Separation of Isotopes

– In this subject, we will discuss the Separation of Isotopes by seven methods

Separation of Isotopes

– Since isotopes have exactly similar chemical properties, their separation by chemical means is out of the question.

– Their difference in those physical properties which depend on the mass of the atom has been utilized to effect their separation.

– The methods commonly employed for Separation of Isotopes 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 below).

– The lighter isotope 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 the 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 electrically 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.

(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.

(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 that 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 sulfur.

– 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 / multicolor edition.