Exp6 Spectroscopy

Experiment # 6 Spectroscopy dela Cruz, Marie Giecel and Pabilane, Alma Group 6, Chem 27.1, SEJ1, Ms. Noime Walican 05 March 2011

I. Abstract Spectroscopy is the branch of science that deals with the interaction of radiation with matter. It measures the amount of incident radiation that “bounces” off the analyte during readings. Using spectroscopy, the molecular composition and/or structure of an unknown sample can be obtained. In this experiment, the unknown sample was measured at two wavelengths, 545 nm and 440 nm. The absorbance readings of the sample at the two wavelengths were compared to the absorbance readings of KMnO4 and K2Cr2O7 at the two wavelengths. It was observed that as the concentration of the sample increases, the absorbance also increases while the transmittance decreases. The experimental concentrations of Mn and Cr of the unknown were 9.471 and 35.82 ppm, respectively. II. Keywords: Spectroscopy, spectrophotometer, absorbance, transmittance, beer-lambert’s law, absorptivity = molar absorptivity III. Objectives b = length of absorbing medium The objectives of this experiment are: c = analytical concentration of absorbing (1) To prepare the absorbance spectrum of species colored species; A = absorption (2) To determine the concentration range for T = transmittance maximum precision; and, (3) To determine by spectroscopy the For mixtures of compounds, Beer’s Law also concentrations of colored species in a apply as follows: mixture. A = 1bc1 + 2bc2 + 3bc3 + … + nbcn IV. Principles Spectroscopy is the study of interactions of In the analysis of the unknown samples, a electromagnetic radiation and matter. It measures 2:8 dilution will be used because the Beer-Lambert’s the amount of radiation produced or absorbed and is law is limited to use in dilute solutions and classified into many types, depending on the part of monochromatic radiation. the spectrum involved. Spectroscopy can provide information such as molecular structure of an V. Experimental unknown compound, and it can also be used in One hundred milliliters each of 0.001 M quantitative and qualitative analyses. KMnO4 and 0.001 M K2Cr2O7 were prepared. Ten The basic premise of spectroscopy is that milliliter solutions were prepared from the 0.001 M the amount of electromagnetic radiation emitted KMnO4 using the following dilution ratio of comes from the amount of energy released during H2O:KMnO4: transition of an analyte from the ground state to the Test tube H2O:KMnO4 volume ratio excited state or vice-versa. (mL) In this experiment, absorption spectroscopy A 10:0 is demonstrated. In absorption spectroscopy, the B 9:1 amount of light absorbed is a function of the C 8:2 wavelength. It means that as the wavelength D 7:3 changes, so does the amount of light absorbed (or E 6:4 the absorbance). The concentration of the F 5:5 substance can be computed from the BeerG 4:6 Lambert’s Law (or Beer’s Law), which states that H 3:7 absorption, A, and the length of path traveled by I 2:8 light determines the amount of decrease in energy J 1:9 per unit area (attenuation) of a beam of radiation. This relationship can be described mathematically as, ,

Chemistry 27.1, Spectroscopy

Table 1. Summary of H2O:KMnO4 volume ratio to be prepared

Prior to usage, the spectrophotometer was warmed for 20 minutes and then, the wavelength was adjusted to 545 nanometers. The cuvette Page 1 of 4

VI. Results A. Determination of Concentration Range for Maximum Precision Dilution

MnO4 ppm Mn

Absorption

2 1.5 1 0.5 0 0

20

40

60

Concentration Mn (ppm) Figure 1. Plot of concentration of Mn (in ppm) versus the absorbance

Concentration vs Transmittance 120 100 80 60 40

T

%T

0.208

0.6194

61.94

-4

10.988 0.489

0.3243

32.43

-4

16.402 0.749

0.1782

17.82

-4

21.976 1.019

0.09572

9.572

-4

27.470 1.285

0.05188

5.188

-4

32.968 1.541

0.02877

2.877

-4

38.458 1.772

0.01690

1.690

-4

43.952 2.050

0.08913

0.8913

Sample Computations: A = εbc where slope=εb Based from the linear regression of the data, εb = 0.0463

-4

49.446 2.229

0.05902

0.5902

Concentration of the unknown:

-4

1 x 10

2:8

2 x 10

3:7

3 x 10 4 x 10

5:5

5 x 10

6:4

6 x 10

7:3

7 x 10

8:2

8 x 10

9:1

2.5

A

1:9

4:6

Concentration vs Absorption

Transmittance (%)

containing the reference liquid (blank liquid) was inserted and the absorbance was zeroed and the transmittance was set to 100%. The absorbance of the succeeding cuvettes containing the different H2O:KMnO4 dilution ratio solutions were recorded. The concentration in molarity and in ppm were computed for all the readings. Afterwards, the results of the concentration were plotted against absorbance (Figure 1) and transmittance (Figure 2). The concentration range for the analysis was determined and then, the absorbance of the unknown at 545 nm was also measured. A 2:8 dilution of K2Cr2O7:H2O was prepared. The absorbance of this dilution was measured at 440 nm and at 545 nm. The absorption of the unknown and the 2:8 KMnO4:H2O dilution at 440 nm were also measured. The concentrations of Mn and Cr in the unknown were computed from the readings.

9 x 10

5.494

Concentration Range for Maximum Precision 10 – 22 (ppm) ppm 2 Correlation Coefficient, r 0.9989 Absorbance of Unknown

0.449

Concentration of Unknown (ppm Mn)

9.698

Table 2. Determination of Concentration Range for Maximum Precision

20 0 0

20

40

60

Concentration Mn (ppm) Figure 2. Plot of the concentration of Mn (in ppm) versus the transmittance (in %)

B. Spectrophotometric Determination of Mn and Cr in a Mixture Known Unknown Mn Cr Mn + Cr A at 545 0.489 0.016 0.449 nm A at 440 0.031 0.089 0.180 nm Concentration of Mn (ppm) Concentration of Cr (ppm)

9.471 35.82

Table 3. Spectrophotometric Determination of Mn and Cr in a Mixture

Chemistry 27.1, Spectroscopy

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Sample Computations: A440 = A440,Mn + A440, Cr A440 = εbcCr + εbcMn A540 = A540,Mn + A540,Cr A540 = εbcCr + εbcMn

Use system of equations to get CMn and CCr: CMn = 9.471 ppm CCr = 35.82 ppm VI. Discussion Spectroscopy was used in this experiment to determine the concentrations of Mn and Cr in the unknown sample. First, the precision of the spectrophotometer was checked by measuring a standard solution of KMnO4 and plotting the concentration against the absorbance values obtained from the readings. From this step, the slope was obtained, which was equal to b, or the molar absorptivity multiplied by the length of the path traveled by the light. Since b = 1, the slope obtained is equal to . For this run, = 0.0463/cm∙ppm. The concentration range for maximum precision was also obtained by estimating the values of the concentration where the graph of the absorption versus concentration is most linear. It should be noted that absorptivity is dependent on factors such as wavelength used and concentration of the solution, therefore, it is necessary that the standardization of the spectrophotometer be carried out to avoid variations in the absorptivity for the same wavelength used. From the Beer-Lambert law, it can be deduced that as absorbance increases, the concentration of the substance should also increase. The graph in Figure 1, obtained from the results of the calibration, illustrates this linear relationship of A and c described by the Beer-Lambert’s Law. Also, in accordance to the Beer-Lambert’s Law, the absorption attenuates, depending on the concentration and the length of path traveled by the light. The transmittance of a solution is the fraction of incident radiation transmitted after some of the light was absorbed by the analyte. Inside the cuvette, where the absorption takes place, the light traveling from the source passes and “hits” the Chemistry 27.1, Spectroscopy

absorbing analyte. This registers as the absorbance reading, and causes a decrease in the intensity of the light, effectively reducing the transmittance. This relationship is described, also by the Beer-Lambert’s Law, as A = -log T, where T is the transmittance. So, if the concentration of the substance is increased, the transmittance should decrease logarithmically, as seen in Figure 2. Again, it should be noted that absorbance could never have a negative value because transmittance is the ratio of the final power (P) of the light after traversing the solution and the initial power (Po) of the light. Since Po > P, the ratio should not have a negative value. In the experiment, the absorbance of the unknown sample was read at  = 545 nm. Using the standardized curve, the concentration of the unknown was computed by dividing the obtained absorbance with the obtained value. For the 0.449 absorbance, the concentration of the unknown obtained experimentally was 9.698 ppm Mn. In the second part of the experiment, an unknown sample containing a mixture of Mn and Cr was read at  = 545 nm and  = 440 nm. This was compared against a standard of 0.001 M KMnO4 and 0.001 M K2Cr2O7. The values were obtained for all readings and the concentrations of the unknown sample were found to be 9.471 ppm Mn and 35.82 ppm Cr. The dilution 2:8 was used because of the limitation of the Beer-Lambert’s Law. The violet color of the Mn is observed because it absorbs the color green ( = 500-560 nm). This would explain why the Mn have higher absorbance at  = 545 nm. Chromium absorbs the color blue (accounting for its yellow color) which occurs at  = 435-480 nm, therefore, Cr should register a higher absorbance reading at  = 440 nm than at  = 545 nm. VII. Answers to Questions 1. How does absorbance vary with concentration? As concentration increases, absorbance increases linearly. 2. If plot A against C is extrapolated, will it intersect the point of origin? Yes because the absorbance was zeroed at the initial concentration (blank). 3. How does % transmittance vary with concentration? As concentration increases, % transmittance decreases, logarithmically. 4. How does curve A vs C compare with that obtained by plotting %T vs C? Explain the difference.

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In figure 1, it was established that as concentration increases, absorbance also increases linearly. From the equation given at the discussion of the principle involved in this experiment, it can be seen that absorbance, A, is equal to the negative logarithm of transmittance, T. This would explain the logarithmic decrease of the transmittance as the concentration of the substance is increased. 5. A 20.0ppm solution of a colored compound gives a 70.0% transmittance in a 1.00cm cell. Find the absorptivity of the solution. c = 20.0 ppm T = 0.700 b = 1.00 cm (

IX. References Bhondwe, A. "Molar Absorptivity." Buzzle Web Portal: Intelligent Life on the Web. 3 Jan. 2011. Web. 03 Mar. 2011. . Skoog, D. et al. “Introduction to Spectrochemical Methods”. Fundamentals of Analytical th Chemistry, 8 ed. 2004. Brooks/Cole – Thomson Learning: California. pp 710, 715-716, 718, 720, 730.

I hereby certify that I have given substantial contribution to this report,

DELA CRUZ, MARIE GIECEL

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) PABILANE, ALMA

6. A solution of a colored species gives 25% transmittance. What will be the % transmittance of the solution whose concentration is 3 times that of the given? Assume linearity of response over the given concentration range. (

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VIII. Conclusion and Recommendations From this experiment, it can be concluded that spectroscopy is a highly accurate and precise way of determining the components of a mixture of compounds. Using only the Beer-Lambert’s Law, the concentration of species in a solution can be obtained by dividing the absorbance with the product of the molar absorptivity and the length of path traveled by the light. It can also be concluded that absorptivity is highly dependent on the concentration of the substances. If the concentration is increased, absorptivity also increases. It can also be concluded that absorptivity is a function of wavelength. It remains constant for a particular wavelength but changes as the value of the wavelength is changed. Possible sources of error in this experiment are the decomposition of KMnO4 during preparation because of exposure to light and inaccurate measurements of weight and volume during preparation of solutions. Chemistry 27.1, Spectroscopy

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