Spectrophotometry Spectrophotometry is a method to measure how much a chemical substance absorbs light by measuring the intensity of light as a beam of light passes through sample solution. The basic principle is that each compound absorbs or transmits light over a certain range of wavelength. The absorption spectroscopy has been widely used for monitoring the rate of chemical reactions. During the reaction, if there is either appearance of colour in a colourless solution or disappearance of colour in a coloured solution or a species which absorbed at a specific wavelength is formed, the spectroscopic technique can be used. Instruments like colorimeters and spectrophotometers are available to cover the visible, near infrared and ultra violet region of the spectrum (200-1000 nm). The absorption spectroscopy is governed by well-known Beer-Lambert’s Law according to which: Absorbance or optical density = I0 /It = Ecl where I0 and It are the intensity of incident and transmitted light, respectively, c is concentration of solution, l is the path length of beam of light within the solution and E is molar extinction coefficient which is characteristic of the absorbing substance. The instruments give the optical density directly. If l remains constant Optical density = Constant × c or Absorbance ∝ c First the Beer-Lambert Law is tested with the solutions of known composition at λmax of solution by plotting (absorbance) versus concentration . The concentration range for which the plot is a straight line is determined and method is applied within this concentration range. The optical density of samples are measured at different times using same cell (to keep l constant) and concentration can be calculated using relation c = A/El. To verify the order of reaction, optical density data can be used directly. If the species is a reactant, then its absorption will decrease as time goes by. If the species is a product, then its absorption will increase as time goes by.Kinetics is the study of how concentrations of reactants and/or products change as time goes by.