Question

Why would CuSO4 have a blue color after the iron reagent is added? Yes, the answer is because it is absorbing the other colors (the spectrophotometer), but WHAT is absorbing the other colors?

(Done in an experiment about spectrophotometry)

Answer 1

The word spectroscopy is used to refer to the broad area of science dealing with the absorption, emission, or scattering of electromagnetic radiation by molecules, ions, atoms, or nuclei. Spectroscopic techniques are probably the most widely used analytical methods in the world today. These techniques are useful in determining the identity of unknown substance or the concentration of a known solute in solution. The measurement of the interaction of electromagnetic radiation with matter is called spectrometry and the device used to make these measurements is called a spectrometer . There are many different types of spectrometers designed to study the different regions of the electromagnetic spectrum. Some of the most widely used are: infrared, visible, ultraviolet, and x-ray spectrometers. The spectrometer you will use can correctly be called a colorimeter, because it measures the absorption of light in the visible spectrum which we perceive as color, and the technique used is said to be colorimetric. How It Works All spectrometers work on the principle that atoms and molecules may exist in only certain specific energy levels, that is, they are quantized. They can absorb only photons having certain energies or wavelengths. The energies of light absorbed by a molecule can be related to motions (energy modes) of the molecule.

The source provides the electromagnetic radiation that will be absorbed by the sample. It is often some sort of light bulb or lamp. The monochrometer selects one particular energy (or wavelength or color) of light. A prism, a diffraction grating or a colored filter can serve as a monochrometer. (A diffraction grating consists of a large number of closely spaced lines etched on a highly polished surface. The lines act as scattering centers for the incoming radiation, separating white light into the colors of the rainbow.)

The detector measures the amount of light that passes through the sample. A phototube (or photo cell) or photomultiplier is often used as a detector. All of these work on essentially the same principle. Light falling on the surface of the detector causes current to flow in a surrounding electrical circuit. The amount of current in the circuit is proportional to the amount of light striking the detector. A spectrometer having a phototube as a detector is often called a spectrophotometer . All the parts of the spectrometer work together as follows (Figure I): Light from the source passes through the monochrometer producing a beam with a single energy or a narrow band of energies. The intensity of this beam, Io, is measured by the detector. The sample is then placed in the beam between the monochometer and the detector. If some of the light is absorbed by the sample, the intensity of the beam reaching the detector, I, will be less than Io. The detector compares the compares the two intensities and reports the result as either percent transmittance (%T) or Absorbance (A). These terms are defined to be: %T = I Io x 100 (the fraction of Io that gets through the sample is called Transmittance) A = -log T = -log    I Io = 2 - log (%T). If the monochrometer is a prism or a diffraction grating, all of the energies (or wavelengths) are available and may be varied. Molecules do not absorb all wavelengths equally well. Consider a colored object. Human sight is the brain's interpretation of photons of electromagnetic radiation in the visual range (light) entering the eye. If all the energies (wavelengths, colors) are mixed they are preceived as white light. If no photons at all enter the eye we "see" black. A color is preceived if only photons of one energy (light of one color, monochomatic light) enters the eye, or if photons of a complementary color are missing from the usual white light mix. A white object then appears to be white because it does not absorb any of the light that strikes it. A black object looks black because it absorbs all of the incident light. A rose looks red if it absorbs all the light except the red or if it absorbs the light of the color complimentary to red - that is, blue-green.