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Separation of Isotopes



Separation of Isotopes

Since isotopes have exactly similar chemical properties, their separation by chemical means is out of question. Their difference in those physical properties which depend on the mass of the atom has been utilised to effect their separation. The methods commonly employed for the purpose are:

(1) Gaseous Diffusion
(2) Thermal Diffusion
(3) Distillation
(4) Ultracentrifuge
(5) Electromagnetic Separation
(6) Fractional Electrolysis
(7) Laser Separation

(1) Gaseous Diffusion

** The rate of diffusion of a gas is inversely proportional to the square root of the molecular weight (Graham’s Law of Diffusion).


** Thus when a mixture of two gaseous isotopes is allowed to diffuse through a porous partition, the lighter isotope passes through more rapidly than the heavier one.

** The isotopes of neon (20Ne, 22Ne) and oxygen (16O, 18O) were separated by this method.

** The mixture of gaseous isotopes is passed through a porous tube sealed in an outer jacket (see Fig blow).

** The lighter isotopes passes into the jacket, while the residual gas becomes richer in the heavier isotope.

** In actual practice a cascade of many ‘Diffusion units’ is used to achieve an appreciable separation.

** This process has been recently used for the separation of the gaseous fluorides 235UF6 and 238UF6. It provides a procedure for effective separation of the isotopes of uranium, namely, U-238 and U-235 (needed for atomic energy).


(2) Thermal Diffusion

** A long vertical tube with an electrical heated wire running down its axis is used.

** When a mixture of gaseous isotopes is introduced into the tube, the lighter particles diffuse more rapidly to the central hot region.

** Here they are carried upwards by convection currents.

** The heavier particles, on the other hand, travel to the cooler inner surface of the tube and sink to the bottom. 

** Thus the lighter isotope collects at the top and the heavier one at the bottom.

** The isotopes of chlorine Cl-35 and Cl-37, have been separated by this process.

** The fluorides of uranium have also been separated by thermal diffusion.


(3) Distillation

** The lighter isotope will be distilled over first, leaving the heavier one behind.

** The isotopes of mercury were separated by this method.

** The frozen mercury from the cooled surface is removed, melted and evaporated under vacuum again.

** The whole process is repeated several times to separate the isotopes of mercury.


(4) Ultracentrifuge

** The mixture of isotopes is rotated in a high speed centrifuge. The heavier isotope is concentrated at the periphery.

** The separation depends essentially on the molecular mass and not its square root, causing better separation.

** The gaseous fluorides of U-235 and U-238 have been separated by this method.

(5) Electromagnetic Separation

** This method uses the principle of the Mass Spectrograph (Dempster).

** For example, the beam of ions of the isotopes of neon (Ne-20, Ne-21, Ne-22) as obtained in the mass spectrograph, is then passed between the poles of a magnet.

** The different isotopes are deflected to different extents and are collected in cooled chambers placed in appropriate positions.

** Although the quantities obtained by this method are very small indeed, the separation is complete.


(6) Fractional Electrolysis

** Here the principle is that the rates of liberation of the isotopes of an element at an electrode during electrolysis are different. This is so because the ions of the heavier isotope move slower, while those of the lighter isotope move faster to the opposite electrode.

** Urey (1931) separated the two isotopes of hydrogen, H-1 and H-2, by the electrolysis of acidified water. H-1 (protium) is liberated five times as rapidly as H-2 (deuterium) at the cathode. The residual water becomes richer in heavy water or deuterium oxide 2H2O or D2O which upon further electrolysis yields gas richer in deuterium

(7) Laser Separation

** A laser is a very fine beam of electromagnetic radiation which consists of photons corresponding to a single wavelength, frequency or energy. All the waves in the beam are in phase with all troughs and peaks moving through space together.

** In recent years, the development of lasers has been used for the separation of isotopes.

** If the laser light is of the appropriate wavelength, one isotope will absorb the energy, while another isotope will not.

** The slight difference in absorption spectra of the two isotopes thus produced has been used to separate the more energetic isotope from the other.

** The laser method has been used successfully for the separation of isotopes of chlorine and sulphur. Potentially, laser isotope separation of uranium is 1000 times more efficient than gaseous diffusion separation.

Reference: Essentials of Physical Chemistry /Arun Bahl, B.S Bahl and G.D. Tuli / multicolour edition. .


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