Ultraviolet visible spectrophotometry. To measure different concentration (one or two substances) of solution, spectrophotometry can determinate an unknown solution.

Compare to other spectral analysis methods, the equipment and operation of UV - visible spectrophotometry are relatively simple, cost less, analysis faster and sensitivity. Such as direct detection of ascorbic acid in the ultraviolet region, its minimum detectable concentration can be achieved 10-6g/mL. By properly measurement conditions, spectrophotometry can determinate the compound of mixture unknown solution because of the high precision and accuracy. In some cases, the relative error can be reduced to 1% to 2%. In addition, spectrophotometry has a wide range of uses such as, the pharmaceutical, chemical, metallurgy, environmental protection, geology and many other fields. UV spectrophotometry can not only be used in quantitative analysis but also in qualitative analysis of the tested substances and structural analysis, the identification of functional groups, molecular weight determination, with material composition and stability constant determination, and so on.

Procedure

Part I

1. Prepare 0.0100, 0.0200, 0.0300 and 0.0400M Cr (III) solutions by dilution of a 0.0500M Cr(III) stock solution. Pipette 5.00, 10.00, 15.00 and 20.00ml volumetric flasks and dilute each flask to the mark with distilled water.
2. Retain the 0.0200M Cr (III) solution after completion of this part of the experiment.
3. Obtain the absorption spectrum for the 0.0100M Cr (III) solution in the wavelength range 350-750nm using the Cary 3 UV-VIS spectrophotometry.
4. Fill two Cuvettes with distilled water, dry outside with a tissue and put the sample to the light.
5. Scan the blank cuvette as a basic line.
6. Scan four remaining standard Cr (III) solution and the unknown Cr (III) solution.

Part II

1. Form the 0.1880M cobalt (II) stock solution, prepare 0.0376M, 0.0752 M and 0.1504 M Co solution in 25ml volumetric flasks.
2. Prepare 0.0500M chromium (III) stock solution and the 0.1880M cobalt solution; prepare 25ml solution that is 0.0200M in Cr and 0.752M in Co.
3. Scan 0.0376M, 0.0752M and 0.1504M solution.
4. Scan 0.0300M Cr(III), 0.0752M Co(II) and Mixture (Cr and Co) solution.
5. Scan the unknown mixture solution.
6. Print out the entire graph.

Results

Part 1

According to the experiment, the absorption and wavelength of Cr shows in table 1.

Table 1 absorption and wavelength of Cr (III) in 0.0100M, 0.0200M, 0.0300M, 0.0400M, 0.0500M and unknown solution

Table 2 absorption at λ=510nm of Cr (III) in 0.0100M, 0.0300M, 0.0500M and unknown solution

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Part 2

According to the experiment, the absorption and wavelength of Co shows in table 2.

Table 3 absorption and wavelength of Co (II) solution

Table 5 absorption and wavelength of Cr(III) and Co (II) solution

In λ=409nm andλ=510nm, the absorption of mixture solution as follow:

Aunknown,mixture, λ= 409nm =0.1784

Aunknown, mixture, λ= 510nm =0.3237

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Calculations

Part1

Because obey the Beer`s Law,

Using wavelength as abscissa and absorbance as the vertical coordinate mapping.

Graph 1. Concentration and absorption of Cr (III) in different standard solution

According to Graph 1, A=16.375CCr+0.0161

Absunknown, Cr=0.356

0.356=16.375 CCr+0.0161

So, CCr= (Absunknown, Cr+0.0161)/ 16.375=(0.356+0.0161)/ 16.375=0.0227M

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Graph 2. Concentration and absorption of Co (II) in different standard solution

A=4.8328CCo-0.0055

Both of Co and Cr graphs show the concentration changes as the absorption.

Part2

According to graph1 and graph2,

Aunknown,mixture, λ= 409nm =0.1784

Aunknown, mixture, λ= 510nm =0.3237

Because Beer`s Law,

Atotal1=ACr (III)λ1 + ACo (II)λ1

λ= 450nm, Atotal1=KCr (III)λ1CCr(III) + KCo (II)λ1CCo(II)

λ= 540nm, Atotal2=KCr (III)λ2CCr(III) + KCo (II)λ2CCo(II)

Solving linear equations with matrix

y=Ax

absorption of Cr and Co solution:

 A=,

concentration of Cr and Co solution:

X=],

absorption of unknown mixture solution:

y=[,

put the values ​​into the matrix:

A=, y=[],

X=y/A

According to the matrix equation:

X=A-1*y

A-1= 

]= []

CCr(III)unknown,mixture= 0.0261M

CCo(II)unknown, mixture=0.0308M

Discussion

In this experiment, Beer`s Law was well obeyed. Five different concentration Cr (III) solutions reflect well on the computer linear. The deviation come from the systematic error and instrument error, because strict adherence to Beer's law is observed only with truly monochromatic radiation. Monochromators are used to isolate portions of the output from continuum light sources; a truly monochromatic radiation never exists and can only be approximated. Otherwise, the fundamental requirement under which then Beer-Lambert Law is derived is that every photon of light striking the detector must have an equal chance of absorption. Thus, every photon must have the same absorption coefficient alpha, must pass through the same absorption path length, L, and must experience the same absorber concentration, c. anything that upsets these conditions will lead to an apparent deviation from the law. Another reason course the deviation is inhomogeneity of solution. Such as micro-organisms in distilled water, resonance, scattering and emission can absorb light absorption..

The absorption and experimental conditions are closely related. Measurement conditions (such as temperature, solvent polarity, pH, etc.), the shape of the absorption spectrum, the absorption peak position, and so on, these all may change the absorption. Otherwise, in order to get good absorption, the solvent should have no significant absorption in the absorption spectrum of the sample.

However, Cr and Co solution shows different color because different substances have different absorption of light. These two substance has not interact to each other. In addition, the danger of UV radiation is damage to the skin causing sunburn or even health issue. The instrument is designed to protect operator from UV.

According to Beer`s Law, because the path length of cuvette is 1cm, the molar absorptive for Cr (III) is rough about 13.67L/mol-1 cm-1 and for Co (II) is 4.8328 L/mol-1 cm-1. In this experiment, using glass cuvette, if using other smaller absorbance cuvette, we can get better results.

Conclusion

In conclusion, the concentration of unknown Cr (III) solution is 0.02081M. According to the experiment, the liner is A=4.8328CCo-0.0055 which is not exactly follow A=εbC because of the experiment deviation. In addition, according to the appendices, graph shows that the higher the concentration, the higher the absorption. This shows both in Cr graph and Co graph.

According to the appendices, the graph shows that absorbance of Cr and Co mixture solution is additive ( Atotal = A1 + A2….). Thus, we can calculate the unknown concentration by the graph. Therefore, in the unknown mixture solution, the concentration of Cr is 0.0261M and Co is 0.0308M.

All in all, in this experiment, all data basically followed Beer`s Law.

References

1. See D.A. Skoog, D.M. West, F.J. Holler and S.R. Crouch “Fundamentals of analytical Chemistry” 8th ed,. Thompson Brooks/Cole, Belmont CA, 2004
2. By: Dulski, Thomas R. “A Manual for the Chemical Analysis of Metals”, ASTM International, 1996
3. Rouessac, Francis& Rouessac, “Annick Chemical Analysis : Modern Instrumentation Methods and Techniques”，John Wiley & Sons, Ltd., 2007
4. Thomas J. Bruno and Paris D. N. Svoronos，“Handbook of Basic Tables for Chemical Analysis”, Second Edition， CRC Press，2004
5. Louis Meites ed., “Handbook of analytical chemistry”, New York, McGraw-Hill, 1963

Appendices