Physico-Chemical Behaviour of Porphyrins in Aqueous Detergent Sohttions 33 



reaction at 60°C. The time required to attain equilibrium at this temperature 

 in the presence of IQ-i m zinc sulphate and 5 X 10"' M DMMP in 0-25% 

 CTAB was approximately 48 hr. The reversibility of the equilibrium was 

 demonstrated by studying both the forward and backward reactions as 

 expressed by equation (2). 



The KJ value determined for the zinc mesoporphyrin equilibrium at 80°C, 

 extrapolated to zero ionic strength is 6-0 X 10~^; the corresponding value at 



IrO 



n 05 



20 



30 



40 



PH 



Fig. 2. Formation curves for the zinc mesoporphyrin complex at 60°C 

 Forward reaction equilibrium points 



O Zn++ + PHg ^ ZnP + 2H+ 



Backward reaction equilibrium points 



• ZnP + 2H+ ^ Zn++ + PHj 



ZnP 



n = , where TpHo is the total concentration of mesoporphyrin present. 



TPH, 



60°C is 2-5 X 10"^. The relationship between the observed equilibrium 

 constants {K^) and their value in water {K^ is given by : 



f (l +^,^) \ 



(6) 



where K^ is the solubilization constant of species x. 



If both species PH2 and MP were equally solubilized then the observed 

 equilibrium constant would correspond to the water value. It seems likely 

 that the solubilization constants will be of a similar order of magnitude, and 

 in any event K^ values are likely to parallel K^ values either when comparing 

 the one porphyrin with different metals or different porphyrins with the 

 same metal. 



