Basics of Colorimetry and Flame Photometry

Introduction to Colorimetry

Colorimetry is the determination of concentration of a substance by the measurement of the relative absorption or transmittance of light in the visible region with respect to a known concentration of the substance.

Instrument used for colorimetric measurement is called colorimeter. This instrument has been named as spectrophotometer since photoelectric cells are generally used to measure the light transmitted by the solution.

When a beam of radiant energy passing through a medium, the energy of the beam is partly altered by reflection, Refraction, diffraction or absorption and the reminder of the energy may be transmitted through the substance.

Spectrophotometry or colorimetry is based on Lambert’s and Beer’s law which states that the decrease in the intensity of light is directly proportional to the concentration of the solution if the thickness of the absorbing medium is kept constant.

Transmittance T=I/Io

Where,

Io is the intensity or energy per cm² per second before passing through substance. I is the intensity of energy after passing through the medium

Absorbance(A) = log 1/T

                           = log Io/l

                          = log Io – log I

                           = log 100-logT

                       A = 2 – log T

The light from the source travels through the field lens, entrance slit, and collimating mirror. Collimating mirror allows the light to fall on the grating chamber which disperses the light. The required wavelength of the light is adjusted by adjustable light control (wave length adjustment). The monochromatic light passes through the exit slit, filter and the sample. Some of the light energy 1s absorbed by the sample and the remaining is transmitted to the photocell which converts light energy into the “electrical energy. This transmittance is measured by galvanometer.

Selection of wave length range

Introduction to flame photometry

The use of flame emission spectroscopy for the estimation of the alkali metals is an important application in routine chemical analysis. For this, low temperature flame photometry provides the most reliable and convenient method. This exploits the fact that compounds of the alkali and alkaline earth metals are thermally dissociated at the temperature of a burner flame and some of the atoms produced are further excited to a higher energy level. When these excited atoms return to the ground state, they emit radiation which, for the alkali and alkaline earth elements, lies mainly in the visible region of the spectrum.

The wavelength of light emitted from the flame is characteristic of the particular element. The intensity of this light is proportional to the amount of the salt solution volatilised in the flame at any moment.

Hence, a flame photometer essentially consists of the following components.

• A flame that can be maintained in a constant form and at a constant temperature.
• A means of transporting a homogenous solution in the flame at a steady rate.
• A means of separating the light of the wavelength to be measured from that of extraneous emission with a monochromator or an optical filter.
• A means of measuring the intensity of the radiation emitted from the flame.

The two characteristics of the alkali metals chiefly facilitating their determination by flame photometry are:

1. Their spectra are excited at a temperature lower than that at which most other elements are excited. This low excitation temperature permits ordinary bottled gas to be used as fuel for the flame
2. Their characteristic wavelengths are widely separated from each other and from those of most other elements. This wide separation of the characteristic wavelengths makes it possible to use simple optical filters rather than the more expensive monochromators.