Experiment No. 6: Spectroscopy Arianne May Balaoing Jana Beatriz Erlano Raissa Athena Ortiz Group#1 Chem 27.1 WEJ3 Ma’am Fatima Joy Cruz Date Submitted: Saptember 28, 2011 I. Abstract Spectroscopy is the study of the interaction interaction of matter and radiation. There are many types of spectrochemical methods, but the one that is applied in the experiment is absorption spectroscopy, because it measures light as a function of wavelength. A spectrophotometer was used to determine the absorbency at certain wavelength of the potassium permanganate and potassium dichromate solutions, which in this experiment, are 545 and 440 nm. Using absorption spectroscopy, the concentrations of Manganate and Chromium ions in the mixture were known and they were founded to be 13.63 ppm with 3.34% error, and 34.44ppm with 10.26% error, respectively. II. Keywords: Keywords: Spectroscopy, Beer’s Law, Absorbance, Wavelength, Spectrophotometer, Molar Absorptivity III. Introduction permanganate ions at these wavelengths would be the sum of the absorbances of the two species Spectroscopy pertains to the interactions of provided there is no rection between the solutes. radiation and matter. It basically aims to measure Hence: the amount of radiation emitted or absorbed by a (2) molecular or anatomic species. It is particularly [ ] [ ] (3) useful in the determination of concentration of [ ] [ ] (4) elements in alloy steel so as to predict its mechanical properties and resistance to corrosion. It is desirable that the wavelengths are such that This experiment used simultaneous the ratio of molar absorptivities is largest. The spectrophotometric determination of two solutes in concentrations follow: a solution, chromium and manganese, in accordance to Beer- Lambert’s Law [ ] (1) (6) [ ] where , absorption measured at wavelength ; , molar abrsoptivities at wavelength ; b, amount [ ] of em radiation impinging on surface (rad/area/time) or cell path length (cm); and C, (7) [ ] concentration. Upon dissolution of steel containing chromium and manganese, we get these elements as their ions, 3+ 2+ Cr and Mn ions, respectively. Hence in laboratory determinations, these are oxidised respectively to such ions. The orange red coloured dichromate shows maximum absorption ( ) at 440 nm while for the pink coloured permanganate is at 545 nm. However, permanganate also absorbs at 440 nm to a smaller extent similarly as dichromate ions also absorbs at 545 nm. Direct determination of both species in the presence of the other is made difficult since their spectra overlap to some extent and there are no wavelength at which either of the ions do not absorb significantly. Thus, the absorption of the solution containing a mixture of dichromate and Chem27.1 Spectroscopy
OR [ ]
[ ]
(8)
Since is constant, Beer- Lambert’s is rewritten as . Beer-Lambert’s plot of Concentration (C) vs. Absorbance ( evaluates the values of molar absorption coefficients ε1 and ε2 as a co Absorbances derived from Beer’s Law or simply measured by spectrometers are also useful in deriving the transmittance (T) or the fraction of radiant energy transmitted when light has been made to pass through the sample at certain wavelength: (9)
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(10)
V. Results and Sample Computations
(11) (12)
wherein I is emergent or transmitted radiation and Io is the incident radiation assumed to be 100%. Spectroscopy is a complex art - but it can be very useful in helping scientists understand how an object like a black hole, neutron star, or active galaxy is producing light, how fast it is moving, and even what elements it is made of. IV. Experimental
Table 1 KCr2O7 at 440nm
A. Determination of Concentration Range for Analysis of Potassium Permanganate in Solution One-tenth of a liter each of 0.001M KMnO 4 and 0.001M K2Cr2O7 were prepared for the experiment. This was done by weighing 0.01580g of KMnO4 and 0.02942g of K 2Cr2O7 and diluting them to 100ml. Ten solutions of each were prepared with varying dilutions with water: 0:10, 1:9, 2:8, 3:7, 4:6, 5:5, 6:4, 7:3, 8:2 and 9:1 proportions.
Figure 1 1 ppm Cr vs A at 440nm
r= 0.9980
m= 0.004370
y-int= 0.01498
The spectrophotometer was warmed up for 20 minutes. A reference blank with distilled water was prepared and placed inside the instrument and the absorbance was set to 0 and the transmission to 100%. Permanganate and dichromate absorbance were measured under 545nm and 440nm. The cuvette containing the sample was placed inside the spectrophotometer and the absorbance of each sample was measured. B. Spectophotometric Determination of Mn and Cr in a Mixture
Figure 1 2 ppm Cr vs T at 440nm
The absorbance of a 2:8 dilution of KMnO 4 was measured at 440nm and the 2:8 dilution of K2Cr2O7 at 545 nm. The absorbance of an unknown solution was measured at both 545 and 440 nm wavelengths.
Chem27.1 Spectroscopy
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Table 2 KCr2O7 at 545nm
Table 3 KMnO4 at 440nm
Figure 2 1 ppm Cr vs A at 545nm
Figure 3 1 ppm Mn vs A at 440nm
r= 0.6324
m= 0.0001521
Figure 2 2 ppm Cr vs T at 545nm
y-int=0.008182
r= 0.9445
m= 0.002249
y-int= 0.01149
Figure 3 2 ppm Mn vs T at 440nm
Table 4 MnO4 at 545nm
Chem27.1 Spectroscopy
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ppm Cr:
Figure 4 1 ppm Mn vs A at 545nm
r= 0.9991
m=0.04033
y-int=-0.02946
% T of Cr at 545: % T of Cr at 440: %T of Mn at 545: %T of Mn at 440:
VI. Discussion A. Determination of Concentration Range for Analysis of Potassium Permanganate (and Potassium Dichromate) in Solution Figure 4 2 ppm Mn vs T at 545nm
Weight KMnO4: =
Weight K2Cr2O7:
2:8 Dilution of KMnO 4:
ppm Mn:
3:7 Dilution of K 2Cr2O7:
Chem27.1 Spectroscopy
From the Beer- Lambert’s plot of Absorbance of Mn at 545nm vs. Concentration (ppm) the slope is 0.040324982 and the y-intercept is -0.029454545 translating to: (-0.029454545)
For the determination of Concentration Range for maximum, the absorbances at 545nm and 440nm of different dilutions of pure solutions of Mn and Cr were measured. Yet, only the absorbance of potassium permangate at 545 and potassium dichromate at 440 was much noted since greatest absorbance of Mn and Cr is at 545nm and 440nm, respectively.
Solving for the concentration x, Absorbance of Mn at 545 therefore From the Beer- Lambert’s plot of Absorbance of Cr at 440nm vs. Concentration (ppm), the slope is 0.004370382 and the y-intercept is 0.014981818 translating to: 0.014981818 Page 4 of 6
Solving for the concentration x, B. Spectophotometric Determination of Mn and Cr in a Mixture Beer’s Law requires the use of monochromatic radiation and it is under the restraint that the linear dependence of absorption and concentration occurs. If 2 or more species in a sample absorb at a specific wavelength, the instrument cannot distinguish between the individual species; it can only determine the total absorbance of the sample as shown by equations 2, 3 and 4. In a spectrophotometric analysis, these equations can be developed from the equation 1.
The main concern of this experiment is the determination of concentrations of manganese and chromate in a sample. By measuring the absorbance at two different wavelengths of a solution containing both ions, it is possible to simultaneously determine the concentration of each ion in the solution. In simultaneous determination of 2 species it is necessary to generate 2 equations in order to determine the 2 unknown concentrations. The 2 equations derived from the Beer-Lambert Law which will allow the simultaneous determination of potassium dichromate and potassium permanganate are equations 3 and 4: [ (3) ] [ ] [ ] [ ] (4) These equations require that the 4 molar absorptivities, the length of pathway, and the total absorbance be known to calculate the concentrations. Since has been known to be known to be constant, they can be treated as a single entity, k, which can be derived from four Beer-Lambert plot of Absorbance versus Concentration, ppm. These plots evaluate єKMnO4b, and єK2Cr2O7b for wavelength 1, and єKMnO4b, and єK2Cr2O7b for wavelength 2 as follows: at 440: at 545: Given the measured absorbance of the mixture at different wavelengths and the evaluated molar Chem27.1 Spectroscopy
absorptivities, the concentration of both Cr and Mn in mixture may be determined based from equations 6 and 7: [ ]
[
]
(6)
[ ]
[ ] [ ]
[
]
(7)
[ ]
The concentrations therefore in the mixture of Mn is 13.19 ppm with 3.34% error and Cr is 31.2 ppm with 10.26 % error.
VII. Conclusion and Recommendation Spectroscopy is an easy method to use in determining the concentration of an unknown species since it only needs the molar absorptivity of the sample, the absorbance, and the cell path length to calculate the concentration of the analyte. A n absorption law called the Beer’s Law governs the overall process of spectroscopy since it gives the relationship of the quantities obtained from spectophotometric data in order to know the quantity of unknown in the sample. The absorptivites directly measured form the spectrometer or derived from the Beer’s Law can be used to derive the transmittance of the substance at certain wavelengths. The BeerLambert’s plot of Absorbance vs. Concentration (ppm) shows positive trend of the graph. The positive trend of the graph is obvious in the positive value of the slope which is actually k which is in turn equal to . On the other hand, the plot of Absorbance vs. Transmittance shows the inverse relationship of the two quantities. This is obvious in the equation which evaluates Page 5 of 6
Absorbance as a function of the negative logarithm of Transmittance. In simulataneous determination of the concentration of two species within a mixture, a direct Beer’s Law cannot be applied but instead a combined Beer’s Law must be used since the absorbance measured is the total absorbance of both ions.
_______________________ Erlano, Jana Beatriz _______________________ Ortiz, Raissa Athena
It is highly advised to keep the cuvettes clean, and wipe them before placing them inside the spectrometer so as not to make discrepancies in the results. Cuvettes made of quartz are also preferred since cuvettes made of plastic has been known to contribute to the absorbance measured. Sulfuric acid is also preferred to be used as blank reference and for dilution over water. Water is only preferred since it is deionized hence it gives a 100% transmittance. But since blank reference will always be set as a zero mark with 100% transmittance regardless of the substance used, it is more ideal to use sulfuric acid since it ensures that the chromium and manganate ions are in their 3+ 2+ 3+ oxidized form which is Cr and Mn . The Cr and 2+ Mn are the species actually absorbed. Also, work on the dark places or use foil to cover the containers or amber bottles to store the sample solutions to avoid light exposure as it may absorbance or reflect the light and decompose that sample. VIII. Reference http://www.britannica.com/EBchecked/topic/55890 1/spectroscopy http://chemistry.niser.ac.in/labhandouts/CL301exp6.pdf http://imagine.gsfc.nasa.gov/docs/teachers/lesson s/xray_spectra/background-spectroscopy.html
Skoog, D.A., West, D.M., Holler, F.J. and Crouch, S.R. (2004). Fundamentals of analytical th chemistry . 8 Ed. Toronto, Ontario: Brooks/Cole – Thomas Learning Inc.
I hereby certify that I have given substantial contribution to this report _______________________ Balaoing, Arianne May Chem27.1 Spectroscopy
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