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THE PHOTOCHEMICAL DECOMPOSITION OF GASEOUS SULPHUR DIOXIDE.
By R. ASHTON HILL.1
14 Paper read before the FARADAY SOCIETY, Monday, April 14th, 1924, SIR ROBERT ROBERTSON, K.B.E,, F.R.S., PRESIDENT, in the Chair.)
Received October 29th, 1923.
The most important generalisation in photochemistry is that first stated by von Grotthus, namely, that only those waves which are absorbed by a substance can be photochemically active.
No simple relation exists between the magnitude of the absorption and the chemical action produced. It is, however, fairly generally believed that when a substance shows chemical activity under the influence of light, then light of any wave-length which is absorbed by the substance (within the region of the band) will be effective in producing activity. The ex- perimental evidence in this connection is however somewhat limited. One of the objects of the present investigation was to test the validity or other- wise of this view.
The reaction chosen was the photochemical decomposition of sulphur dioxide gas, which is known to form sulphur and oxygen, the latter being very largely used up to form sulphur trioxide." The method adopted was to employ a source of radiation which would give a line spectrum extend- ing generally over the same region as that occupied by the absorption band of sulphur dioxide, and then to proceed to cut out lines from the ultra- violet end of the spectrum until it was found that the substance was no longer decomposed. By comparing the results with the known absorption of the gas it should be possible to test the statement referred to.
The source of light was a Schott uviol mercury vapour lamp 40 cm. in length. The lines emitted by this lamp cover the region 700μμ to 253μμ which includes the first or "near" band of sulphur dioxide with its head at 2916μp. The lamp was burned on a constant current of 2'4 amp., the p.d. across the terminals being 35 volts.
The sulphur dioxide gas, freed from sulphur trioxide and partially dried by passage over phosphorus pentoxide, was contained in a uvio tube (395 cm. in length, external diameter 4 cm., thickness of glass wall, 1 mm.), placed at a distance of 5 cm. from the lamp. The initial pressure of SO, was in all cases 600 mm. of mercury. The time of exposure varied from 4 to 8 hours.
After exposure, caustic soda solution was run into the tube, the sulphate being determined analytically, the sulphur dioxide present initially being obtained from the pressure and temperature of the gas, this result being also checked by direct analysis. The quantity of free oxygen
1 Communicated by Prof. W. C. McC. Lewis. *Cf, Coehn, Zeitsch, Elektrochem.. 21. 545, 1907.
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