B.Tech-Chemistry

ANALYTICAL TECHNIQUES, FLAME PHOTOMETRY,INSTRUMENT FOR FLAME EMISSION , SPECTROPHOTOMETRY

ANALYTICAL TECHNIQUES

Now a day’s several techniques are available in the field of analytical chemistry. These techniques include spectroscopy, spectrophotometry, X-ray electron diffraction methods, mass spectroscopy, Conductometric titrations, Flame photometry, calorimetry, TGA, DTA, DSC etc.These techniques are versatile and provides appropriate information for understanding chemistry in a better way.

Some analytical techniques as per syllabus are discussed below:

FLAME PHOTOMETRY

It is a type of emission spectroscopy widely used for analysis of metals particularly sodium and potassium in biological fluids, tissues and other samples. The concentration of an element in a solution is determined by measuring the absorption, emission or fluorescence of electromagnetic by its monatomic particles in gaseous state in the flame.

Principle

A Metal is analyzed on the basis of radiation emitted by it when the sample is atomized into a flame. When a solution containing a metallic salt is sprayed into a flame; the salt gets vaporized. This causes some of the metal atoms to get excited to an energy level. When these electrons return to the lower energy level, they radiate energy of different wavelengths. The radiated energy, on passing through optical filter, emitted the characteristic wavelength radiation which is amplified and recorded. The emission is proportional to the number of excited atoms, which is proportional to the total number of atoms in the flame i.e. the sample concentration.

Flame Spectra- The spectra of gaseous, atomic particles consist of well defined narrow discrete lines arising from electronic transition of outermost electrons. Since there are no bonds, atoms undergo electronic transition only, no vibrational or rotational transitions. The energy to which the atoms are subjected must be less than the ionization potential.  The resonance wave - length (at which the most intense absorption and emission occur) is:    671 nm for lithium,    589 nm for sodium and   767 nm for potassium.

The number of atoms of an element excited by the flame depends on :

1- Flame temperature

2- The energy difference between the excited and ground states.

Accordingly

-The number of excited atoms in the flame is considerably small, even in the case of alkaline metals which are easily excited.

-Any increase of the flame temperature is accompanied by great increase in the number of excited atoms.

INSTRUMENT FOR FLAME EMISSION

1 Flame atomizer.

2 Monochromator

3 Detector.

4 Readout meter. 

Applications

  1. It is useful in industries for detection of elements present in fuels, cement etc.
  2. It is used for analysis of Na, K. Ca etc.
  3. Soil, water and plant materials are analyzed.
  4. Blood and urine samples can be analyzed.

Limitations

  • The number of excited atoms in flame is very small.  It is the alkaline and alkaline earth metals that can be practically determined.
  • It needs perfect control of flame temperature.
  • Interference by other elements is not easy to be eliminated.

Heavy and transition metals, the number of absorption and emission lines is enormous and the spectra are complex.    

SPECTROPHOTOMETRY

It is a device which detects the percentage transmittance of light radiation when light of certain intensity and frequency range is passed through the sample. It compares the intensity of transmitted light with that of incident light

Principle

On absorbing UV or visible light, electrons get excited from ground state to higher energy state. Ground state electrons have paired spin. If on excitation the spin remains paired it is called excited singlet state. If on excitation the spin becomes parallel it is called excited triplet state. Triplet state is of lower energy and hence more stable. Absorption of UV or visible light results in transition with retained spin of electrons. The transition (excited singlet state to excited triplet state) involves emission of energy as light.  

Schematic diagram of a spectrophotometer

Diagram showing Spectrophotometer