ACTION OF LIGHT ON SULPHUR BY G. A. RANKIN
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Previous Work It was first discovered by M. A. Lallemandl that, when sunlight concentrated with a lens is passed through a saturated solution of sulphur in carbon bisulphide, amorphous sulphur is precipitated as a fine powder along the path of the light rays. On analyzing the light after it passes through the solution it was found that all the light of the spectrum from A to G is transmitted while that from G through the ultraviolet is absorbed by the solution. Lallemand concluded that the work necessary to transform the rhombic sulphur into the amorphous state is done by the light absorbed. Berthelot2 verified the results of Lallemand and found further that the arc light, which contains a large amount of ultraviolet light, could be used to replace the sunlight. He also found that the presence of hydrogen sulphide in the solution prevented the precipitation of the amorphous sulphur, and that when some of the amorphous sulphur was shaken with an alcoholic solution of hydrogen sulphide, it went over to the soluble form. Berthelot determined the heat of reaction when rhombic sulphur was transformed into the amorphous state. He found that a small amount of heat was evolved, and concluded that the light acted simply as an exciting agent and did not effect the work of transformation. Petersen3 obtained an evolution of heat of 910 calories, but there seems to be a question whether his amorphous sulphur was the same as Berthelot's. The question now arises as to what effect the use of solvents other than carbon bisulphide, the variation of concentration, temperature, and intensity of light would have on a
Comptes rendus, 70, 182 (1870). Ibid., 70, 941 (1870). Zeit. phys. Chem., 8, 611 (1891).
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the precipitation of the amorphous sulphur? In the present paper some attempt has been made to answer this.
Verification of Previous Work A saturated solution of sulphur in carbon bisulphide containing an excess of sulphur was placed in the sunlight. The amorphous insoluble sulphur precipitated and the excess of soluble sulphur began t o dissolve. After some time it all dissolved and the solution became unsaturated with respect to the soluble sulphur, due to the amount of insoluble sulphur thrown out of solution. To show that only light of the violet or blue end of the spectrum effected the precipitation of the amorphous sulphur the following test was made : Two bottles were taken, each containing a saturated solution of sulphur in carbon bisulphide. One was placed in a beaker containing a- solution of potassium bichromate, which absorbs all the light of the spectrum from the violet t o the light green, while the other bottle was placed in a similar beaker containing a solution of copper sulphate which absorbs all the light of the spectrum from the red to the light green. On placing them under the direct action of the sunlight, it was seen that the oottle in the copper sulphate solution was immediately filled with a cloud of amorphous sulphur while the other bottle remained perfectly clear. This shows clearly that it is the violet light which causes the precipitation. Influence of Concentration As the concentration of sulphur in carbon bisulphide is varied, the intensity of light necessary for precipitation varies. Thus with a very dilute solution the intensity of light required is very great in comparison with the intensity required for a saturated solution, the temperature being constant. Influence of Temperature If the concentration is held constant and the temperature varied, it is found that the intensity of light required for precipitation of the amorphous sulphur increases with rising temperature, .
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Influenee of Solvent If amorphous sulphur separates pure from a solution, we are dealing with the displacement of equilibrium between two solid phases, rhombic sulphur and amorphous sulphur. The nature of the solvent can therefore have no effect. When light was allowed to act on solutions of sulphur in such solvents as toluene, benzene, carbon tetrachloride, and acetone, a precipitation of amorphous sulphur was obtained in every case. Berthelot’s contention that light acts only on dissolved sulphur is wrong. The reaction takes place more rapidly in the presence of a solvent because no surface film is formed. The Reverse Reaction The reaction between rhombic sulphur and amorphous sulphur, R. S. E+ A. S. takes place only under the action of light. If it is a reversible one, the reverse reaction A. S. E-+ R. S. would take place only in total darkness or very feeble light. Such was found to be the case. A thin coating of amorphous sulphur was precipitated from solution on the sides of a vessel. The vessel was then placed in the dark for twelve hours, after which time the amorphous sulphur had R. S. entirely disappeared, showing that the reaction A. S. had taken place. We may then represent this reversible light reaction thus : R. S. (Dark) A. S. (Light) The rhombic,sulphur is the stable form in the dark while the amorphous insoluble form is stable in the light. If we add to the solution some substance which accelerates sufficiently the rate of the reaction A. S. E-+ R. S., no precipitation of amorphous sulphur will occur even in bright sunlight. Berthelot found such a substance in hydrogen sulphide which, however, does not prevent precipitation of amorphous sulphur completely in the most intense light. It has been shown by Professor Smith, of Chicago,l that hydrogen sulphide and ammonium hydroxide increase the rate a t which
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Jour. Ani. Chem. SOC.,27, 993 (1905).
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fused amorphous sulphur changes into the other modification, and it was to be expected that a similar catalytic action would be found a t lower temperatures and in presence of a solvent. Following out this idea, experiments were also made with ammonia gas. As this reacts with carbon bisulphide, another solvent had to be used. When a solution of ethyl alcohol saturated with ammonia gas was shaken with flowers of sulphur, which had been extracted with carbon bisulphide, the amorphous sulphur went over into the soluble state in a very short time. On crystallizing from carbon bisulphide rhombic sulphur was obtained. The displacement of the equilibrium is more marked with ammonia than with hydrogen sulphide. Equilibrium Diagram: After the above preliminary work had been done, it was thought that it might be interesting to determine with some accuracy the conditions of concentration, temperature and light intensity at which the precipitation of the amorphous sulphur first takes place. Since the light beyond G can be treated as a new independent variable, it must, be possIble to obtain one isothermal curve for the solubility of rhombic sulphur and another for the solubility curve of aniorphous sulphur. At the intersection of these two curves we should have rhombic and amorphous sulphur co-existing in stable dynamic equilibrium with solution and vapor. An electric arc focused with a system of lenses was used as the source of light. It answered the purpose better than any other source obtainable, as it contains large amounts of violet and ultraviolet light and can be kept constant within certain limits, The average intensity of the focus was found to be 1600 candle-power. To determine the intensity of the light at any point beyond the focus, the diameter of the beam of light at that point was measured, and knowing that the intensity varies as the square of the diameter of the beam of light, i t can be determined. Table I gives the values for the intensity of the light as calculated for various distances from lens. From
these data a curve was plotted as shown. The focus was cm from the lens.
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'40
70
Fig.
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80
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TABLE I Distance from lens Intensity in candlepower per sq. cm. cm
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36 39 44 56.8 74 84
109
76.3 44.6 I 7.0 8.0
6.5
In performing the experiment the temperature was maintained constant while the concentration and light intensity were varied. Various concentrations of sulphur in carbon bisulphide were made up and the intensity of light a t which each first started to precipitate was determined. By working in this way, we eliminate all difficulties as to the intensity of the light in the interior of the flask. Data for two different temperatures were obtained and are given in Tables I1 and 111. From the above data two curves were plotted, ordinates being taken as intensities of light and abscissas as grams of sulphur per hundred grams of carbon bisulphide. The values of the ordinates on the left refer to the 22.5' curve; those on the right t o the 40' curve.
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Fig. 2
TABLE I1 Solvent : Carbon Bisulphide Light : Electric Arc Temperature, 22.5' C Intensity in candle-power
5 .O 6.4 6.4 10.0
10.4 11.0 12.2 23.2
Grams sulphur per IOO grams carbon bisulphide
44.7 37.2 15.0 12.0 11.2
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8.0 4.8 3.0
TABLE 111 Temperature, 40' 70.8
8 .o
45.8
49.6
Along the curve xy amorphous sulphur is in equilibrium with the solution and vapor. The dotted line xz is the solubility curve for rhombic sulphur, it being taken for granted that the solubility of the rhombic sulphur is not affected by the light, A t the point x rhombic sulphur, amorphous sulphur, solution and vapor are in dynamic equilibrium. In the field above the curve, amorphous sulphur is the stable form while below the curve rhombic sulphur is the stable form.
The light necessary to maintain amorphous sulphur in equilibrium with the solution increases as the concentration decreases until finally the curve approaches the line of ordinates asymptotically as the concentration approaches zero. As the concentration increases, the light necessary for equilibrium decreases until the point is reached, at which, for a given temperature, the solvent is saturated with respect to both forms of sulphur. This quadruple point for a temperature of 22.5' is not at the point of total darkness as can be seen from the curve 6 u t a t an intensity of about 5 candlepower. As the temperature for any given concentration is increased, the intensity of light required for equilibrium increases rapidly, showing that the rate of change of the amorphous sulphur increases with a rise of temperature. At 40' it takes about 45 candle-power to produce the first precipita; tion of amorphous sulphur from a saturated solution. No attempt has been made to calculate a formula for either curve. The values for the intensity of the light are only approximate a t best and refer to the total light, whereas the effective light is that beyond G. Summary (I) Amorphous insoluble sulphur is precipitated from solutions of rhombic sulphur by the action of violet and ultraviolet light. ( 2 ) As the concentration increases the intensity of light required for precipitation decreases. (Temperature constant.) (3) If the concentration remains constant and the temperature rises, a more intense light is necessary to cause precipitation. (4) Tbe reaction is a reversible one, the direct action being favored by the light, the reverse by the dark. R. S. (Dark) A. S. (Light) (5) Ammonia and hydrogen sulphide tend to accelerate the reverse action and to prevent precipitation even in bright sunlight,
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(6) Two isothermal solubility curves were obtained, which give the equilibrium conditions of concentration and
intensity of light for carbon bisulphide solutions of sulphur at 22.5' and 40' C. (7) Amorphous and rhombic sulphur are in equilibriuni when acted on by a 5 candle-power light at 22.5' and by a 45 candle-power at 40'. (8) All the phenomena are covered by the phase rule classification provided we treat the light as a new, independent variable. The work was suggested by Professor Bancroft and carried on under his supervision. Cornell University, August, 1906.
