Colorimetry vs Spectrophotometry
Spectrophotometry and colorimetry are techniques, which can be used to identify the molecules depending on their absorption and emission properties. This is an easy technique to determine the concentration of a sample, which has a color. Though the molecule does not have a color, if we can make a colored compound from it by a chemical reaction, that compound can also be used in these techniques. Energy levels are associated with a molecule, and they are discrete. Therefore, discrete transitions between the energy states will only occur at certain discrete energies. In these techniques, the absorption and emission arising from these changes in the energy states are measured. This is the basis of all spectroscopic techniques.
In a basic spectrometer, there is a light source, absorption cell and a detector. The radiation beam of the tunable light source passes through the sample in a cell, and the transmitted intensity is measured by the detector. Variation of the signal intensity as the frequency of the radiation is scanned is called the spectrum. If the radiation does not interact with the sample, there will not be any spectrum (flat spectrum). In order to record a spectrum, there has to be a difference in population of the two states involved. On the microscopic scale, the ratio of the equilibrium population in two states separated by an energy gap of ∆E is given by the Boltzmann distribution. The absorption laws, or in other words, Beer’s and Lambert’s laws indicate the extent to which the intensity of the incident beam is reduced by the light absorption. Lambert’s law states that the degree of absorption is proportional to the thickness of the sample, and the Beer’s law states that the degree of absorption is proportional to the concentration of the sample. The principle behind the spectrophotometry and the colorimetry are the same.
This is the technique used to determine the concentration of a solution having color. It measures the intensity of color and relates the intensity to the concentration of the sample. In colorimetry, the color of the sample is compared with a color of a standard in which the color is known. Colorimeter is the equipment used to measure the colored samples and give the appropriate absorptions.
Spectrophotometer is the instruments used in this technique. It has two main parts, the spectrometer, which produces the light with a selected color, and the photometer, which measures the intensity of light. There is a cuvette where we can place our liquid sample. The liquid sample will have a color, and it absorbs the complementary color of it when a light beam is passed through that. The color intensity of the sample is related to the concentration of the substance in the sample. Therefore, that concentration can be determined by the extent of absorption of light at the given wavelength.
What is the difference between Colorimetry and Spectrophotometry?
• A colorimeter quantifies color by measuring three primary color components of light (red, green, blue), whereas spectrophotometer measures the precise color in the human-visible light wavelengths. .
• Colorimetry uses fixed wavelengths, which are in the visible range only, but spectrophotometry can use wavelengths in a wider range (UV and IR also).
• Colorimeter measures the absorbance of light, whereas the spectrophotometer measures the amount of light that passes through the sample.