THE PHYSICAL CHEMISTRY OF VISUAL PURPLE 



and the active hydrogen atom so formed can propagate a chain by 

 reacting with an undissociated chlorine molecule as follows, 



H + CI2 -> HCl + CI (b) 



The new CI atom can then act as in equation (a) and the new H atom 

 as in equation (b) and the chain continued until it is terminated by, 



H + H -> H2 or 



CI + CI ^ CI2 



Such atomic-chain (and energy-chain) reactions are sensitive to the 

 concentrations of reactants and to the temperature. 



Wherever there is the possibility of a chain reaction (causing high 

 yields) or alternatively, where de-activation processes are predomi- 

 nant (causing low yields) the overall quantum efficiency is usually 

 affected by the concentrations of reactants and by the temperature. 

 Conversely, where these variables are without effect on the yield this 

 is generally considered to be evidence that the reaction mechanism is 

 a simple one, having an overall quantum efficiency in the region of 

 unity. 



THE METHOD OF PHOTOMETRIC CURVES 



DARTNALL, GOODEVE and LYTHGOE (1936, 1938) made a quanti- 

 tative analysis of the photochemical bleaching of visual purple 

 solutions in monochromatic light using a new method subsequently 

 termed (goodeve and wood, 1938) the method of photometric 

 curves. This method consisted of an analysis of the transmission/time 

 curves of bleaching solutions. The apparatus used is depicted in 

 Fig. 3.2. The source was a 1000 c.p. tungsten arc, 'Pointohte,' lamp 

 5, the light from which was focussed on to the entry slit, S^, of a 

 monochromator by means of the condensing lens and the totally 

 reflecting prism R. After passing through the monochromator lens 

 L, the hght entered the front face of the prism P and then, after 

 reflection from the silvered back face, was brought to a focus by L, 

 as a spectrum in the plane of the slits. The dominant wavelength of 

 that part of the spectrum which emerged from the exit slit S2 could 

 be altered by rotation of the prism, while the purity of the Hght 

 delivered was governed by the widths of the exit and entry slits. 



After emerging from the exit slit, the monochromatic beam passed 

 through a small lens, F, which formed a circular image of the lens, 

 L, on the aperture of the cell holder, H. This was made with passages 



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