2.5.2 Absorption
In the last laboratory you noticed that thermal or electrical excitation of atoms caused electrons to be promoted to higher levels of lying energy. The subsequent relaxation of the excited electrons to their ground energy levels was followed by photon emissions in the visible electromagnetic spectrum field. The distinctive colors emitted during this relaxation phase can in some cases be used to classify the individual responsible for the spectrum of emissions. You will be turning this phenomenon around for the next two test times by using the absorption spectrum of a transition metal ion in aqueous solution. The transition metal ions are known to show a wide variety of colours. Yes, many transition metal ions are responsible for the colored pigments in paints, the color of gemstones such as rubies or sapphires and stained glass colors. Absorption spectra of chemical species (atoms, molecules or ions) are produced when a beam of electromagnetic energy (i.e. light) passes through a sample and a portion of the photons of electromagnetic energy passing through the sample is absorbed by the chemical species. A perfect example of this phenomenon is our understanding of colour. Consider the case where a ray of white light (i.e., sunlight) passes through a chlorophyll-containing sample solution (the compound responsible for leaf colour). In the blue and red regions of the visible portion of the electromagnetic spectrum the chlorophyll molecules absorb only a few select photons. The energies of such absorbed photons cause electrons to be excited in the chlorophyll molecule and the energy of these excited electrons is used in the plant cell to drive the conversion of carbon dioxide and water into glucose. More important for our purposes is that when the red and blue photons which chlorophyll absorbs are subtracted from white light, the resulting light beam that leaves the solution appears green to our eyes and this is why leaves appear green to us. If we were able to calculate the total number of photons of all colors entering the sample and compare it with the total number of photons of all the colors leaving the sample, we would find fewer photons exiting the sample than entering it. This is consistent with the fact that some of the photons from the white light beam that entered the solution were absorbed by the chlorophyll molecules.A spectrophotometer is an instrument for doing this measurement. Using some well-understood electronics this tool effectively "counts" the number of photons entering a sample and compares it to the number of photons leaving a sample. Moreover, the device is capable of taking white light and splitting it into its constituent colors (i.e. much like Copyright © 2010; John Hardesty and Bassam Attili Page 2 a prism), enabling the user to analyze the light absorption of different wavelengths with a resolution of almost 1 nm. In optics, the portion of physics which deals with light properties The Intensity, I, is the calculation of the amount of photons emitted at a point in a given time unit. (Higher intensity could be regarded as' brighter' and lower intensity could be regarded as' dimmer;' thus high intensity light would be bright and low intensity light would be dim.) When we calculate the light beam intensity entering our sample (Io) and equate it with the light beam intensity leaving our sample (I) then we can use the I / Io ratio to get an estimate of what proportion of the light entering the sample was found leaving the sample. The ratio is called the Transmission Transmittance: T I o We can translate this ratio to a percentage by multiplying it by 100 to get Percent Transmittance (percent T): percent Transmittance: percent T I o 100 Thus, if the light intensity leaving our sample is 76 and the light intensity entering our sample is 100, the transmittance will be 0.76 and the transmittance percentage will be 0.76 This will be 76%, suggesting that 76% of the photons that enter our sample find their way out of our sample. For our purposes the interpretation of a new term, Absorbance (A) is mathematically convenient: Absorbance: A log10 I o Absorbance is a direct measure of how much light our sample absorbs. If you play with the formula in your calculator you can note that absorbance will take on values between 0 (at 100 percent transmittance) and around 2.0 (at 1 percent transmittance); hence high absorbance values are correlated with very little light passing through the sample entirely and low absorbance values (i.e. those approaching0) Are related to most light that passes through the sample entirely. The Beer-Lambert Law: Consider a chemical product solution that absorbs light of a given wavelength. We might picture two fascinating situations. Next, if we pass through a relatively dilute solution a ray of light of the correct wavelength, We might picture photons meeting a limited number of absorbing chemical compounds, so we could anticipate a high percentage of transmission and low absorption. Alternatively, if we move through a highly focused solution the same beam of light, we may assume that the photons would encounter a large number of copyrights © 2010; John Hardesty and Bassam Attili Page 3 Chemical species absorbing, so we should anticipate a low percent transmittance so strong absorption. Accordingly, absorbance is proportional to sample concentration. Third, We might assume that if we allow the light beam to encounter a solution for a long period of time, we might expect a low percentage of transmittance and high absorption; while if the light beam was allowed to encounter the solution for a short period of time, we could expect a high percentage of transmittance and low absorption. As light travels at a constant velocity, c= 3.0x 108 m / s, this means that the absorbance should also be proportional to the beam's path length across the sample. These two considerations allow us to define the following proportionality: A k l c where, k= constant proportionality, l= length of path, and c= concentration of the absorbing chemical species If the length of the path is measured in centimeters and the concentration of the absorbing species is expressed in molarity, the proportionality constant is called molar absorption, with units ofM-1 cm-1; And our proportionality reduces to the Beer-Lambert Law: A l c This strategy is used not only by chemists but by scientists in many fields. The Beer-Lambert law helps you, the scientist, to measure the absorption of a given sample and to deduce the solution concentration from that calculation! Currently, The concentration of a specific chemical species in a solution can be determined as long as you know that the species absorbs light of a specific wavelength. [24]