FLAME REACTIONS : SELENIUM AKD TELLURIUM I N T H E HYDROGEN-AIR FLAME BY JACOB PAPISH
In a recent paper by the author1 experiments were described in which elementary selenium and tellurium were precipitated from a Bunsen flame charged with the oxides of these elements. From these experiments it has been concluded that the characteristic flame reactions of selenium and tellurium are due to the existence of the free elements in the flame. I n order to throw more light on the nature of these reactions, it was deemed desirable to extend the experimental work to the behavior of some of the compounds of selenium and tellurium, and of the elements themselves, in the hydrogenair flame. Selenium Dioxide.-This substance, prepared as described elsewhere12was placed in a hard-glass tube provided with a platinum tip. Hydrogen, generated from zinc and sulphuric acid and washed through a solution of silver nitrate, was passed through the tube and ignited above the platinum tip. The selenium dioxide was now heated with a flattened Bunsen flame. After a short while the flame of the burning hydrogen, which had become charged with vaporized selenium dioxide, was seen to consist of a deep blue inner zone surrounded by a pale blue middle zone. The outer zone was greenish. But when the selenium dioxide in the hard-glass tube was heated more highly, thus causing more selenium to volatilize, the outer zone became red in color. No selenium was deposited on the cold object when the outer zone was depressed. The red variety of selenium was obtained when the cold object was introduced in the middle Jour. Phys. Chem., 22, 430 (1918). Ibid., p. 431, The method of preparing tellurium dioxide is also given on the same page. , Elementary selenium and tellurium used in the following experiments were prepared by dissolving the purified oxides in hydrochloric acid and precipitating the elements with sulphur dioxide. They were then thoroughly washed and fused. 2
Flame Reactions
641
and in the inner zones. Vapors of selenium dioxide were seen to escape from the outer zone of the flame. The selenium dioxide, which had been volatilized in a current of hydrogen, was not reduced by the hydrogen to such an extent as to allow free selenium to enter the hydrogenair flame. The mass which condensed just beyond the space in the glass tube where the selenium was heated, was contaminated very slightly with amorphous selenium, indicating slight reduction, but i t was followed by a ring of pure crystalline selenium dioxide. When the flame above the platinum tip was blown out without interrupting the heating of the selenium dioxide in the glass tube, fumes of selenium dioxide were seen to escape. These fumes were allowed to condense in a long glass tube and were found to consist of pure selenium dioxide, as the deposit was not discolored by any of the varieties of selenium. The purity of the deposit also tends to prove that no hydrogen selenide found its way into the flame. Tellurium Dioxide.-When the experiment just described was repeated with tellurium dioxide in the place of selenium dioxide, the flame was seen to consist of the following: A slender inner zone, green in color, and surrounded by a lilaccolored middle zone. This lilac zone was surrounded by a green outer zone, which constituted the upper half of the flame. Vapors, undoubtedly tellurium dioxide, were seen to escape from the green zone of the flame. On the introduction of the cold object in the outer zone no deposition of tellurium took place. But when the cold object was introduced in the middle zone and especially into the green inner zone, a voluminous deposit of tellurium in the form of a dark mirror was obtained. No conclusions can be drawn from the above experiments as to the behavior of tellurium dioxide in the hydrogen-air flame, because the tellurium dioxide was readily reduced when heated in the current of hydrogen. The substance that entered the flame and imparted to i t the colors described was in the main elementary tellurium. Hydrogen Selenide.-The introduction of hydrogen selenide
Jacob Papish
642
in the hydrogen-air flame reminds one of Marsh’s test for arsenic and antimony. Selenium dioxide when introduced in the hydrogen generator is reduced very readily by the nascent hydrogen according to the equation : SeOz
+ 4H
=
2H2O
+ Se,
but the quantity of hydrogen selenide formed is negligible, although it can be detected by the characteristic odor and by the colored flashes it imparts to the hydrogen-air flame. Por this reason i t was found necessary to mix hydrogen and hydrogen selenide, the latter generated from aluminum selenide and water, and burn the mixture as it was issuing from a glass tube provided with a platinum tip. The details of the experiment are as follows: Aluminum selenide was prepared by heating, in a covered porcelain crucible, a mixture of 5 parts by weight of finely powdered aluminum and 2 2 parts of powdered selenium. The reaction, which is exothermic in nature, takes place energetically. FonzCs-Diaconl suggests the use of a magnesium ribbon to start the reaction. This was found satisfactory for small quantities of the mixture, but in the case of larger quantities the reaction was far from complete. As aluminum selenide is very readily acted upon by moisture, i t was sealed off in glass tubes for subsequent use. Hydrogen, generated from zinc and sulphuric acid and washed through a solution of silver nitrate, was allowed to bubble briskly through an Erlenmeyer flask of 500 cc. capacity half filled with water. This flask, fitted with a three-hole rubber stopper, was provided with a safety tube of 8 mm inner diameter and with a platinum-tipped glass tube for the burning of the escaping gases. The apparatus is no more than a modification of the kind used for the production of the “philosopher’s candle.” The stream of hydrogen was adjusted and ignited a t the point of escape. The result was the ordinary, barely visible, hydrogen flame. Coarsely granuComptes rendus, 130, 1314 (1900).
Flame Reactions
643
lated aluminum selenide was introduced into the Erlenmeyer flask through the safety tube. Shortly after, the flame above the platinum tip assumed a characteristic color and was seen to consist of three distinct zones. The inner zone, which was comparatively short and slender, was very faintly blue. The middle zone, comprising the main portion of the flame, was deep blue, and the outer zone surrounding it was greenish. When the outer zone was depressed with the cold object no deposit of selenium was obtained, but the middle blue zone furnished large quantities of amorphous selenium when subjected to the same treatment. The structure of the flame can be very strikingly illustrated by depressing the flame longitudinally and horizontally with a glazed paper. The image thus obtained will be seen t o consist of a colorless inner cone, of a deep red middle cone, and of a faint red outer cone. Colorless, deep red and pale red rings are obtained when the flame is depressed horizontally. From this i t is to be inferred that the inner zone contains the unburned hydrogen selenide, while the middle zone is rich in elementary selenium. Hydrogen Telluride.-Aluminum telluride, which was used as the source of hydrogen telluride, was prepared by fusing together a mixture of powdered aluminum and tellurium in proportion of I .6 of the former to 11 of the latter. The combination between these elements takes place with explosive violence, and for this reason, small portions of the mixture should be fused a t a time. The apparatus was set up as in the case of hydrogen selenide, and the coarsely granulated aluminum telluride was introduced into the Erlenmeyer flask through the safety tube. As hydrogen telluride is still more unstable than the corresponding selenide, a large quantity of i t was decomposed with the precipitation of tellurium before the flame became colored. At last the flame was seen to consist of a slender inner zone, sky-blue in color, surrounded by a lilac zone. The outer zone which formed the upper half of the flame was green in color. When the cold object was introduced into the outer green zone, no deposition of
644
Jacob Papish
tellurium took place. But when the middle zone or the blue inner zone was depressed, a bright metallic mirror of tellurium was deposited on the cold object. Selenium.-Selenium was heated in a stream of hydrogen and introduced in the hydrogen-air flame. The inner zone of the flame became blue, while the surrounding middle zone assumed a pale violet color. The outer zone which was not very distinct, except a t the top, was greenish, while white fumes, undoubtedly selenium dioxide, were seen to escape from the flame. When the flame was extinguished without interrupting the heating of the selenium, a slight odor of hydrogen selenide could be detected from the escaping gases. This indicates that in addition to free selenium some hydrogen selenide found its way into the flame. The formation of hydrogen selenide in this way is in accordance with the investigations of Pklabonl and of Bodenstein. No deposit of selenium was obtained when the greenish outer zone was depressed with the cold object, but a voluminous deposit of amorphous selenium was obtained from the middle and inner zones. Tellurium.-When the preceding experiment was repeated with tellurium in the place of selenium the following results were obtained: The inner zone was green; the middle zone was lilac; and when more tellurium was volatilized by applying higher heat, this zone was seen to be resolved into a red zone surrounded by a blue zone. On extinguishing the flamk a slight odor of hydrogen telluride was noticed. The results obtained on depressing the flame with the cold object were as follows: No tellurium was obtained from the outer green part of the flame. A mirror of tellurium was obtained when the middle and inner zones were depressed. A summary of the results is given in the following table : Comptes rendus, 118, 142; 119, 73 (1894); (1897).
Zeit. phys. Chem., 29, 429 (1899).
121, 401
(1895); 124, 360
Flame Reactions
-I
Substance volatilized
I
1
Inner zone
1
Middle zone
Outer zone
Deposit Deposit. Lumineson cold Luminescence on cold cence object object
Luminescence
SeO2 Deep blue HzSe Very faint blue Blue Se TeOz Green HzTe Blue Green
645
-I
Se None Se Te Te Te
I
Pale blue Deep blue Violet Lilac3 Lilac Lilac4
-
I
l
Se Se Se Te Te Te
l
Deposit on cold object
-I
Greenish1 Greenish Greenish2 Green Green Green
None None None None None None
Becomes red if much selenium dioxide is volatilized. Occasional red flashes. * Accompanied by red flashes. Also red surrounded by blue when much tellurium is volatilized.
1 2
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Jacob Papish
the dioxide in the current of hydrogen, and for this reason the only substances to be considered are tellurium and hydrogen telluride. Here again processes of dissociation, reduction and oxidation are back of the luminescences of the different zones. Department of Chemistry Purdue University Lafayette, Indiana
