Tests for Characteristic Elements

The results of tests for characteristic elements such as nitrogen, sulfur and halogens may serve to roughly indicate the nature of the unknown material, including the nature of the base polymer and additives, if present. The following tests may be performed for qualitative detection of N, S, and halogens. 

The elements are obtainable in a state of very high purity but some of their physical properties are nonetheless variable because of their dependence on mechanical history. Their colours (Cu reddish, Ag white and Au yellow) and sheen are so characteristic that the names of the metals are used to describe them. Gold can also be obtained in red, blue and violet colloidal forms by the addition of vtirious reducing agents to very dilute aqueous solutions of gold(III) chloride. A remarkably stable example is the Purple of Cassius , obtained by using SnCla as reductant, which not only provides a sensitive test for Au but is also used to colour glass and ceramics. Colloidal silver and copper are also obtainable but are less stable. 

Snow, especially its water-soluble fraction, is one of the most sensitive and informative indicators of mass-transfer in the chain air - soil - drinking water. Therefore analytical data on snow-melt samples were selected for inter-laboratory quality control. Inter-laboratory verification of analytical results estimated in all the groups have shown that relative standard errors for the concentrations of all the determined elements do not exceed (5-15)% in the concentration range 0.01 - 10000 microg/1, which is consistent with the metrological characteristics of the methods employed. All analytical data collected by different groups of analysts were tested for reliability and... 

Pu may be considered as an ideal tracer nuclide for these studies because of its decay characteristics. Its existence or absence in the early solar system can be considered as a crucial test for or against the theories of the synthesis of chemical elements in stars. 

Elements heated by a flame glow their characteristic color. This is commonly called a flame test and is used to test for the presence of an element in a sample. When viewed through a spectroscope, the color of each element is revealed to consist of a pattern of distinct frequencies known as an atomic spectrum. 

With these compounds the presence of the halogen will have been detected in the tests for elements. Most acid halides undergo ready hydrolysis with water to give an acidic solution and the halide ion produced may be detected and confirmed with silver nitrate solution. The characteristic carbonyl adsorption at about 1800 cm -1 in the infrared spectrum will be apparent. Acid chlorides may be converted into esters as a confirmatory test to 1 ml of absolute ethanol in a dry test tube add 1 ml of the acid chloride dropwise (use a dropper pipette keep the mixture cool and note whether any hydrogen chloride gas is evolved). Pour into 2 ml of saturated salt solution and observe the formation of an upper layer of ester note the odour of the ester. Acid chlorides are normally characterised by direct conversion into carboxylic acid derivatives (e.g. substituted amides) or into the carboxylic acid if the latter is a solid (see Section 9.6.16, p. 1265). 

There is also a standard test method for determination of major and minor elements in coal ash by inductively coupled plasma (ICP)-atomic emission spectrometry (ASTM D-6349). In the test method, the sample to be analyzed is ashed under standard conditions and ignited to constant weight. The ash is fused with a fluxing agent followed by dissolution of the melt in dilute acid solution. Alternatively, the ash is digested in a mixture of hydrofluoric, nitric, and hydrochloric acids. The solution is analyzed by (ICP)-atomic emission spectrometry for the elements. The basis of the method is the measurement of atomic emissions. Aqueous solutions of the samples are nebulized, and a portion of the aerosol that is produced is transported to the plasma torch, where excitation and emission occurs. Characteristic line emission spectra are produced by a radio-frequency inductively coupled plasma. A grating monochromator system is used to separate the emission lines, and the intensities of the lines are monitored by photomultiplier tube or photodiode array detection. The photocurrents from the detector... 

The characteristic of the halides of the elements of the lithium group is that they are all soluble white salts, crystallising in cubes. In dilute solution they are all ionised, and even in strong solution a large percentage of ions are present. Hence they all react as metal ions and as halogen ions. Thus, for instance, with silver nitrate, which is the usual test for ionic chlorine, the following reaction takes... 

Purity tests for salts are much more problematic than are purity tests for solvents. The only way to follow the salt purity rigorously is by careful element analysis by a combination of methods, which include atomic absorption, ICP (in solutions), and, to some extent, MS. Water contamination can be detected relatively easily by IR spectroscopy. All the above-mentioned salts have characteristic IR spectra in which the hydration water has specific bands. 

Powder characteristics of a physico-chemical nature, such as solubility, wettability, dispersability, and measures of the instant properties of powders (sludge, slowly dispersible particles, hot water sediment and coffee test), are determined by a variety of empirical physical tests, some of which incorporate subjective elements. These and the other tests for whole milk powders identified above are described in detail by Westergaard (1994) and Pisecky (1997). 

---