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13 Radiochemical Separation Techniques
13.3.1.3 Hold-Back Carrier
Hold-back carrier can be either isotopic or non-isotopic type. Coprecipitation of
unwanted radioactive species during the precipitation is sometimes reduced by diluting them with a stable isotope of the same element, in the same chemical form. For
example, precipitation of radioactive isotope
131 I while precipitating
35 S as barium
sulfate from a mixture containing
131 I- and
35 S-ions is prevented by adding 0.1M KI
into the mixture before carrying out the precipitation. Potassium iodide holds back
131 I in the solution while Ba
35 SO 4 is precipitated. Such type of carriers are called as
“hold-back” carriers.
A combination of the hold-back carrier with a scavenger works as an artificial filter
paper that can remove micro-concentration of impurities, which have no carrier, from
a micro-component. However, although the radiochemical purity of the nuclide is
considerably increased by this procedure, its specific activity is lowered.
13.3.2 Condition for Effective Use of Carrier
Carrier can only be effective if the chemical nature of the carrier is the same as that
of the tracer. This applies to both the isotopic and non-isotopic carriers. For example,
iodide ions can be exchanged for iodide ions only and not for iodate ions. Hence,
if there is uncertainty in the chemical state of the tracer present in the sample, it is
always better to add the carrier irrespective of its valence state and then carry out
three or four times the oxidation and reduction processes of the sample after adding
the carrier, so that all the substance with tracer come back to its one chemical state.
13.4 Solvent Extraction
In solvent extraction, the substance to be separated is mixed with two immiscible
solvents (normally organic solvent with aqueous solvent) and shaken vigorously
for some time and then two layers are allowed to settle. The radioactive isotope
gets distributed between the two solvents. Since the distribution coefficient for the
radioactive isotope with two solvents is different, one of the solvents will get concentrated with the radioactive isotope leaving behind other constituents in other solvents.
Seaborg and Graham have shown that the micro-components distribute themselves
between the two immiscible solvents with the same value of partition coefficient as
was present in macro-amounts. Hence, the separation by this technique can be carried
out without any use of the carrier. This method can be applied to almost all soluble
substances, organic or inorganic, and can be made quantitative and often remarkably
specific. Solvent extraction methods are useful in radiochemical separations because
of the ease and speed of manipulation.
13 Radiochemical Separation Techniques
13.3.1.3 Hold-Back Carrier
Hold-back carrier can be either isotopic or non-isotopic type. Coprecipitation of
unwanted radioactive species during the precipitation is sometimes reduced by diluting them with a stable isotope of the same element, in the same chemical form. For
example, precipitation of radioactive isotope
131 I while precipitating
35 S as barium
sulfate from a mixture containing
131 I- and
35 S-ions is prevented by adding 0.1M KI
into the mixture before carrying out the precipitation. Potassium iodide holds back
131 I in the solution while Ba
35 SO 4 is precipitated. Such type of carriers are called as
“hold-back” carriers.
A combination of the hold-back carrier with a scavenger works as an artificial filter
paper that can remove micro-concentration of impurities, which have no carrier, from
a micro-component. However, although the radiochemical purity of the nuclide is
considerably increased by this procedure, its specific activity is lowered.
13.3.2 Condition for Effective Use of Carrier
Carrier can only be effective if the chemical nature of the carrier is the same as that
of the tracer. This applies to both the isotopic and non-isotopic carriers. For example,
iodide ions can be exchanged for iodide ions only and not for iodate ions. Hence,
if there is uncertainty in the chemical state of the tracer present in the sample, it is
always better to add the carrier irrespective of its valence state and then carry out
three or four times the oxidation and reduction processes of the sample after adding
the carrier, so that all the substance with tracer come back to its one chemical state.
13.4 Solvent Extraction
In solvent extraction, the substance to be separated is mixed with two immiscible
solvents (normally organic solvent with aqueous solvent) and shaken vigorously
for some time and then two layers are allowed to settle. The radioactive isotope
gets distributed between the two solvents. Since the distribution coefficient for the
radioactive isotope with two solvents is different, one of the solvents will get concentrated with the radioactive isotope leaving behind other constituents in other solvents.
Seaborg and Graham have shown that the micro-components distribute themselves
between the two immiscible solvents with the same value of partition coefficient as
was present in macro-amounts. Hence, the separation by this technique can be carried
out without any use of the carrier. This method can be applied to almost all soluble
substances, organic or inorganic, and can be made quantitative and often remarkably
specific. Solvent extraction methods are useful in radiochemical separations because
of the ease and speed of manipulation.