Radiochemical analysis

The use of radioactive isotopes has been of great value in the development of analyses for low concentrations of materials. An isotope is a form of an element that has the same chemi­cal properties, but slightly different physical properties. A radioactive isotope is thus chem­ically identical with any corresponding stable isotope of the same element. It undergoes all the same chemical processes. It can also be followed by tracing its radioactive emissions. Often, only traces of the radioactive isotopes are needed. They can be incorporated into chemical compounds to produce "labeled" (measured) substances. These can be used and followed throughout in various reactions. The radioactivity at any step in a process can be measured. This establishes how efficient that step has been by the proportion of radioactiv­ity carried through to the next step.

Isotope dilution analysis is a special appli­cation of radioactivity. A known weight of a ra-dioactively labeled compound is added to a mixture containing an unknown amount of the same—but unlabeled (unmeasured)—com­pound. After complete mixing, a small amount of the mixture is removed. It is treated selec­tively for the particular chemical compound. By measuring the degree of dilution of the ra­dioactive isotope, it is possible to calculate the amount of nonradioactive compound.

In activation analysis, a nonradioactive sam­ple is irradiated (treated) with high-energy neutrons. This irradiation converts some of the

3r]⁺ (mass 94) -3r]+ (mass 96) ■

60-

3r+ (mass 81)

40"

⁹Br+ (mass 79)

20-

inactive atoms into radioactive isotopes. By measuring the amount of radioactivity pro­duced under defined conditions, it is possible to calculate how much of that element was originally present.

A thermal balancere­cords the loss in weight of a substance as its tempera­ture is gradually increased. A counterbalanced sample of the substance is sus­pended in a furnace and heated. A thermocouple de­tects the rise in tempera­ture. As the sample loses weight, the other end of the balance beam falls. This moves a shutter and allows

Thermal analysis

Modern thermal analysis includes various techniques. These techniques measure some physical property of a material (such as its weight) as its temperature is increased. The measurements are often carried out on very small samples. They can be used to give an idea of how substances will change over ex­tended periods under normal conditions of temperature.

Thermogravimetry

Thermogravimetry is the best-known form of thermal analysis. It consists of measuring the

light to fail onto a photo-multiplier. The signal from the photomultiplier is am­plified. It is then passed through a coil fixed to the pivot of the balance beam. The coil is also situated in a magnetic field. Thus, the current flowing in it causes it to rotate slightly. This twists the suspension, re­storing the balance and making the shutter interrupt

weight change of a sample as the temperature is steadily increased. The resulting weight/temperature graph is characteristic for the material being studied. Changes in weight usually occur sharply at specific temperatures. The changes correspond to the breaking of chemical or physical bonds. They are often as­sociated with the loss of volatile substances, such as water, carbon dioxide, or oxygen, from the molecules of the sample.

A thermal gravimetric graph provides a large amount of information from the sizes and shapes of the changes in the line. Straight lines left to right on the graph indicate no weight change and hence no decomposition. Slopes and curves up or down show that a weight change has taken place due to some material loss. The curve is also quantitative. This means that weight losses between one level and another correspond to losses of defi­nite parts of the molecular structure.

The measurement of thermogravimetry is carried out on a thermal balance, a special type of balance that can operate in furnace temperatures as high as 3000° F. (1650° C). The accuracy of the results depends very much on the use of small samples. These have to be well ground to a uniform particle size. Ther­mogravimetry is used to study the decomposi­tion of pure materials. It may also be em­ployed to determine the percentage composition of mixtures of two known sub­stances. The individual weight loss at each step on the curve can usually be related to one or the other of the two components.

Differential analysis

There are two other forms of thermal analysis. In differential thermal analysis (DTA), the mate­rial under examination is compared with a cor­responding amount of reference material. The reference material is usually alumina, a natural mineral that is also a source for aluminum. The two materials are heated together with a steady rise in temperature. As the heating progresses, any change in the sample results in the release or absorption of energy. This is measured as a temperature difference be­tween the sample and the reference. This dif­ference, which DTA measures, gives rise to a graph consisting of a series of peaks (the exo-therms) and troughs (the endotherms). The for­mer correspond mainly to chemical changes in compounds. The latter indicate physical changes in the crystalline structure or fusion of molecules. The widths of the peaks and troughs are also of importance in indicating the changes in the sample.

By contrast, differential scanning calorime-try (DSC) measures the difference in energy re­quired to keep both the sample and reference substances at the same temperature. For exam­ple, when an endotherm would occur on DTA, that heat difference in DSC is made up by the instrument adding more heat to the sample. In this case, the instrument measures the amount of extra energy that has to be added to either the sample or reference container. This is done in order to maintain both at the same temperature. In all cases, the shapes of the peaks obtained are affected by the rate of heating, the sample size, and even the shape of the sample containers.

Analytical chemistry: Thermal analysis 135

water in two stages. It first becomes the monohydrate, with one molecule of water (B). It then becomes the an­hydrous salt (C). On contin­ued heating, anhydrous copper sulfate decomposes to one type of copper oxide (D) and then another type (E). Thermal analysis,which records weight loss with ris­ing temperature, gives a graph (right) with "steps" corresponding to each of the reactions.

The equipment for DTA and DSC consists of an electrically heated solid metal block. This block contains two identical recesses for the sample and reference capsules. The tempera­tures of the two substances are measured as the block is heated. The whole system is oper­ated according to a preset temperature pro­gram. This is often controlled by a microcom­puter. Applications of thermal methods of analysis are expanding very rapidly. They have been used to determine deterioration in cer­tain types of cement containing alumina. Ther­mal methods can establish the best tempera­tures for making epoxy resins and for fusing mixtures of inorganic compounds.

Date: 2015-12-11; view: 1885