IONIC INTERZCTIONS 



31 



[Pr~] and [MPr] will be constant for equitoxic concen- 

 trations of the different rnetals. Consequently if K^, 

 were known we could calculate the values of the equi- 

 toxic concentrations of the different metals, which are 

 represented by [M^] . Now the equilibrium constant is 



rN^ 



*3 



volts 



+2 



■H 



° o^ i. ° 



\ dBi 

 • ■ o 



^^« 



oo 



0.1 001 0001 0.0001 000001 0.000001 



Molarity 



Fig. 6. The toxicity of heavy metals to various systems. The logarithms 

 of the equitoxic concentrations of various metals are plotted against 

 their standard electrode potentials. ■ 50 % inhibition of papain; a 50 % 

 inhibition of catalase; A killing of Paramecium: % killing of Fundulus 

 egg; o killing oi Polycellis nigra (planarian). 



related to the standard electrode potential of the metal 

 concerned (the reasons for this are given elsewhere, 

 Danielli, 1946). If we substitute for K^ in equation 

 (3) an appropriate term for the standard electrode po- 

 tential Em, and rearrange our equation we end up with 



Em = 0.058 log [M] + constant, (4) 



i.e. if we plot log[M] against E^ we should obtain a 



