TOXICITY OF HEAVY METALS 73 



The copper electrodes were freshly plated and calibrated, "for subsequent 

 correction of the readings, with reference to any difference in potential 

 which might exist between them when both were placed in the simple 

 Cu(XO 3 ) 2 solution. 



The difference in concentration of copper inns in the two solutions thus 

 compared was calculated from the observed difference in electrical potential 

 by the following formula : 



V = 0.059 1 logio-?^ 

 *t 



in which V is the observed Electromotive force, in volts ; C and Q are 

 the concentrations, respectively, of the copper ions in the two solutions, 

 and the quantity 0.0591 is a constant (Loomis and Acree [n], page 591). 

 In each of the tests with which we are at present concerned three entirely 

 separate determinations were made, for each of which the reading was 

 several times repeated, and the results were in excellent agreement. It was 

 found that the difference in electric potential between the 0.008111 solution 

 of Cu(XO :j ) 2 and the same solution containing also a 0.05111 concentration 

 of Ca(XO.j) 2 , was 0.006 volt, the simple solution having the higher 

 potential. From this observation it appears, by substitution in the formula 

 given above, that 



C 

 r =1.26, 



^i 



or that the concentration of copper ions in the combination solution is 



^, or 0.794, as great as that in the simple' one. Xow, from Jones' [12] 



conductivity tables (page 57), it is found by interpolation that the 

 Cu(XO :! ) 2 in our simple solution (0.008111) is about 90.6 per cent, disso- 

 ciated at 25 C. It thus appears that the Cu(XO 3 ) 2 in the combination 

 solution here considered must be dissociated to an extent equal to 0.794 X 

 90.6, or 71.9 per cent. 



It has been shown above (page 71) that the Ca(NO 3 ) 2 in this particular 

 combination should decrease the dissociation of the copper salt, on account 

 of the common ion, to 76 per cent. There is thus a difference of about 

 4 per cent, between the dissociation of the Cu(XO 3 ) 2 in this mixture as 

 calculated from the isohydric principle and that derived from the potenti- 

 ometer measurements. This difference may, of course, be due to the forma- 

 tion of a double salt, following the hypothesis of Clark already mentioned, 

 but it makes no difference in the present discussion whether it be 76 or 

 72 per cent, of the Cu(XO 3 ) 2 that is here dissociated; as has been stated, 

 the solution would still be toxic enough to inhibit germination in these 

 spores if only one per cent, of the total Cu(XO 3 ) 2 were dissociated. 



The remaining two concentrations of Cu(XO 3 ) 2 (0.0004111 and o.oooirn, 



PHYSIOLOGICAL RESEARCHES VOL. I, NO. 2, SERIAL NO. 2, 

 AUOUST, 1913. 



