Uses of Artificial Isotopes
A very large number of isotopes which do not occur naturally can now be made fairly readily by neutronThe uncharged particle in an atomic nucleus; its mass is similar to the mass of a hydrogen atom. capture using an atomic reactor or a laboratory neutron source. Many of these artificial isotopes have proved very useful in chemistry since they provide a way of identifying atomsThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume. from a particular source, a technique known as labeling or tracer study. This technique is particularly easy to use if the isotope is radioactiveDescribes a substance that gives off radiation‐alpha particles, beta particles, or gamma rays‐by the disintegration of its nucleus.. Thus, for example, if a small quantity of the radioactive isotope 131I is added in the form of iodide ionAn atom or covalently bonded set of atoms that carries an overall net charge. to a saturatedDescribes 1) a solution that contains the equilibrium concentration of a solute, or 2) an organic compound that contains no double or triple bonds (such as an alkane). solutionA mixture of one or more substances dissolved in a solvent to give a homogeneous mixture. of lead iodide, one soon finds that the solidA state of matter having a specific shape and volume and in which the particles do not readily change their relative positions. lead iodide in contact with the solution, as well as the solution, become radioactive. This clearly demonstrates that the solution equilibriumA state in which no net change is occurring, that is, in which the concentrations of reactants and products remain constant; chemical equilibrium is characterized by forward and reverse reactions occurring at the same rate.
- PbI2 Pb2+ + 2I– (1)
is a dynamic process involving the constant interchange of iodide ions between the solution and the solid.
Tracer studies are also possible with isotopes which are not radioactive. The isotope 18O is often used in this way, since no convenient radioactive isotope of oxygen is available. In naturally occurring oxygen the isotope 18O is only 0.2 percent of the total. If extra 18O is added, its presence can be detected by massA measure of the force required to impart unit acceleration to an object; mass is proportional to chemical amount, which represents the quantity of matter in an object. spectrometry. An interesting and important example of the use of 18O is in the study of photosynthesis. If the water in this reaction is enriched with 18O, then the isotope is found in the oxygen produced:
6CO2(g) + 6H2O(l) → C6H12O6(s) + 6O2(g) (2)
By contrast, if the carbon dioxide is enriched with 18O, none of this enrichment appears in the oxygen produced. Another example of the use of 18O comes from inorganicPertaining to the chemistry of elements other than carbon and compounds containing at most a small amount of carbon. chemistry. It is the reaction between the sulfite ion and the chlorate ion in aqueousDescribing a solution in which the solvent is water. solution:
By labeling the oxygens in the chlorate ion, it is found that all the 18O lost from the one species is gained by the other and none of it is transferred to the solventThe substance to which a solute is added to make a solution. water. The mechanism of this reaction must thus be a direct transfer of oxygen and is quite unrelated to the two half-equations we conventionally use when balancing the redox equation:
- SO32– + H2O → SO42– + 2H+ + 2e–
2H+ + 2e– + ClO3– → ClO2– + H2O
Neutron Activation Analysis
An important use of radioisotopes in detecting small amounts of certain elements in a sample is neutron activation analysis. The sample being analyzed is irradiated by a neutron source. Nuclei of the element being analyzed capture neutrons, and an unstable nucleusThe collection of protons and neutrons at the center of an atom that contains nearly all of the atoms's mass. is formed which emits a γ ray. Since the wavelengthThe distance between the crests of adjacent waves (or between any adjacent corresponding points in waves); used in the context of electromagnetic radiation. of this γ ray is characteristic of the element, it can be distinguished from other elements in the sample. This method of analysis has the advantage of being nondestructive. The sample being analyzed is scarcely altered by being irradiated. Neutron activation is also among the most sensitive of analytical techniques. As little as a pictogram (10–12 g) of arsenic, for example, can be detected. This is about 10 000 times more sensitive than Marsh’s test—so often used by the fabled detective Sherlock Holmes. Neutron activation analysis is used by many modern detectives to find evidence of air and water pollutionThe contamination of the air, water, and earth by personal, industrial, and farm waste. as well as the types of crimes with which Holmes was involved.