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Spectrum of Arsenic

The line emission spectrum of arsenic has been the subject of much investigation throughout the whole range of radiations accessible to photography and under varying conditions of excitation. It is characterised by a great number of lines and is difficult to analyse because many important lines lie in the far ultraviolet or in the infra-red.

hartleys apparatus
Hartley's Apparatus
In the arc spectrum there are no lines in the visible region, but many occur between λλ 3000 and 2000 A. The method employed to obtain the arc spectrum is usually to place metallic arsenic in the hollowed ends of carbon or metal poles. Thus Rao used arsenic contained in carbon or aluminium poles and photographed the spectrum between 8800 and 1370 A.; he used the arc in nitrogen for investigating the Schumann region down to 1650 A., and below this the arc in vacuum between carbon poles containing arsenic, and the spark between metallic arsenic electrodes in hydrogen; later the arc in helium and neon enabled him to obtain measurements from 2800 to 500 A. Meggers and de Bruin vaporised arsenic in graphite or copper arcs and by means of concave grating and quartz prism spectrographs measured wavelengths ranging from 10,023.98 to 1889.85 A. Hartley, using solutions of arsenious chloride to moisten graphite electrodes, arranged as shown in fig., found the following to be the most persistent lines:

Solution AsCl3

1 per cent2859.72779.52350.12288.9
0.1 per cent2779.5

Using the condensed spark, de Gramont gave the following as the raies de grande sensibilite, those marked with an asterisk being the raies ultimes:

2860.5; 2780.2*; 2745.0; 2349.8*; 2288.1

Rao observed a strong ultraviolet triplet at 1972.6,1937.7 and 1890.5 A., which he stated constitutes the raies ultimes of the whole emission spectrum.

The spark spectrum exhibits a great many lines throughout the whole range of wavelengths. It includes the lines of the arc spectrum, but the relative intensities are different. Many of the lines have fine structures, and careful analysis of these has made possible a classification of a number of the lines according to whether they belong to the neutral atom (As I), to an atom simply ionised (As II), doubly ionised (As III) or in a higher state of ionisation (As IV, As V, etc.), and the electronic configurations and transitions involved have been derived. The classification has been facilitated by photographing the spectrum obtained from an electrodeless discharge in arsenic vapour. Gartlein photographed the spark spectrum using various amounts of inductance in series with the spark and observed the presence of As II lines even with large amounts of inductance, but increase in the inductance resulted in a decrease in the intensity of lines from the higher states of ionisation; the spectrum was also characterised by " long lines " due to glowing arsenic vapour. The nuclear moment of the arsenic atom has been calculated to be 3/2.

The table gives the more intense lines of the emission spectrum, the wavelengths given being the weighted mean values derived from the most reliable data available. The lines present in the arc spectrum and those due to the un-ionised atom are indicated. The scale of relative intensities is an arbitrary one - 1 weakest, 10 strong, 15, 20, etc. very strong.

Most of the lines in the arc spectrum are easily reversed. In order to differentiate the arc and spark spectra Buffam and Ireton used an under-water oscillatory condenser discharge with a suitable condenser capacity in the circuit; the spectra were produced between poles of metallic arsenic in a vessel through which water circulated continuously, and were photographed by means of Hilger spectrographs. The arc lines were inverted on a dark continuous background, while the spark lines were not.

It has been found that in spectral analytical investigation the interrupted and flaming arc methods are respectively 100 and 10 times more sensitive than the condensed spark method. As little as 2×10-5 mg. of arsenic can thus be detected in solid alloy electrodes and, in solutions free from heavy metals, 0.01 per cent, of arsenic can be detected. Micro-methods for the spectrographic determination of small amounts of arsenic have been described.

The emission spectrum of arsenic vapour shows a group of bands, attributable to As2 molecules, in the region 2700 to 4200 A., whilst several bands in the region 2148 to 3047 A. appear to be due to As atoms. The extinction coefficient of arsenic vapour at various temperatures and over the range 3000 to 3900 A., has been measured.

Emission spectrum of Arcenic

Wavelength λ(Å)Relative Intensity Spark.#
I 3119.67/4
I 3075.325/2
I 3032.848/4
I 2990.994/2
I 2898.7310/4R
I * 2860.468/4R
I * 2780.2310/8R
I 2745.005/6R
I 2381.205/4R
I * 2349.846/10R
I * 2288.143/10R
* 19724 R
* 1936.95
* 1889.94 R
I = Lines emitted by neutral atom. R = Easily reversed. * = Most persistent lines. # -Spark/Arc intensities if any

The Zeeman effect of arsenic spectra has been studied, and wavelengths, classifications and Zeeman patterns have been determined for 11 lines in As I, 64 lines in As II and 2 lines in As III.

A fluorescence spectrum of arsenic vapour, after exposure to a mercury lamp at a high temperature has been observed.

The absorption spectrum of arsenic vapour has been examined by passing the light from the arc or spark of arsenic through the non-luminous vapour. Eighty absorption bands between 2200 arid 2750 A. have been enumerated in the spectrum obtained by passing ultraviolet light through the vapour heated to 1100° C., and can be assigned to the diatomic molecule.

The ultraviolet absorption spectrum of arsine gives an absorption limit at 2390 A. There appears to be a definite gradation in the absorption spectra of the hydrides of nitrogen, phosphorus, arsenic and antimony; the ammonia spectrum has well-defined predissociation bands, that of phosphine weak ones, while arsine and stibine show only continuous absorption. The infra-red absorption spectra of the first three of these gases are similar in that each has two sequences of harmonic oscillation bands, but arsine and phosphine also exhibit a third sequence peculiar to themselves. Thus while the structure of the three molecules appears to be essentially similar, each possesses distinctive features.

The Raman spectra of arsenious chloride, in the liquid and gaseous states, of "light" and "heavy " arsine and of sodium arsenite and sodium arsenate, have been examined and frequencies obtained. The Raman spectra of the chloride and bromide in solution in ether or benzene consist of the spectra of the pure solute and pure solvent only, indicating that chemical combination does not occur in the solution. With solutions in methyl and ethyl alcohols, the frequencies of the latter are unchanged, but those of arsenious chloride are lowered somewhat.

The X-ray and series spectra, and also the β-ray spectrum of radioactive arsenic, have been examined.

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