PHOTOCHEMISTRY ,4ND THE PHASE RULE. BY WILDER D. BANCROFT
Draper’ was the first to point out that only those rays, which are absorbed by a substance, can produce a chemical change in that substance. It does not follow from this that chemical change is produced by all rays which are absorbed. There appears to be a conversion factor for the change of radiant energy into chemical energy and we know practically nothing about the conditions determining the value of this conversion factor. For cases of reversible equilibrium we can predict the direction of the change. Under the influence of light of a given wave-length, we shall have formed the system having the lower conversion factor for the light in question. A special case is the one in which the new compound or compounds do not absorb the particular wavelengths which are acting on the system. This conclusion has already been formulated in other terms a number of times’ and its truth is self-evident. We are dealing with an application of the theorem of Le Chatelier. ,4n interesting experimental confirmation of this has recently been given by Regenere3 Ozone has a maximum absorption in the ultraviolet at 2 j7/1/1 and light of 200-300/1/1 converts ozone rapidly into oxygen. On the other hand, oxygen4 absorbs waves of less than 193pp and Regener‘ finds that the conversion of oxygen into ozone is caused very largely by light of a wavelength less than 200,up. The color changes of the so-called silver photochloride’ are in line with this, though the absence Phil. Mag. [3], 19, 195 (1841). Elder: Chem. News., 65, 513 (1892) ; Wiener : Wied. Ann., 55, 257 (1895) ; Luther : Zeit. phys. Cheni., 30, 628 (1899) ; Wilderniann : Ibid., 42, 257 (1902) ; Bauer : Ibid.. 45, 625 (1903). Wildermann’s formulation is the exact opposite of what the text shows that h e intends, Drude’s Ann., 2 0 , 1033 (1906) : Cf. Russ : Elektrochemie, 12,409 (1906). 4 Kreusler : Drude’s Ann., 6, 419 (1901). 6 Regener : Ibid., 2 0 , 1040 (1906). Wiener : Wied. Ann., 55, 225 (1895) ; Bauer : Zeit. phys. Chem., 45, 613 (1903). 2
I
of change in the dark is a point which calls for further investigation. The change of dissolved sulphur, into so-called insoluble sulphur is caused by violet and ultra-violet light, and these solutions absorb the light beyond G. In this particular case, the reverse reaction is not known to be affected by light and the same has been shown by Luther and Weigert' to be true for the change of dianthracene into anthracene. While the difference in the unknown conversion factors for the reacting substances and the reacting products determines the direction of the reaction, there is another factor which is very important in determining the magnitude of the displacement of the equilibrium. Luther2 has pointed out that a reversible equilibrium under the influence of light is a case of dynamic equilibrium. The displacement of the equilibrium depends on the rate of the reverse reaction. If this is high, the effect due to light is small.3 If it is low, the effect due to light may be large. Adding anything that will increase the reverse rate will decrease the displacement of equilibrium caused by light of a given wave-length and a given intensity. An admirable illustration of this is to be found in the equilibrium between oxygen and ozone. At ordinary temperatures and under ordinary conditions ozone changes back slowly into oxygen and it is easy to convert several percent of the oxygen into ozone. At low temperatures the rate of decomposition is much less, and very much higher percentages of ozone can be obtained. With rising temperature, the rate of decomposition increases very rapidly and above zooo practically no ozone is formed by the silent discharge. All this is quite independent of any displacement of the straight chemical equilibrium due to ozone being an endothermal compound. So far as we know, this last is not an appreciable factor at any temperature. 1
Zeit. phys. Clieiii., 51, 29; ; 53, 38j (1905). Ibid., 53, 404 (rgoj). Cf. Haber : Zeit. Elektrochemie, 11, 849 (1905).
Photochemslvy am?the Phase Rule
723
Alexander Smith' has shown that hydrogen sulphide and ammonia accelerate the rate at which fused amorphous sulphur changes back into ordinary melted sulphur. Berthelot2 states that no insoluble sulphur is formed when sunlight acts on a solution of sulphur in carbon bisulphide saturated with hydrogen sulphide gas. This statement is too sweeping, as a matter of fact; but the presence of hydrogen sulphide does decrease the amount of sulphur which can be converted into soluble sulphur by light. It has been shown by Mr. Rankin, in my laboratory, that the addition of ammonia gas t o solutions of sulphur in other solvents than carbon bisulphide checks the production of insoluble sulphur by light. The bearing of this on the phase rule relations is now clear. In cases of reversible equilibrium, light acts as another variable and n 3 phases are required for an invariant system. We have the varying intensity of light introducing another degree of freedom. Strictly speaking, we have as many degrees of freedom as we have kinds of active light; but for most purposes we can treat a beam of light as though it were homogeneous, provided we vary the intensities of the constituent rays uniformly. One case is already known and others will doubtless be discovered in which light of one wavelength and a given intensity produces a given equilibrium while light of another wave-length and the same intensity produces another equilibrium. If now we act on the system with a ray made of these two wave-lengths, and vary the relative intensities of the two monochromatic lights in the compound ray we shall introduce two degrees of freedom, and m degrees of freedom if we have m kinds of active monochromatic light, the relative intensities of which are independently variable. These, however, are refinements which will present no difficulties when the concrete cases arise. When the independent variables of a system are the n components, the pressure, the temperature, and .m kinds of active light, n + m + 2 phases constitute an invariant system. The
+
Jour. Am. Chem. SOC.,27, 797, 979 (1905). Comptes rendus, 70, 941 (1870).
number of phases constituting an invariant system reduces to n + 3 when dealing with absolutely monochromatic light or when the relative intensities of the m kinds of active light depend solely on the intensity of the composite ray. For a given initial mass of oxygen a t a given pressure and temperature, the equilibrium percentage of ozone is fixed, provided these are the only variables; but it is not fixed if we introduce ultra-violet light as an independent variable. For ultra-violet light of zero intensity, the equilibrium percentage of ozone is practically zero. For a given, though unknown, intensity, of ultra-violet light a t 20' Regener' found an equilibrium percentage of 3.4 percent ozone. It is evident that any intermediate value could be obtained by using intermediate intensities of the ultra-violet light. In one case Regener obtained equilibrium with 2 . 2 percent of ozone, but it is not, certain whether this variation was due to a decreased intensity of the ultra-violet light or to a change in its composition. For the present that is immaterial. The essential thing is that a t constant pressure and temperature we get a dynamic equilibrium between oxygen and ozone and that this equilibrium varies with variations in the ultra-violet light. Reference has previously been made in this paper to the fact that light of less than 200pp converts oxygen into ozone 'while light of 200-3oopp changes ozone into oxygen. If we vary the relative intensities of these two kinds of light independently, we are introducing two new variables instead of one and n + 4 phases constitutes an invariant system. The experiments of Regener were not made with this in mind, but they are sufficient t o establish the proposition. Since the rate of the reverse reaction increases with rise of temperature, we should expect to find less and less ozone at higher temperatures. Reference has already been made to the fact that practically no ozone is obtained above 200'. For what are presumably similar conditions of exciting light, Regener finds 3.4 percent ozone at 20°, 3.15 percent ozone at 40°, and 2.7 percent ozone at 54'. Drude's Ann.,
20,
1041(1906).
Photochemistyy a d the Phase R u k
725
With oxygen and ozone, no solid and liquid phases can be obtained at any except very low temperatures. With sulphur, however, it would be a comparatively simple matter to realize a change of freezing-point with changing intensity of light. Berthelotl states that solid rhombic sulphur is not changed into soluble sulphur by light but this is obviously a case of experimental error. If insoluble sulphur precipitates pure from a solution, the nature of the solvent cannot affect the equilibrium between the rhombic and the insoluble sulphur. Berthelot was probably misled by the formation of a surface film which prevented further action. In the absence of light, insoluble sulphur is never a stable solid phase, though an instable quadruple point has been realized with monoclinic and insoluble sulphur in contact with vapor and melt. Under the influence of light, insoluble sulphur can be made the stable phase at ordinary temperature. By adjusting the intensity of the light, rhombic sulphur and insoluble sulphur may be made to coexist in stable equilibrium over a wide range of temperatures. More interesting than this is the case of a solvent and a light-sensitive substance which occurs in two modifications in solution. Depending on the assumptions made as to the action of light, we can, of course, construct a number of solubility diagrams. Two instances will be sufficient to show the general method. As the simplest possible case we will postulate that neither modification affects the solubility of the other and that the action of light is to displace the equilibrium without affecting the solubility of either modification. There are then only two possibilities: that the light causes increased formation of the less soluble modification, or that it causes the formation of the more soluble modification. The isotherms for these two possibilities are given schematically in Pig. I . The abscissas are solubilities of the lightsensitive substance and the ordinates are intensities of light. The curve ABC represents the case in which the less soluble Cornptes rendus, 70, 941 (1870).
form is made stable by light while DEF is the reverse case. The point A is the solubility point as usually determined. With increasing intensity of light, there is an increased formation in solution of the other modification. At B the second modification precipitates and we have a quadruple point stable under the influence of light only. The curve
Fig.
I
AB is a straight line parallel to the axis of ordinates if the solubility of the second modification is zero. Along BC the second modification is solid phase. This curve can never pass to the left of a point representing a saturated solution containing only the second modification. The higher the rate of the reverse reaction, the farther to the right will C lie. If the solubility of the second modification is zero, the curve BC may approach the axis of ordinates asymptotically. The second case in Fig. I requires no special discussion. The points D, E and F correspond to the points A, B and C respectively. For extreme in solubility in the dark D would lie a t the origin. No case is definitely known in which light causes the formation of a more soluble modification, and Roloff’ believes that no such case can occur. This seems to me an unjustifiable conclusion. We have already seen that certain wavelengths change oxygen into ozone and that certain others change ozone back into oxygen. There is no apparent reason Zeit. phys. Chem., 26, 345 (1898).
Photochewistvy aizd the Phase Rule
727
why this should occur only in cases where there is no measurable amount of one modification when the system reaches equilibrium in the dark. Yet if this limitation does not hold, we shall eventually find cases in which the solubility isotherm corresponds to the curve DEF in Fig. I . The curve ABC in Fig. I could doubtless be realized by studying the equilibrium between anthracene and dianthracene in a suitable solvent a t a suitable temperature. The special case in which the second modification is practically insoluble, has been worked out in my laboratory by Mr. Rankin for sulphur and carbon bisulphide. The results obtained furnish a gratifying confirmation of the applicability of the phase rule classification to equilibria of this class. Dutoitl has recently shown that the conductivity of halide salts dissolved in methyl ethyl ketone decreases when the solution is exposed to light. In a n / 6 2 2 solution of sodium iodide, the value for p was 84.9 in the dark and only 57.8 in bright sunlight. The action is reversible and the original conductivity is regained when the solution is put back in the dark. It seems possible that this change of conductivity might be accompanied by a change of solubility and it is to be hoped that this experiment may be tried. This discussion leads up to a new way of looking a t the equilibrium between CO, H,O, CH,, C,H,, etc. If the silent discharge or if ultra-violet light acted only as a catalytic agent, we should reach a final equilibrium which would depend solely on the percentage composition of the gas, provided the pressure and temperature were fixed. It would be entirely independent of the nature of the gases with which we started. For the simpler two-component, two-phase system of hydrogen in presence of an excess of carbon, Bone and Jerdan2 found that at high temperatures the final equilibrium was the same whether they started with hydrogen, methane or acetylene. Such a result obviously does not open upmuch Zeit. Elektrochemie, 12,642 (1906). Jour. Chem. SOC.,7 1 , j g (1897).
728
Photochemistvy and the Phase R u l e
in the way of new fields for systematic work. We now see, however, that the equilibrium reached by means of ultraviolet light will depend on the nature and the intensity of the ultra-violet light. If certain substances contain specific absorption bands we can lessen the decomposition of these substances by using light which does not contain wave-lengths corresponding to the absorption bands in question. While the problem of photo-synthesis or plant-synthesis is admittedly not an easy one, the phase rule line of attack seems by far the most promising one. Cornell University.
