72 LON A. HAWKINS 



in the presence of NaCl was advanced by Clark [01], who considered that 

 a double salt was formed, such as Na 2 HgQ 4 , and supposed this to be 

 only slightly dissociated in the presence of an excess of NaCl. This writer 

 also worked with various copper salts in the presence of KNO 3 , K 2 SO 4 , 

 NH 2 SO 4 , etc., and suggested that the decreased toxicity of the copper salts, 

 which was uniformly observed in such combinations, was likewise due to 

 formation of double salts. 



In view of the work of Hosford and Jones, 17 on the dissociation of 

 double salts in dilute solution, it seems very improbable that the lowering 

 of the toxicity of Cu(NO 3 ) 2 brought about by Ca(NO 3 ) 2 , as shown in 

 the present study, is due to decreased ionization of the toxic compound, 

 brought about by double salt formation. It is, however, possible to obtain 

 direct evidence on this question by comparing the concentrations of copper 

 ions in the various solutions, with and without the calcium salt. The per- 

 centage of dissociation occurring in simple solutions of Cu(NO 3 ) 2 has been 

 determined experimentally for many concentrations, and these data are 

 available in published tables. 18 From such data may be derived the con- 

 centration of copper ions present in any solution containing only Cu(NO 3 ) 2 , 

 but no published data are yet available from which might be ascertained 

 the concentrations of copper ions in the binary mixtures here dealt with. 



The determinations here required of the relative concentrations of copper 

 ions in solutions of Cu(NO 3 ) 2 with and without addition of the calcium 

 salt, may be made by means of the potentiometer. 10 With this instrument 

 the solutions involved in table I were tested, and the differences in 

 electrical potential between the combination solutions of Cu(NO 3 ) 2 and 

 Ca(NO 3 ) 2 and the corresponding simple solutions of Cu(NO 3 ) 2 were 

 measured. The two solutions to be tested (for example, the simple solu- 

 tion containing o.ooSm Cu(NO 3 ) 2 and the combination solution containing 

 o.ooSm Cu(NO 3 ) 2 and o.O5m Ca(NO 3 ) 2 ) were placed, respectively, in the 

 two arms of a U-tube, separated below by a saturated solution of ammonium 

 nitrate. Into each arm of the U-tube was introduced a copper electrode 

 and these were connected with a potentiometer (Leeds and Northrup, type 

 K). The potentiometer was so calibrated that the difference in voltage 

 between the two copper solutions might be read directly on the instrument. 



17 Hosford, H. H., and Jones, Harry C., Conductivity, temperature coefficients of conductivity and 

 dissociation of certain electrolytes. Amer. Chem. Jour. 46: 240-278. ign. 



18 Jones, Harry C., Electrical conductivity, dissociation and temperature coefficients of conduc- 

 tivity, from zero to sixty-five degrees, of aqueous solutions of a number of salts and organic acids. 

 Carnegie Institution of Washington Publ. 170. Washington. 1012. 



19 Loomis, N. E., and Acree. S. F., A study of the hydrogen electrode, of the calomel electrode, and 

 of contact potential. Amer. Chem. Jour. 46: 585-620. 191 1. 



It is a pleasure to acknowledge here, that the suggestion of this method, as applicable to the prob- 

 lem in hand, is due to Professor S. F. Acree, of the Chemistry Department of the Johns Hopkins Uni- 

 versity, and that much valuable help in making the determinations was received from him and from 

 Dr. C. N. Myers. This part of the work was carried out at the chemical laboratory. 



