The Rare Earths: Their Occurrence, Chemistry, and Technology

by Stanley Isaac Levy

[185] Ann. Chim. Phys. 1900, [vii.], 19, 184.

The Spectrum Examination

In no department of chemistry have the methods of spectrum analysis proved of more value than in the field of the rare earths. They provide the chemist with a means of following and controlling his processes of separation which is far more delicate and decisive than the older method of determining the equivalent weight. Whilst the examination of emission spectra, and especially of arc spectra, is of decisive value in every case, it has the disadvantage of requiring delicate and complicated apparatus and great experimental skill; wherever possible, therefore, the examination of the absorption spectra is preferred, though this is useful only for a few of the elements, and varies considerably with the conditions employed.

The Absorption Spectra.

—Absorption in the visible region of the spectrum is observed only with those rare earth compounds which are coloured, and is of value, therefore, chiefly for identification in the case of praseodymium and neodymium among the cerium elements, and of erbium among the yttrium metals; these give characteristic absorption bands, even in dilute solution. The absorption spectra of the rare earth compounds are highly characteristic, the bands being well defined and sharply bounded, whereas coloured compounds of the common elements show general absorption, or at best diffuse bands, under the same conditions.

In observing an absorption spectrum, the light from a Nernst lamp, or incandescent burner, is passed through a layer of a suitable solution of the coloured compound, of known concentration and thickness, and after collimation is analysed by a suitable prism; the spectrum is observed by a telescope in the ordinary way. Where accurate readings are not required, as, for example, in testing for the presence or absence of a particular element, the position of the bands may be read to a sufficient degree of accuracy by means of a scale, the image of which is adjusted to coincide with the spectrum as seen through the eyepiece; but in mapping a spectrum accurately, more refined methods must of course be used. The photographic method, in which a photograph of the spectrum is taken on a plate which bears, for purposes of measurement, a comparison spectrum of known lines, is very convenient for examining the absorption in the violet and ultraviolet regions.

The intensity, and to some extent also the position, of bands in an absorption spectrum may vary considerably, according to the conditions employed. Of the various factors which must be considered, the concentration of the solution, the thickness of the layer used, the nature of the solvent, and of the acid radicle, and the presence of other earths are the most important. The concentration of the solution, and the thickness of the layer, which together constitute the Optical Density, must be so adjusted that the absorption is neither too strong nor too weak; in the first case the sharp bands tend to merge into broad diffusion areas, and details are obscured, whilst in the second case the presence of coloured compounds which do not show strong absorption bands may be overlooked.

The nature of the acid radicle has considerable influence on the position of the absorption maxima, the general rule being that the bands are shifted towards the red end of the spectrum as the molecular weight of the compound used increases. Naturally, also, the nature of the solvent has an important effect, all the usual phenomena which must be considered in the measurement of the physical properties of substances in solution coming into play; electrolytic dissociation, hydration, dissociation and the formation of complexes, for example, are all important factors. The presence of colourless earths has also been found to cause important differences. It follows, therefore, that for the chemist, the absorption spectra can be considered as a valuable aid only in detecting the presence or absence of the three elements which give the strongest and most characteristic absorption bands, viz. praseodymium, neodymium, and erbium, and that conclusions regarding the quantitative composition of mixtures must be drawn with the utmost caution.

The Emission Spectra: Spark Spectra.

—The factors which tend to limit the value of the absorption spectra for analytical purposes, for the most part disappear when the emission spectra are employed. In the case of the spark spectra, indeed, great differences are observed according to the conditions and method of experiment; but the arc spectra are practically invariable under all conditions, and hence they constitute the ultimate test in all cases. The spark spectra are observed when one terminal—the cathode—of an induction coil is embedded in the oxides to be examined, and the discharge then passed. The discharge is also frequently passed between platinum poles partly immersed in a strong solution of a salt of the element under examination; a form of apparatus very suitable for this method of observation has been described by Sir W. Crookes.[186] The spectra so obtained are in a high degree characteristic, but they vary very considerably with the form and dimensions of the coil, the length and cross-section of the wires, the potential difference employed, and so on. An entirely new spectrum also is obtained in many cases by mere reversal of the current; under these conditions, a phosphorescent appearance is observed, the spectrum of which—reversed spark spectrum of de Boisbaudran—has been found in many cases to resemble the cathode luminescence spectra of Crookes.