ה.
76
2
THE PHOTOCHEMICAL DECOMPOSITION OF
was inappreciable as was to be expected in view of Coehn's observation and the relatively small value for the total decomposition obtained under the conditions employed.
After exposure a very slight deposit of solid was observed in the tube. A series of ten suc- This was shown to be sulphur in the following way,
After each cessive experiments, each lasting 4 hours was carried out. exposure the vessel was simply evacuated and refilled wim sulphur dioxide. Finally the vessel was evacuated and refilled with dry air three or four times and washed with water and alcohol to remove all trace of acid. sulphur was definitely shown to be present by the method of Gil.1
Free
The actual primary photochemical process may either be a dissociation of the SO molecule into atomic sulphur and molecular oxygen, or may In either simply consist in the activation of a sulphur dioxide molecule. case the final result is expressed by-
3502
=
*SO, + S.
Experimental Results.
Coehn, using a quartz mercury lamp on 6 to 9 amps. has shown that sulphur dioxide, enclosed in an annular space surrounding the lamp, is de- composed to the extent of 67 per cent., but when a uviol sheath is inter- This indicates that posed, the decomposition is reduced to 14 per cent.
wave-lengths as long as 253μμ are effective for the decomposition, but since the first absorption band of SO, extends to 318μu on the long wave side of the head, Coehn's result is far from showing that radiation of any For this reason wave-length within the band limits would be effective. measurements were carried out using the uviol lamp with a series of filters, the details of which are given later. The values for the decomposition of the gas under the influence of different qualities of light are given in the table below.
TEMPERATURE OF GAS, 16-18° C.
INITIAL PRESSURE, 600 MM.
Reference No.
of Experiment.
Nature of Light, Filter and Time of Exposure.
Total Decomposition.
Per cent.
3'5
3'4
0'0
3'4
3'4
Three uviol sheaths, 8 hours
3'2
White transparent glass, 8 hours
3*2
p-toluidine filter, 8 hours
3*3
9
B-napthol filter, 8 hours
0'0
10
Lamp alone, acting on specially dried gas for 8 hours
2'S
I
Lamp alone, 8 hours
2
Lamp alone, 4 hours
3
Plate glass filter, 8 hours
4
One uviol sheath, 8 hours
Two uviol sheaths, 8 hours
(In the above table percentage decomposition represents grams per 100 grams of original SO..)
All the experiments, with the exception of No. 10, were carried out with gas which had been passed fairly rapidly over phosphorus pentoxide. A further increase in the water vapour content was found to be without effect upon the results. When the sulphur dioxide is specially dried, by
1Zeitsch, anal. Chem., 33, 54, 1894.
2 Loc. cit.
GASEOUS SULPHUR DIOXIDE
3
passing it slowly over PO, the decomposition value of the photostationary state dropped from 3'5 per cent. to 28 per cent.
NATURE AND EFFECTIVENESS OF THE FILTERS. Experiments Nos. 1 and 2. Vessel Wall without Additional Filter: It will be observe that practically the same amount of decomposition was obtained after 4 hours exposure as after 8 hours. This indicates a photo- stationary state corresponding to the physical and dimensional conditions employed. This is attained in less than 8 hours. In all subsequent ex- periments an exposure of 8 hours was employed to obtain comparable results. Photographs were taken, on a Hilger quartz spectrograph, of the spectrum of the uviol lamp alone and also with one uviol sheath as filter. The line 253 is shown "strong" in the spectrum of the lamp alone, but "very weak" in that of the lamp with one sheath, whilst other lines are diminished in intensity. It is to be remembered that it is under the latter condition that the present experiments have been carried out, as the gas itself is enclosed in a uviol sheath. The general trend of subsequent results strongly suggests that no line which is not found to be "moderately strong" can effect decomposition.
1
The results of experiments 1 and 2, when we take into account the corresponding spectrum photograph indicate that sulphur dioxide can be decomposed by radiation as long as 274. Although the uviol lamp emits wave-lengths as short as 253, 265, and 269μμ, these barely get through a sheet of uviol glass, ie. they cannot effectively penetrate the reaction vessel.
no
Experiment 3. Vessel Wall Plate Glass Filter-In this case decomposition was observed. The plate glass transmitted no line of wave-length shorter than 334μ. This line is however transmitted moderately strongly. It follows that the decomposition must be brought about by one or more of the lines 313, 302, 297, 289, 280, and 274μμ, known to be emitted by the lamp.
Experiment 4. Vessel Wall + One Uviol Filter-The corresponding spectrum photograph was taken through two uviol sheaths in order to allow for the vessel wall, and similar procedure was adopted in the case of additional filters. The lines transmitted in this case are four in num- ber, namely, 313, 302, 297, and 289. The observed decomposition amounted to 3'4 per cent.
Experiment 5. Vessel Wall + Two Uviol Filters.-Under these condi- The tions the decomposition is identical with that of the previous case. lines transmitted were found to be 313, 302, and 297μμ.
Experiment 6. Vessel Wall + Three Uviol Filters.-A slightly lower decomposition value was obtained, namely, 3'2 per cent. The spectrum Obviously photograph is indistinguishable from that of the previous case. further use of uviol sheaths would be of little value.
Experiment 7. Vessel Wall + White Transparent Glass.-The spectrum photograph showed that this specimen of glass transmitted no light of wave-length shorter than 297pp and the latter only feebly as compared with the effect of the uviol sheaths. The decomposition with this filter
The terms "strong," "weak," etc., refer to an approximate system of estimating the relative intensities of the various lines in the subsidiary spectrographic measure- ments which were carried out in connection with the investigation of the photochemical change. Since the relative intensities have not been obtained in quantitative and numerical form, details of this portion of the work have been omitted.
77
No comments yet.
Private notes are available after approval.