresponds to the loss of the two 4s electrons (as it is easier to lose the 4s electrons than the 3d electrons). Transition metals can have variable oxidation states because the ionization energies allow for up to two 3d electrons to be lost.

Because transition metals are relatively small in size, the transition metal ions attract species that are rich in electrons – ligands (neutral molecules or negative ions that contain non-bonding pair of electrons – which when covalently bonded with and form complex ions. Because the d orbitals usually split up into two groups (high and low) in transition metal complex ions, the energy required to promote a d electron into the higher split level corresponds with a particular wavelength in the visible region, which is absorbed when light passes through the complex ion. Transition metal usually then exhibits the remaining energy/light – the complementary colour.

In this investigation, the different absorbance of these coloured solutions will be investigated by varying the number of moles of the transition metal in the solution. According to the Beer-Lambert law, absorbance is directly proportional to the concentration and that there is a logarithmic dependence between the absorbance and the concentration of the substance, this relationship is as shown in figure 1 and 2.

In the graph representation of the Beer-Lambert law, the logarithmic relationship can evidently be seen – as the concentration of the solution increases, the calibration curve becomes less linear and more flat. This is probably due to the saturation of colour of the solution. In addition, the graph also indicates that the relationship starts at the origin and is generally linear at lower concentrations.

In this investigation, Nickel (II) Sulphate will be used as the transition metal and H2O will be used as the ligand. The complex ion formed will therefore be a hexaaquanickel(II) complex ion, Ni (H2O) 6 2+. It has a coordination number of 6 and is of an octahedral shape. (Microsoft Encarta, 2007)

Aim

To investigate how the concentration of hexaaquanickel(II) ions (Ni (H2O) 6 2+) in solution affects the absorbance of red light (660nm) by measuring it with a colorimeter.

Hypothesis

As the concentration of hexaaquanickel(II) ions increases, the absorbance of red light will also increase. This is so because as ...

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Aim

To investigate how the concentration of hexaaquanickel(II) ions (Ni (H2O) 6 2+) in solution affects the absorbance of red light (660nm) by measuring it with a colorimeter.

Hypothesis

As the concentration of hexaaquanickel(II) ions increases, the absorbance of red light will also increase. This is so because as stated in the Beer-Lambert law, the absorbance of light is directly proportional to the concentration. Furthermore, as the concentration increases, there are more molecules of the complex ions within the solution to interact with the light that is being transmitted – hence an increased absorbance at higher concentrations. In addition, despite the logarithmic relationship, I expect my data to show a linear relationship instead because the number of moles I am measuring red absorbance against is rather low (maximum 0.5 moles), so while it would be insufficient to see the clear logarithmic curve; the linear increase in the beginning would still be evident.

Variables

Independent – Concentration of hexaaquanickel(II) ions (0.0313mol, 0.0625mol, 0.125mol, 0.250mol, 0.500mol)

Dependent – Absorbency of red light (660nm)

Controlled – Volume of solution (25cm³ per different mol solution)

Equipment

Method

Measure 6.57g of nickel sulphate with an electronic balance and place in a 250cm³ beaker
Measure 50cm³ of deionised water with 50cm³ measuring cylinder and pour into the 250cm³ beaker with the nickel sulphate to create a 0.5mol nickel sulphate solution
Mix the solution thoroughly with a glass stirring rod, make sure the solution is transparent (not murky) and no remnants of the nickel sulphate should be present in the solution
Label the five 50cm³ volumetric flasks: 0.03125mol, 0.0625mol, 0.125mol, 0.25mol and 0.5mol
Pipette 25cm³ of the previously made nickel sulphate solution from the 250cm³ beaker and place into volumetric flask labeled “0.5mol”
Pipette another 25cm³ from the beaker and place into volumetric flask labeled “0.25mol”
Measure and pipette 25cm³ of deionised water and add into “0.25mol”
Mix thoroughly
Measure and pipette 25cm³ from “0.25mol” and add into “0.125mol”
Repeat steps 7 to 8 but add the water into “0.125mol”
Measure and pipette 25cm³ from “0.125mol” and add into “0.0625mol”
Repeat step 10 but add into the water “0.0625mol”
Measure and pipette 25cm³ from “0.0625mol” and add into “0.0313 mol”
Repeat step 10 but add into the water”0.0313mol”
Connect the PASPORT colorimeter to the computer
Select to measure red (660nm) absorbance
After all five solutions have been made, label five cuvettes the same labels as the volumetric flasks (place on lid, careful not to have any of the label on the cuvette itself)
Fill each labeled cuvette with its corresponding volumetric flask label with a dropper
Fill the remaining unlabeled cuvette with water
Place the cuvette with water into the colorimeter and press green button to calibrate, do not do anything until the green light switches off by itself
Place the cuvette labeled “0.03125mol” into the colorimeter – press start and stop after getting a constant reading
Record the data
Repeat steps 21-22 until all labeled cuvettes have been measured for red absorbance

Data Table

Uncertainties

Graphs

Discussion and Conclusion

It can be seen from the graph that there is a linear relationship between the amount of red light absorbed and the concentration of hexaaquanickel(II) ions. It can also be deduced that as the concentration increases, the red light absorption increases at twice the rate. However, it is interesting to note that the line of best fit does not start at the origin, but at (0, 0.0623) as the equation derived from the line of best fit states, suggesting that despite showing a clear linear trend, my data is precise but not accurate. This is possibly due to equipment imperfection, for example the cuvette, which will be discussed in the evaluation.

However, it is still evident that, as stated in my hypothesis, as the concentration increases, the chances of light interacting with the complex ion molecules also increase, hence yielding a higher light (red, in this case) absorption. While it is true that the Beer-Lambert law states the relationship between concentration of a substance and its absorbency has a logarithmic relationship, my data is linear because the concentrations of my tested solutions were rather low, so if I were to continue my experiment and create more concentrated nickel sulphate solutions, I would expect to see the curve become non-linear as concentration increases because the solution will eventually become saturated. Therefore, in conclusion, my hypothesis corresponds with the results: the relationship between red absorbance and concentration of hexaaquanickel(II) ions is quite clear – as the concentration increases, the red absorbance also increases.

Evaluation

One aspect I can improve my method is using the same cuvette and in the same direction each time for measuring all the different solutions, as it has been noted that the cuvettes we have been currently using are not perfectly constructed and may differ with the distance as light passes through. This will help improve the accuracy of the results and an important aspect to take into consideration, because also stated in the Beer-Lambert law, the length in which the light passes through also makes a difference in the absorption of light (the longer the container is, the more chances of light interacting with the molecules of the solution).

Another aspect was in the preparing the different solutions, because I had diluted each solution using the same solutions from before, so the uncertainty of each would naturally continuously build up (final uncertainty of 4.31%) – for example, if I had accidentally created a 0.052 mol nickel sulphate solution, then the next solution I diluted from that solution would not be 0.025 mol as intended. One way to see through this limitation is to perhaps prepare each solution separately to avoid a build up of uncertainties.

In addition, another way to make this investigation more conclusive and detailed could be increasing the different amounts of concentration of the nickel sulphate solution, as I only had 5 different concentrations.

Bibliography

Clark, J. (2007). The Beer-Lambert law. In Absorption spectra. Retrieved January 15, 2008, from

http://www.chemguide.co.uk/analysis/uvvisible/beerlambert.html

Microsoft® Encarta® Online Encyclopedia. (2007). Complex. Retrieved January 17, 2008, from

http://au.encarta.msn.com/encyclopedia_781538720/Complex.html

Neuss, G. (2007). Determining the concentration of an element. In Chemistry course companion (p.

276). Oxford University Press.

Because nickel sulphate solution is green in colour, red light will be used to measure the absorbency of the solution as it is the complementary colour.