COLORIMETRY-SPECTROPHOTOMETRY 1 1 1 



quantum) or occasionally as photons. A quantum is simply a certain 

 amount of energy, whose energy equivalent could be measured in cal- 

 ories, electron-volts, or ergs. The size of the quantum increases as the 

 frequency increases. The extremely long wavelength and low frequency 

 radio waves contain relatively little energy per quantum, while gamma 

 radiation, with its short wavelength and very high frequency, contains 

 a great deal of energy per quantum. Planck was able to show that the 

 relationship is in strict proportion and was able to arrive at a value for 

 the proportionality constant (/i) of the equation 



E = hv (9-1) 



If we know the frequency, we can use Planck's constant, approximately 

 6 X 10"^'^ erg-sec, and arrive at the energy (E) in ergs per quantum. 

 Since a single quantum is an extremely small amount of energy, we 

 usually deal in terms of N (Avogadro's number) quanta. By way of 

 illustration it can be pointed out that N quanta of red light are equivalent 

 to about 40 kilocalories of energy, while N quanta of blue light are 

 equivalent to about 70 kilocalories. 



Colorimetry and spectrophotometry depend upon the interaction be- 

 tween light and matter. Almost every material will absorb radiant energy 

 of some wavelengths. If these wavelengths happen to be within the 

 visible region, we say that the material has color because the eye sees 

 only those parts of the spectrum which are not absorbed but are reflected 

 or transmitted. If the absorbed wavelengths happened to be in the in- 

 frared or ultraviolet, and relatively little of the visible light were ab- 

 sorbed, the material would be white or colorless. If nearly all the visible 

 light were absorbed, we would call the material black. 



When a molecule absorbs light energy it must absorb one and only 

 one full quantum. It cannot absorb parts of quanta or several at a time. 

 When the atom or molecule accepts this bundle of energy, so that it 

 contains more than the usual amount of energy, it goes into an "excited 

 state." The most likely explanation on the basis of atomic structure is that 

 one of the orbital electrons is lifted from a stable configuration into a 

 less probable, higher energy state. The amount of energy required for 

 this transformation is exactly the amount in the particular quantum. In 

 any kind of atom or molecule there may be several possible ways in 

 which the different electrons can be displaced. Each of these would re- 

 quire its own characteristic amount of energy. Therefore the atom or 

 molecule could absorb several different wavelengths of radiant energy. 

 If we examine the material by means of the spectroscope we see dark 



