Useful Terminology Colorimetry Electromagnetic Spectrum Color Wheel Visual Colorimetry
Goldman, Jane, Founder and Editor In Chief has reference to this Academic Journal, PHwiki organized this Journal Colorimetry is the use of the human eye to determine the concentration of colored species. Spectrophotometry is the use of instruments to make the same measurements. It extends the range of possible measurements beyond those that can be determined by the eye alone. Note: This experiment will demonstrate both techniques on the same set of dyes. Useful Terminology Visual Observations Because colorimetry is based on inspection of materials with the human eye, it is necessary to review aspects of visible light. Visible light is the narrow range of electromagnetic waves with the wavelength of 400-700 nm. = the mnemonic used to remember the colors of the visible spectrum. Colorimetry ROY G. BIV
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Visible light is only a very small portion of the electromagnetic spectrum. Note: Frequency () in addition to Energy (E) are directly proportional whereas Frequency () in addition to Wavelength () are inversely proportional. Electromagnetic Spectrum longer wavelength, lower energy; shorter wavelength, higher energy. Electromagnetic radiation is characterized by its wavelength, , Frequency, in addition to energy, E: E = h= hc / c = Where h = Plancks constant & c = speed of light in a vacuum.
Color Wheel (ROYGBIV) Complementary colors lie across the diameter on the color wheel in addition to combine to as long as m white light, so the color of a compound seen by the eye is the complement of the color of light absorbed by a colored compound; thus it completes the color.
Intensity: For light shining through a colored solution,the observed intensity of the color is found to be dependent on both the thickness of the absorbing layer (pathlength) in addition to the concentration of the colored species. For One Color: A series of solutions of a single color demonstrates the effect of either concentration or pathlength, depending on how it is viewed. Side view Top view (a.k.a. Birds eye view) Visual Colorimetry For more than one color: the ratio of an unknown mixture can also be determined by matching the shade of the color to those produced from known ratios. In this example, the ratio of a mixture of red in addition to blue can be determined visibly by comparing the mixture to purples produced from known ratios of red in addition to blue. Ratio used Purple produced Visual Colorimetry Dilution Factor (constant pathlength) 3 drops of dye std + 5 drops water 8 drops total volume Recall: C1V1= C2V2 Then as long as the dilution, Cdiluted x Vdiluted= Cstd x Vstd Cdiluted = Cstd x (Vstd / Vdiluted) Since Vdiluted = Vtotal Cdiluted = Cstd x (Vstd / Vtotal) Substituting the volumes: Cdiluted = Cstd x (3 drops / 8 drops) If the original concentration is 5.88 ppm, then: C diluted = 5.88 ppm x (3 / 8) C diluted = 2.21 ppm
Intensity: When the product of the concentration in addition to the pathlength of any two solutions of a colored compound are the same, then the same intensity or darkness of color is observed. Duboscq visual colorimeter Adjustable Path Lengths Spectrophotometry Spectrophotometer – an instrument that measures the amount of light absorbed, or the intensity of color at a given wavelength. The intensity of color can be given a numerical value by comparing the amount of light prior to passing it through the sample in addition to after passing through the sample. These quantitative measurements of light absorbed are the Transmittance in addition to the Absorbance. A = abc A is the absorbance Beer-Lambert Law (a.k.a. Beer’s law) – the linear relationship between absorbance in addition to concentration of an absorbing species. Absorbance Main use of Beers Law is to determine the concentration of various solutions. (We used Beers Law to calculate concentration in the equilibrium experiment.) c is the concentration of the sample in (mol/L) a is molar absorptivity in L/[(mole)(cm)] Also called extinction coefficient or ; it is dependent on the material being studied. b is the path length in cm The diameter of the cuvette or sample holder which is the distance the light travels through the absorbing sample. b is a constant when the same size cuvette is used as long as all samples.
Transmittance is given by the equation: T = I/Io where I is the intensity of the light after it has gone through the sample & I0 is the initial (time = 0) light intensity. Absorbance is related to the %T: A = -logT = -log(I/ Io) Transmittance is Related to Absorbance Equation Summary T= (I/I0) = 10-A %T = (I/I0) x 100 A = -logT = log(1/T) Note the scale as long as Absorbance: 9/10th of the scale is from 0-1 in addition to 1/10th is from 1-2. For this reason, the spectrometers have been calibrated in % Transmittance in addition to all readings will be taken in %Transmittance. Sample Calculation If %T = 95%, then A = log(100/95) = log(1/.95) = -log(.95) A = 0.02227 Spectronic 20 (a.k.a. Spec-20) Spec-20 – A single-beam visible light spectrophotometer. Tungsten filament lamp emits visible wavelengths of light. Blank is inserted to adjust 100%Transmittance at each wavelength.
Simple Spectrophotometer Schematic The lamp emits all colors of light (i.e., white light). The monochromator selects one wavelength in addition to that wavelength is sent through the sample. The detector detects the wavelength of light that has passed through the sample. The amplifier increases the signal so that it is easier to read against the background noise. 1. With sample chamber empty, set desired wavelength then adjust to 0%T with right knob on front panel. 2. Insert blank solution, close lid in addition to adjust 100%T with right knob on front panel. 3. Insert dye solutions, read in addition to record %T values. 4. Change wavelength, repeat steps 2-4. NOTE: The filter must be changed periodically to coordinate with the wavelength range studied: blue (400-449), green (450-549) in addition to orange (550-749). Spectronic 20 Instructions (Directions below will be available next to each instrument) Sample Chamber Mode Knob (set to Trans) Digital Display Wavelength Knob 0-100%T Knob Filter Lever Post Lab: 4 Plots of Absorption Data Plots similar to the 3 below will need to be generated using a computer program such as Excel. You will also need to make a plot of your unknown blue or red which will look similar to 1 or 2. 1 2 3 This plot is shown here simply to demonstrate the underlying colors of the purple graph.
Joseph von Fraunhofers initial desire was to create a glass lens that did not produce an image that was fringed with a rainbow of colors. He realized the problem was that the glass lens bent some colors more than others. He began searching as long as a source of light of a single color. In 1814, he developed a spectroscope to study the spectrum of the light given off by the sun. He was amazed to discover that in the midst of the rainbow of colors was a series of black lines. These dark lines were later determined to be the result of the absorption of selected frequencies of the electromagnetic radiation by an atom or a molecule. Development of the Spectroscope Joseph von Fraunhofer (March 6, 1787 June 7, 1826) Fraunhofer lines observable in the Solar Spectrum 390 nm 700 nm Fraunhofer also completed an important theoretical work on diffraction in addition to established the laws of diffraction. One important innovation that Fraunhofer made was to place a diffraction slit in front of the objective of a measuring telescope in order to study the solar spectrum. He later made in addition to used diffraction gratings with up to 10,000 parallel lines per inch. By means of these gratings he was able to measure the minute wavelengths of the different colors of light. (Diffraction gratings will be discussed more later.) Development of Diffraction Gratings
Gustav Robert Kirchhoff (March 12, 1824 October 17, 1887) German Physicist Robert Wilhelm Eberhard Bunsen (March 31, 1811 August 16,1899) German Chemist Bunsen in addition to Kirchhoff further developed the spectroscope by incorporating the Bunsen burner as a source to heat the elements. In 1861, experiments by Kirchhoff in addition to Bunsen demonstrated that each element, when heated to inc in addition to escence, gave off a characteristic color of light. When the light was separated into its constituent wavelengths by a prism, each element displayed a unique pattern or emission spectrum. 1855-1860 – Gustav Kirchhoff in addition to Robert Bunsen The emission spectrum seemed to be the complement to the mysterious dark lines (Fraunhofer lines) in the sun’s spectrum. This meant that it was now possible to identify the chemical composition of distant objects like the sun in addition to other stars. They concluded that the Fraunhofer lines in the solar spectrum were due to the absorption of light by the atoms of various elements in the sun’s atmosphere. Emission Spectra Complement Absorption Spectra Johann Jakob Balmer (May 1, 1825 March 12, 1898) Swiss Mathematician & Honorary Physicist In 1885, Johann Jakob Balmer analyzed the hydrogen spectrum in addition to found that hydrogen emitted four b in addition to s of light within the visible spectrum. His empirical as long as mula as long as the visible spectral lines of the hydrogen atom was later found to be a special case of the Rydberg as long as mula, devised by Johannes Rydberg. Hydrogen Spectrum The Balmer Series
The permitted energy levels of a hydrogen atom. Where v = frequency n = the quantum number R = (Rydberg constant) R = 3.29 1015 Hz 1 Hz = 1 s-1 C = (speed of light) C = 2.9979108 m/s Part A: Calculating the Balmer & Lyman Series The four b in addition to s of light calculated by Balmer can be simply calculated using the Rydberg equation: These equations will be used on page 159. = c / The Nobel Prize in Physics 1922 as long as the investigation of the structure of atoms in addition to of the radiation emanating from them. Niels Henrik David Bohr Oct. 7, 1885 Nov. 18, 1962 Danish Physicist In 1913, Bohr developed a quantum model as long as the hydrogen atom. Proposed the Solar System model of the atom where the electron in a hydrogen atom moves around the nucleus only in certain allowed circular orbits. These orbits then correspond to the energy levels seen in the Balmer series. (p 167) https://www.youtube.com/watchv=-YYBCNQnYNM 3. Measure the line spectrum of the gas tubes set up in Room 201. Note: The fastest/easiest way to do this is have one partner view the lines in addition to the other write down the observations. 4. Compare your results with NIST literature values. For the fluorescent light compare it to the element mercury. PART B: Emission spectrum of other compounds using The STAR Spectrophotometer. View the line spectrum through the STAR Spectrophotometer – point slit towards the light in addition to view to the right. 2. Verify that the scale is lined-up accurately by looking at the fluorescent light. In addition to other lines, you should see a green doublet as long as mercury at ~570 nm (the scale on the bottom).
Neon Helium Atomic Spectra of Hydrogen & the Noble Gases The Atomic Spectra will be determined as long as Hydrogen in addition to the Noble Gases by looking at the gas discharge tubes. Hydrogen Colorimetry & Spectrophotometry Checkout Visual Portion Spec-20s 1 – 12 well plate 5 – cuvettes in a 3 – 12 well strips test tube rack 5 – Beral pipets 2 of which need to be at least 9 wells long. Dont have to be returned. Dyes – Located in Lab: Record Concentrations Blue std. = — ppm Red std. = — ppm Waste (We are using FDA food dyes in addition to distilled water.) Atomic Spectra Checkout STAR Spectroscope Set of Crayons ROYGBIV Gas discharge tubes set up in Room 201. ( as long as viewing by STAR spectroscope) For April 21-23 Turn In: 1.) Colorimetry & Spectrophotometry pp 51-58 + 4 Graphs 2.) Atomic Spectra pp159-161 & 167-169 Read Over: Radiochemistry pp 119-136 in Lab Packet & remember to bring your student id.
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