CHM260 LABORATORY REPORT
Experiment Number: 2
Title: UV-Visible Determination of an Unknown
Concentration of KMnO
4Solution
Name: Nurul Adilah Bt. Mohd Noor
Student no.: 2011893478
Lab group: 2
Date of Report Submission: 9/1/2014
Lab Partner’s Name: Nor Hasniza Bt. Ab Majid
Saidatul Fathini Bt. Shobri
Lecturer’s Name: Miss Dhia Azliana
EXPERIMENT 2
UV-Visible determination of an unknown concentration of KMnO4 solution.
OBJECTIVES
1. To determine the maximum wavelength of potassium permanganate. 2. To plot the calibration curve of potassium permanganate.
3. To determine the concentration of an unknown solution of potassium permanganate
INTRODUCTION
All absorbance spectrophotometers contain a light source, a sample compartment, and a detector. Many spectrophotometers also contain one or more monochromators, a device used to separate light into its component wavelengths. Spectrophotometers that measure in the UV and visible region are of two general types : scanning and diode-array. In contrast, a scanning UV-Vis spectrophotometer contains a monochromator, usually consisting of holographic gratings, which allows light of individual wavelength to be sequentially imparted to the sample.
Spectroscopy involves the observations of absorption or emission of electromagnetic radiation resulting from transitions of atoms or molecules from one energy level to another. When a molecule at its “ground state” (the state of lowest energy) absorbs energy of some type, the molecule is said to undergo a “transition” to a higher energy state. The higher energy state is referred to as an “excited state”. A molecule can only absorb energy if the input energy exactly matches a molecular transition from one energy level to another. Many organic and biological molecules have transitions that occur between energy levels of electronic states of atoms or molecules. Therefore, our focus will be on the visible and ultraviolet regions of the electromagnetic spectrum in this lab. The visible and ultraviolet region of interest is found between 170 and 800 nm, though the most useful region for
experimental use is between 250 and 700 nm.
APPARATUS
Beaker, burette, glass rod, Volumetric flask 100 mL and dropper.
CHEMICALS
PROCEDURE
During this laboratory experiment, a series of dilutions is made and the absorbance of the dilutions at λ max is taken. The permanganate ion is absorbed at 534 nm and the absorbance
values for the solutions is determined at this wavelength. In this experiment, an initial permanganate stock solution is prepared and the solutions to be measured are diluted from a dilution of the stock. Once the absorbance values are taken, a Beer’s law plot for KMnO4 is
generated and the concentration of the unknown solution is determined.
A. Preparation of the KMnO4 Standard Solutions
1. 0.01 g KMnO4 is weighed accurately to the nearest mg on a weighing paper. The
reading is recorded. The solid is transferred to a 100 mL volumetric flask by using a funnel.
2. The solid is dissolved with a few mL of distilled water. The flask is stoppered and shaken. The distilled water is added to the mark, and the last few drops is added by using a medicine dropper. The flask is stoppered and shaken several times to homogenize the solution.
3. The ‘stock’ solution is poured into a beaker. The beaker is labelled as ‘100 ppm’. 4. 5.00 mL of the ‘stock’ solution is pipetted and diluted with distilled water in a 100 mL
volumetric flask.
5. The solution is transferred into a beaker and the beaker is labelled as ‘5 ppm’. 6. Step 4 is repeated by using 10 mL, 15 mL and 20 mL stock solution and then are
transferred into small beakers.
7. The beakers are labelled as ’10 ppm’, ’15 ppm’, and ’20 ppm’ respectively.
B. Preparation of the Unknown
1. Between 5.00 to 20.00 mL of the ‘stock’ KMnO4 solution is pipetted and diluted with
distilled water in a 100 mL volumetric flask.
2. The solution is transferred into a beaker and it is labelled as ‘Unknown’.
C. Operation of the UV-Vis Spectrophotometer
Instrument : Varian/ Cary 50 UV-Vis Spectrophotometer. Operating instructions
1. Cary Win UV icon is selected. 2. ‘Scan’ is clicked.
3. Setup is chosen, the ‘CARY’ is clicked and then the required start and stop scan wavelength (nm) is keyed in.
i. Y-mode (min = 0 and max = 1). ii. X-mode = 800-200 nm
iii. Beam mode = Dual Beam
4. Cycle mode is selected if more than 1 cycle is required. 5. “Scan Control Speed” is selected to fast.
6. ‘Baseline’ icon and check ‘Baseline Correction Function’ is clicked.
7. ‘Accessories 1’ is clicked and check on the Use Cell Charger cells is selected. 8. At the peak table option, maximum peak is selected.
9. In the Auto Store icon, ‘storage on (Prompt At Start)’ is set. 10. To save the method, go to the file and the scan method is saved. 11. The ‘BLANK’ cuvette solution is filled with distilled water. 12. The ‘BLANK’ cuvette solution is put and ‘Baseline’ is clicked. 13. The ‘BLANK’ cuvette is removed and the ‘SAMPLE’ cuvette is put. 14. ‘Start’ icon is clicked to start the measurement.
D. Determination of the Unknown Concentration
1. The concentration icon is clicked.
2. Setup is chosen, Cary icon is clicked, then maximum wavelength, λ max is keyed in.
3. Replicate = 3 is selected.
4. ‘Standard’ icon is clicked, the ‘Calibrate During Run’ function is checked.
5. The calibration standard unit (mg/L) and the number of the standard samples is set. 6. The Fit Type (Linear Direct) is selected.
7. Sample icon is chosen, the number of the samples are selected and the unknown is keyed in.
8. Report icon is clicked, operator name and the comment is keyed in. 9. In the Auto Store icon, ‘Storage on (Prompt at start)’ is set.
10. To save the method, ‘File Save Method As Ok’ is clicked. 11. The ‘BLANK’ cuvette is put and ‘Zero’ is clicked.
12. The ‘BLANK’ cuvette is removed and the ‘SAMPLE’ cuvette is put. 13. ‘Start’ icon is clicked to start the concentration measurements.
PRE-LABORATORY QUESTIONS
1. Show how you will prepare a 5 ppm solution from a 100 ppm KMnO4 stock solution
using a 100 mL volumetric flask. Briefly describe the procedure.
5 mL of the ‘stock’ solution is pipetted and is diluted with distilled water in a 100 mL volumetric flask. The solution is transferred into a beaker and labelled as it as ‘5 ppm’.
2. What is the expected wavelength at maximum absorption, λ max, for the KMnO4
solution? What is the ε value at this wavelength?
The expected wavelength at maximum absorption, λ max, for the KMnO4 solution is
525.0 nm. Value of ε is equal to 5.63 x 10-9 Lmol-1 cm -1 .
A = εbc 0.178 = ε (1 cm)(5 x 106 mg/L) c = mol/L mol L =5 ×10 6mg L × 1× 103g 1 mg × 1 158.032 mol g mol L =3.1639 ×10 7mol L A = εbc 0.178=ε(1 cm)(3.1639 ×107mol L ) 0.178=ε
(
3.169 ×107mol cm L−1)
0.178
3.1639 ×107L mol−1cm−1=ε
ε=5.63× 10−9Lmol−1cm−1
QUESTIONS
1. Why is glass not a suitable cell material for use in UV spectroscopy?
Glass is not a suitable cell material for use in UV spectroscopy because the glass will absorbed some of the UV light and will cause an inaccurate result for the experiment as the reading will be higher than the exact one.
2. State one advantage of using the UV-Vis Spectrophotometer compared to a Spectronic 20 for this analysis.
A UV-Vis Spectrophotometer’s main advantage over the Spectronic 20 is that the scanning spectrophotometer can rapidly scan a range of wavelengths and record absorbances at each wavelength while Spectronic 20 is only well suited for quantitative absorption measurements at a single wavelength.
CALCULATION
Show the sample calculation for the preparation of standard 3:
x
100 mL×100 ppm=15 ppm
x= 15 ppm
100 ppm×100 mL
x=15 mL
DISCUSSION
The objectives of this experiment are to determine the maximum wavelength of potassium permanganate, to plot the calibration curve of potassium permanganate and to determine the concentration of an unknown solution of potassium permanganate. At first in this experiment, an initial permanganate stock solution is prepared and the solutions to be measured that are 5 mL, 10 mL, 15 mL, 20 mL and also unknown solution are diluted from a dilution of the stock. Once the absorbance value are taken, Beer’s law plot for KMnO4 is
generated and the concentration of unknown solution can be determined. We used 15 mL of KMnO4 stock solution for unknown solution in this experiment.
The instrument that we have used to do this experiment is UV-Vis Spectrophotometer. UV Visible spectroscopy measures the response of a sample to ultraviolet and visible range of electromagnetic radiations. Molecules and atoms have electronic transitions while most of the solids have interband transitions in the UV and Visible range. The instrument used in
ultraviolet-visible spectroscopy is called a UV/VIS spectrophotometer. It measures the intensity of light passing through a sample (I) and compares it to the intensity of light before it passes through the sample (Io). The ratio I/Io is called the transmittance and usually is
expressed as a percentage (%T). The absorbance, A is based on the transmittance that is A= - log T. The UV-visible spectrophotometer can also be configured to measure reflectance. In this case, the spectrophotometer measures the intensity of light reflected from a sample (I) and compares it to the intensity of light reflected from a reference material (Io) such as white
tile. The ratio I/I0 is called the reflectance and usually is expressed as a percentage %R).
Instruments for measuring the absorption of UV or visible radiation are made up of the following components that are light source, spectroscope, sample containers and a detector. Two kinds of lamps, a Deuterium for measurement in the ultraviolet range and a tungsten lamp for measurement in the visible and near-infrared ranges are used as the light sources of a spectrophotometer. In tungsten lamp, a continuous spectrum of 300-3000 nm is emitted while in deuterium lamp, only a continuous spectrum of 168-500 nm, with maximum energy at 250 nm is emitted. Other component in this instrument is spectroscope. A spectroscope plays a role in selecting a monochromatic light from a light source (white light).
Spectroscope include Filter type, Prism type and Grating (diffraction grating) type. In filter type, a single wavelength can be extracted with a filter. A filter is also used in combination with diffraction grating for filtering out stray light. For prism type, a spectrum of 175-2700 nm can be dispersed. The degree of dispersion varies with the wavelength. Then, for grating
type, dispersion is homogenous at the entire wavelength and a wide-range wavelength can be obtained with a diffraction grating. In addition, a constant spectrum featuring a constant slit breadth can be obtained. Next, the other component is sample container. A container that contains a sample is usually called "cell" and two types are available that are glass and quartz cells. Since light in the ultraviolet range with a wavelength of 340 nm or less hardly passes through a glass cell, it is used for measurement in the visible range of 340 nm or more. On the other hand, although a quartz cell allows passage of light in the entire wavelength in the ultraviolet and visible ranges, it is mainly used for the measurement in the ultraviolet range due to its high price. The last component is a detector. A detector plays a role in changing the light transmitted from a sample into an electric signal. The detector is typically a photomultiplier tube, phototube, photodiode and photodiode array.
The maximum wavelength of potassium pemanganate that we obtained in this
experiment is 525.0 nm. The concentration of unknown solution of potassium permanganate that we obtained is 14.6 mg/L since we used 15 mL of KMnO4 stock solution. In this
experiment, some precautionary steps has been taken to avoid error from occurred that will affect the results. For example, all the apparatus have been washed properly with distilled water to avoid the solution from being contaminated with other substances. Then, when I want to measure solution by using pipette and volumetric flask, my eyes is perpendicular to the calibration mark to avoid parallax error from occurred.
CONCLUSION
As a conclusion, the calibration curve of potassium permanganate has been plotted. The maximum wavelength of potassium permanganate that we obtained for this experiment is 525.0 nm and the concentration of an unknown solution of potassium permanganate is 14.6 mg/L.
REFERENCES
1. (2013). In N. A. Faridah Hanum Hj Badrun, Basic Instrumental analysis Laboratory
experiments for an Introductory Course in Instrumental Analysis.
Zailuddin Ariffin, F. N. (2013). Basic Instrumental Analysis Laboratory Experiments
for an Introductory Course in Instrumental Analysis.
2. Scanning spectrophotometer. (n.d.). Retrieved from Chempage Laboratory resources:
http://chem.wisc.edu/deptfiles/genchem/lab/labdocs/modules/scanspec/scanspecdesc.ht m
3. UV-Vis/NIR Spectrophotometer Basic Course. (n.d.). Retrieved from Hitachi High-Tech: http://www.hitachi-hitec.com/global/science/uv_vis/uv_basic_8.html
