THE PHYSICAL CHEMISTRY OF VISUAL PURPLE 



function <^ is a measure of the reduction in the rate of photodecom- 

 position due to the presence of absorbing impurities and products of 

 bleaching. At the end of an experiment, that is when all photo- 

 chemical change has been completed, /^ is equal to If and ^ is given by 



000 = J 



^Oge II If 



Thus (f>, since it is nearly independent of 7^, depends almost exclusively 

 upon the values of / and If, that is on the final transmissivity. 



At wavelengths shorter than about 420 m/u (in alkahne solution) 

 the photoproducts absorb more strongly than the visual purple. 

 When measured with short wavelength Hght therefore, the trans- 

 mission of bleaching visual purple solutions decrease with time. In 

 such cases If is always less than /^ and the appropriate </> functions are 

 given by the dotted portions of the curves in Fig. 3.5. Also, the term 

 loge IJ(If — Q in the L.H.S. of equation (16) must be replaced by 

 logg Itl\lf — It\ where |/y: — /J, the modulus of (/, — 7^), is reckoned 

 as a positive quantity irrespective of its real sign (goodeve, lythgoe 

 and SCHNEIDER, 1942). The plot of logg7J|7^ — 7^| against time 

 then yields a straight Hne as in the cases where the transmission 

 increases with time. 



EXPERIMENTAL RESULTS 



The equation 



logio T— ^ = ^ TTl • ^ + constant (17) 



If — Ji Z'5A 



(equation (16) restated in terms of decadic logarithms) is a general 

 one, expressing the progress of any photo-chemical change. By 

 plotting experimental values for the L.H.S. of the equation against 

 time, a straight line is obtained having a slope equal to <^ay7/2-3y4. 

 The quantity IjA, namely the quanta per second incident on each 

 square centimetre of solution can be measured, and (j) obtained from 

 the transmissivities. Consequently ay, the product of the extinction 

 coefficient and the quantum efficiency can be calculated, goodeve 

 and WOOD (1938) proposed the term 'photosensitivity' for this 

 product which is of greater practical importance than either the 

 extinction coefficient or the quantum efficiency considered separately. 

 Equation (17) occupies the same role in photochemistry as do the 



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