Experiment#14: Spectroscopy of Cobalt II Ion Purpose: The purpose of the experiment was to preparation of a standard solution, quantitative dilutions, and use of a spectrometer to determine an absorption spectrum and a Beer’s law plots. In the learning object of light, we know about the structure of atoms and molecules come from learning our interaction with light. The light may be considered to have both wave nature and particle nature. A photon is the basic unit of light that demonstrates both of these properties. Which have the equation, speed of a wave to its wavelength and frequency, V= C/? ? is wavelength, V is frequency, C is speed of light c=3. 00×108 m/s). Also, energy of a photon to the frequency, ? E=hv (E is energy, v is frequency, h is a constant factor h=6. 63×10-34 J. S) As we can see light at night by neon light or the firework in the sky that is being released by atoms in them. We can learn about how light interacts with atoms and understand the arrangement and behavior of electrons in atoms. Structure of atoms and molecules are described in the scientific literature and based sound experimental evidence.
The most compelling evidence for structure comes from spectroscopic experiments, as will be demonstrated in this paper. Spectroscopy is one of technique, which uses the interaction of energy with a sample to perform an analysis. Spectrometer produces light of a specific wavelength. Light bulbs produce white light with all colors 400-700nm. Diffraction grating splits white light into different wavelength. The slip allows only narrow bond of light, which is wavelength. Photometer detector measures intensity of light passing through the sample.
Also, spectrophotometry measure the amount of light is absorbed material. Amount of light absorbed is not dependent on wavelength and use one wavelength at a time. Monochromatic light is light made of photons of only one wavelength ?. Wavelength at which greatest absorbance occurs is different for each species. When we correlate absorbance to concentration, we use wavelength maximum to give the most accurate results. We have equation to help us to find the wavelength maximum (? max). They measure the absorbance of a sample at each wavelength by using a blank to set zero first.
Then they find wavelength with the highest absorbance. Beer’s Law equation: A=? bc (A is absorption, c is concentration (mole/l) or molarity of the absorbing species, b is path length of light through a sample, and ? is a constant which is found by the nature of the absorbing species). When graphing the absorption and concentration, students calculate the slope of the graph. They use standard samples to make the graph and use the graph to determine the concentration of unknown sample. Then they can find the constant ? by knowing absorption, the path length, and the concentration).
Spectroscopy of Cobalt (II) ion is one of the experiments that they do in the laboratory. Preparing a standard solution, quantitative dilutions, and using a spectrometer to determine the absorption spectrum and a Beer’s la plot in the experiment did this. Indeed, in this experiment students will use the spectrometer to find absorption and concentration of five different amount Cobalt (II) solutions with different wavelength and different volume, and used this determine to concentration of the cobalt (II) solution ion in the unknown. Besides, other purpose of this experiment was to experience stoichiometric calculations.
The students will learn how to calculate molarity of Co(NO3)2 will be determine from finding mass of beaker, mass of Co(NO3)2. 6H2O, and mole of Co(NO3)2. 6H2O. Material and Methods: The experimental procedure consisted of three parts: preparation of the solutions absorption spectrum, Beer’s law plot, and determination of the concentration of the cobalt (II) ion in the unknown A. the detailed experimental procedure can be found in the primary reference of experiment, Murov, S. L. Experiments in General Chemistry (Fifth Edition), Thomson Brooks/Cole: Florence, KY, 2004; pp 163-164.
In the experiment Spectroscopy of Cobalt (II) ion, Cobalt (II) ion was selected because it has an absorption maximum in the visible that help us distinguish the tube with different concentration. Thus, we can easily prepared the solution but we need to be careful with cobalt (II), it is a little toxic. Moreover, the concentrations of Cobalt (II) ion that can be determine using spectroscopy technique but there are some limitations. Because of the concentration is too high and molecules in the solution might collide with each other. That is the reason why makes the result accurate less.
In another examination, visible spectroscopy can be analyzed quantitatively with other cations and anions such as chromium (III), iron (III), nickel (II), Copper (II), dichromate, and permanganate, ferricyanide. Those are the absorption spectra of all colored species are unique in both extinction coefficients at any wavelength. To begin the procedure, in the preparation of the solution, we used cobalt nitrate hexahydrate to dissolve in 25ml-deionized water in a baker to prepare 50ml of 0. 150M cobalt nitrates. Cautiously, we transferred the solution into the 50ml volumetric flask.
Then we rinse the beaker with deionize water and add the washing into the flask. After that, we deliver the solution with buret to six tubes with six-differences amount. For example, from tube 1 to tube 5, we deliver 1. 00ml solution for tube 1, 2. 00ml solution for tube 2, 3. 00ml solution for tube 3, 4. 00ml solution for tube 4, and 5. 00ml solution to tube 5. In addition, we repeat the same procedure by adding water to each tube. We add 5. 00ml for tube 0, 4. 00ml for 1, and 3. 00ml for tube 2, 2. 00ml for tube 3, and 1. 00ml for tube 4. Be sure, we have to check all tube have the same level 5. 0ml solution with water and mix the contents of each one. Also, we prepare one tube unknown A of cobalt nitrate. Second, we used absorption spectrum method with test tube 5 to figure out the wavelength of maximum absorption. We started the method by set the wavelength to 430nm and make sure that the transmission reads zero. Next, we inserted tube zero that is 5. 00ml of deionized water and set it down to zero absorption. Then, we can inserted tube 5 read the absorption, removed it, and repeat the above steps for all wavelength from 460nm to 610nm. Finally, we graph the absorption which is y-axis versus the wavelength is x-axis.
Third, in the Beer’s law plot, we used 5 test tubes and an unknown to record all absorption and concentration values of them all. Graph the absorption that is y-axis, and the concentration of Cobalt (II) ion, which is x-axis. Results: The results of the molarity of cobalt nitrate for determining the mass of cobalt nitrate hexahydrate and the mole of cobalt nitrate hexahydrate showed in the table 1 excellently. Table1: the calculations of mass and mole of cobalt nitrate hexahydrate to determine molarity of it. Formula mass of Co(NO3)2. 6H2O| 291. 05 g/mol| Mass needed to prepare 50ml of 0. 50M Co(NO3)2| 2. 183 g| Mass of beaker + Co(N3)2. 6H2O| 65. 202g| Mass of beaker| 63. 017g| Mass of Co(NO3)2. 6H2O| 2. 185g| Moles of Co(NO3)2. 6H2O| 7. 507×10-3 mol| Molarity of Co(NO3)2| 0. 1501M| Before performing the concentration of the unknown, the absorption of cobalt (II) ion must be found. The spectrum was determined, and we identify absorption profiles with proportionally lower absorption value. After all, the spectrum selects the best wavelength for the concentration. The Beer’s law is used to determine the molarity of the cobalt (II) ion in the solution of the unknown concentration.