THE PHYSICAL CHEMISTRY OF VISUAL PURPLE 



on plotting log /J(/y. — Q against time. This showed that the assump- 

 tions made in the theoretical treatment, viz. (1) that visual purple 

 solutions obey Beer's and Lambert's laws, and (2) that the quantum 

 efficiency does not depend on the concentration, were justified. 

 Between 20°C and 60°C, ay, the photosensitivity was constant and 

 had the value 9 x 10^^ cm^ per quantum absorbed. At 5°C the value 

 was slightly less (7 x 10~^^). Subsequently, broda and goodeve 

 (1941) showed that even at — 73°C a glycerol- water solution of visual 

 purple had a photodecomposition rate of the same order as at room 

 temperature. 



The photosensitivity is a measure of the rate of a photochemical 

 reaction. The value for visual purple is among the highest recorded 

 for any substance. It is comparable, for example, with that for the 

 hydrogen-chlorine reaction which, though having a quantum effici- 

 ency (y) of between 10^ and 10^ yet has an extinction coefficient (a) 

 of only about 10~^^ cm^. 



The quantum efficiency (y). Since the absorption characteristics 

 (a) of visual purple do not change markedly with pH nor with 

 temperature, the constancy of ay over wide ranges of these variables 

 indicates a constant value for y, the quantum efficiency. The lower 

 values obtained at 5°C suggest that some deactivation may have set 

 in at this temperature. 



The constancy of ay also provides a strong argument against the 

 secondary process being a chain reaction, for such reactions are 

 almost invariably influenced by temperature, and commonly by 

 concentration. One may therefore conclude that y is not greater than 

 unity. It is less easy to estimate the relative probability of deactiva- 

 tion (and hence of a low value for y) except to say that a high 

 deactivation fraction could not easily be reconciled with the observed 

 constancy of the photochemical bleaching rate irrespective of 

 variations in temperature, pH, and solvent. Thus y is either equal to, 

 or not much less than, unity (dartnall, goodeve and lythgoe, 

 1936, 1938). 



The variation of photosensitivity with wavelength. The spectral 

 variation of the photosensitivity of frog visual purple was measured 

 by goodeve, lythgoe and Schneider (1939, 1942). The results 

 obtained are shown in Fig. 3.6 (ffiled circles). Trendelenburg's 

 (1904) bleaching values — as corrected by henri and larguier des 

 BANCELS (1911) for the energy distribution of the Nernst lamp 

 spectrum, and brought to an equal quantum intensity basis — are 



77 



