THE PHYSICAL CHEMISTRY OF VISUAL PURPLE 



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1^ — -Clamp 

 Glass sleeve 



Glass rod 



■Rubber connector 



unbleached with 2 drops of bleached solution and so on to the last, 

 which was unbleached solution alone. The 1 1 standards so prepared 

 were pipetted into similar tubes to those used in the bleaching 

 experiments and were protected from light by black rubber tubing. 

 In the colorimetric comparisons the experimental solution was 

 similarly jacketted. The matching 

 was done by the light of a 0-04 

 candle power artificial daylight 

 lamp having a uniform circular 

 surface of 30 mm diameter. This 

 matching light was used only 

 momentarily. 



The experimental tube, arranged 

 as in Fig. 3.1, was exposed to 

 light of known intensity, the tube 

 being rotated by hand at roughly 

 10 r.p.m. by means of the glass 

 rod. After a certain interval the 

 tube was removed from the bleach- 

 ing Hght for comparison with the 

 standards, after which it was re- 

 turned for further bleaching. Con- 

 centrations were estimated to the 

 nearest 5 per cent, i.e. to half 

 the difference between standards. 

 HECHT (1920) found that the varia- 

 tion of concentration of visual purple with time of exposure could be 

 expressed by the equation for a monomolecular reaction, namely 



where a was the initial concentration of the solution (100 per cent) 

 and X the concentration after the time / (minutes). 



A further series of experiments was carried out in which the curve 

 of concentration against time was obtained by exposing a number of 

 exactly similar solutions for increasing intervals. In this way each 

 solution was brought to a stage of decomposition by a continuous 

 exposure as opposed to a series of intermittent exposures of the same 

 total duration. No difference was found between the curves obtained 

 in this way and those obtained by the intermittent exposure method. 



61 



• Visual purple solution 



Fig. 3.1. Hecht's experimental 



arrangement for bleaching visual 



purple solutions. 



{After Hecht, 1920) 



