584 



SCIENCE. 



[N. S. Vol. XX. No. 514. 



The law of chemical mass action here again 

 shows itself entirely inapplicable to the 

 phenomena connected with the ionization 

 of salts. That some investigators have 

 thought that the deviations from this law 

 indicated by the conductivity were only 

 apparent, and that they were attributable 

 to variations in the migration velocity, has 

 arisen, no doubt, from the fact that they 

 have confined their attention to di-ionie 

 salts, and have failed to recognize, on the 

 one hand, the striking divergences from it 

 exhibited by tri-ionic salts, and, on the 

 other, the substantial correspondence of 

 the conductivity and freezing-point results. 



Combining this principle in regard to 

 the ionization of mixed salts in solution 

 with the empirical concentration law of 

 Storch for single salts, we are led to the 

 conclusion that the ratio of the concentra- 

 tion of the un-ionized part to the product 

 of the concentrations of the two ions (but 

 in the case of tri-ionic salts not raised to a 

 power corresponding to the requirements 

 of the mass action law) is a function of 

 the sum of the equivalent concentrations 

 of all the ions in the solution and of that 

 alone.* This ratio is, moreover, roughly 

 inversely proportional to the square root 

 of the total ion-concentration. 



The correctness of this principle is 

 further demonstrated by the fact that with 

 its aid, also, the conductivity of a mixture 

 of two salts without a common ion can be 

 computed from their separate conductivi- 

 ties. This is shown by the conductivity 

 measurements, made by Archibald and 

 more recently by Sherrill, upon mixtures of 

 potassium chloride and sodium sulphate, 



* This is expressed mathematically by the fol- 

 lowing equation, in which Ci and c, represent the 

 equivalent concentrations of the two salts, and 

 7i and y, their degrees of ionization in the pres- 

 ence of each other ; 



c,(i-r,) 



= K,{c,y, + c,y,y'- 



or of sodium chloride and potassium sul- 

 phate. Up to at least 0.2 normal concen- 

 tration, the agreement between the ob- 

 served and calculated values is within 0.5 

 per cent. On the other hand, the diverg- 

 ence of the observed values from the re- 

 quirement of the mass-action law amounts 

 to many per cent. 



It seems appropriate at once to supple- 

 ment these principles in regard to the form 

 of the concentration function by a state- 

 ment of tM'o general rules which have been 

 found to express the magnitude of the ioni- 

 zation of salts of different types. These 

 rules, unlike the preceding principles, are 

 only crude approximations ; but, neverthe- 

 less, they prove of some assistance in rough 

 applications of the ionic theory, and un- 

 doubtedly possess an important theoretical 

 significance, not yet recognized. They may 

 be stated as follows: (1) The decrease of 

 ionization with increasing concentration is 

 roughly constant in the case of different 

 salts of the same type; and (2) the un^-ion- 

 ized fraction at any definite molal concen- 

 tration is roughly proportional to the 

 product of the valences of the two ions in 

 the case of salts of different types. Thus, 

 at 0.1 normal concentration the mean value 

 of the degree of ionization for 17 uni-uni- 

 valent salts measured at 18° is 83.3 per cent., 

 the average deviation of the separate values 

 from this mean is 2.1 per cent., and the 

 maximum deviation of any of them is 5.4 

 per cent, of the mean value; while for 

 fourteen uni-bivalent salts the mean value 

 is 69.8 per cent., the average deviation 5 

 per cent, of this, and the maximum devia- 

 tion about 10 per cent, of it. The un-ion- 

 ized fraction in 1/20 molal solution is 13^ 

 per cent, for these univalent salts; 30 per 

 cent., or about twice as great, for the uni- 

 bivalent salts; and 60 per cent., or about 

 four times as great, for the three bi-bivalent 

 salts investigated (zinc, magnesium and 

 copper sulphates). The salts of mercury 



