THE PHYSICAL CHEMISTRY OF VISUAL PURPLE 



namely at 502 mw (the Amax for visual purple) and at the isosbestic 

 wavelength appropriate to the pH of the solution. An isosbestic 

 wavelength (see Fig. 2.11) is a wavelength for which the density of 

 visual purple is the same as that of its product of bleaching (indicator 

 yellow). Any density loss at such a wavelength showed that the 

 indicator yellow was itself thermally decomposing. When this was 

 the case, suitable corrections (calculated from the magnitude of the 

 observed loss and the known absorption spectrum of indicator 

 yellow at the particular pH) were made to the changes observed 

 at 502 mjii. 



The changes measured at 502 (after correction where necessary) 

 were thus due to the decomposition of visual purple only. Density 

 losses due to photochemical bleaching by the light used in measure- 

 ment were negligibly small. 



LYTHGOE and QUILLIAM found that under all conditions of pH 

 (from 4-2 to 10-7) and temperature (from 31° to 56°C) the time course 

 of thermal decomposition could be described by the equation 



where D^ was the density of the solution at time t (sec), Dq the initial 

 density, and Df the final density when all the visual purple had been 

 bleached; Dq — Df was thus a measure of the initial concentration 

 of visual purple and Dt — DfSL corresponding measure at time t. The 

 value of the constant k depended on the pH and temperature. 



In Fig. 3.9 the logarithms of the velocity constants k so found are 

 plotted against the reciprocal of the absolute temperature. It is 

 evident that for any given pH the data fall on a straight line and, 

 consequently, that the Arrhenius equation is obeyed. This states that 



log, k=C - EIRT 



where C is a constant, R is the gas constant, T is the absolute tem- 

 perature and E is the heat of activation of the reaction. The heat of 

 activation can be calculated from the slopes of the straight lines in 

 Fig. 3.9. For all lines except that for pH 4-2 the slopes are roughly 

 the same and correspond to an activation energy of approximately 

 44,000 calories per gram-molecule. The results in Fig. 3.9 show that 

 visual purple is most stable in neutral solution. A high salt content 

 (see lower dotted line in Fig. 3.9) also reduces the rate of thermal 

 decomposition. 



85 



