OTHER PROPERTIES OF ELECTROLYTIC SOLUTIONS 305 



magnitude of the energy effects, as derived from conductance-temperature 

 measurements, agrees with those derived from the heats of dilution of 

 solutions of weak electrolytes. The ionization constants of acetic acid 

 and ammonia, for example, have maxima in the neighborhood of ordi- 

 nary temperatures, indicating that the energy change accompanying the 

 ionization process is zero; correspondingly, the heats of dilution of 

 solutions of these substances have small, although uncertain, values. In 

 general, weak electrolytes have a greater heat of dilution than strong 

 electrolytes and, correspondingly, their ionization changes more largely 

 with temperature. 



The heats of dilution of strong electrolytes unquestionably have 

 very small values. Correspondingly, the ionization of strong electrolytes 

 at ordinary temperatures changes but little with temperature. The 

 ionization of certain salts, such as magnesium sulphate, decreases 

 markedly at higher temperatures; and it is to be expected that solutions 

 of these salts will exhibit an appreciable heat of dilution even at rela- 

 tively low concentrations. Experimental determinations of these quan- 

 tities, however, are lacking. In view of the uncertainty of the thermal 

 data, it cannot be stated that the commonly accepted ionic theory leads 

 to results which are in contradiction with the thermal properties of 

 electrolytic solutions. 



Recently, careful determinations of the heats of dilution of a number 

 of electrolytes have been made by a number of investigators. Accord- 

 ing to Randall and Bisson, 35 the heat of dilution of sodium chloride from 

 0.28 N to zero concentration amounts to only two calories. At higher 

 concentrations the heat of dilution, although small, is quite marked. 

 The heats of dilution of a number of salts, as well as of mixtures of 

 salts, have been determined by Stearn and Smith, 36 and Smith, Stearn 

 and Schneider. 37 The heats of dilution for sodium and potassium 

 chlorides were found to be very nearly the same, although varying 

 slightly at high concentrations. At low concentrations, the heat of dilu- 

 tion, in all cases, approaches a value of zero, as might be expected. The 

 heats of dilution are not in all cases of the same sign, since that of 

 strontium chloride is opposite in sign from that of sodium and potassium 

 chlorides. The heats of dilution for mixtures of two electrolytes in 

 general differs markedly from the mean heat of dilution of the con- 

 stituents. Stearn and Smith suggest that this result may be due to the 

 fact that complex compounds, whose formation presumably would be 

 accompanied by an energy change, are formed in mixtures of salts. For 

 sodium and potassium chlorides the heat of dilution is negative, which 



8 Randall and Bisson, J. Am. Chem. S<tc. 42, 347 (1920). 

 86 Stearn and Smith, J. Am. Chem. Soc. 1&, 18 (1920). 

 " Smith, Stearn and Schneider, ibid., #, 3$ (1920). 



