2/8 Conductivity of Aqueous Solutions. Part VIII. 



equations were obtained, from which the values of At/ and a in calories 

 were calculated and found to be 14,170 and -32.5, respectively. From 

 these constants the value of At/ is found to be -4,750 calories at 18 and 

 -10,070 calories at 100. These values represent the heat absorbed when 

 the reaction HS0 4 _ = H + -f- S0 4 " takes place in the direction from left 

 to right, the negative sign showing of course that heat is in reality evolved. 



To compute this same energy-change by the second method mentioned 

 above, we used the thermochemical data of Thomsen* which show that, 

 when at 18 one mol of NaOH dissolved in 100 mols of water is added 

 to one mol of NaHS0 4 dissolved in 200 mols or 3600 grams of water, 

 16,630 calories are evolved. Subtracting from this the mean value, 

 13,770 calories, found by Wormannf at 18 for the heat of neutralization 

 of nitric and hydrochloric acids by sodium and potassium hydroxides, we 

 get 2860 calories, which represents approximately the heat evolved by 

 the ionization of that quantity of HS0 4 which exists as ion and as un- 

 ionized KHSO,. or H 2 S0 4 in the acid sulphate solution employed, it being 

 assumed that the small heat-effect attending the dissociation of the 

 un-ionized KHS0 4 or H 2 S0 4 into HS0 4 _ and K + or H + is nearly com- 

 pensated by the heat-effect attending the formation out of its ions of the 

 new quantity of un-ionized K 2 S0 4 resulting after the neutralization. 

 Now, the acid sulphate solution under consideration was about 0.28 

 molal; and our results recorded in table 119 show that a 0.05 molal solu- 

 tion contains 41 per cent and a 0.1 molal solution 47 per cent of the salt 

 in the three forms just mentioned. From these last data we may con- 

 clude that the corresponding percentage in a 0.28 molal solution would 

 almost certainly lie somewhere between 54 and 60. Assuming the mean 

 value of 57 per cent, we obtain for the heat absorbed when one mol of 

 hydrosulphate-ion dissociates at 18 -2,860/0.57 or -5,020 calories, a value 

 which agrees with that (-4,750 calories) derived from the ionization-con- 

 stants fully as closely as could be expected, considering the character of 

 the data involved. 



The ionization-constant of the hydrosulphate-ion is still so large at 

 156 (115 X 10" e ) that neutral sulphates of strong bases would not be 

 appreciably hydrolyzed at this temperature, even at a concentration of 

 0.002 normal. To determine whether this is still the case at the higher 

 temperature of 218 has an important bearing on the interpretation of 

 the conductivity results obtained with potassium sulphate by Noyes and 

 Melcher (section 44, Part IV). Assuming that the heat of ionization of 

 hydrosulphate-ion continues to change with the temperature according 

 to the linear equation derived from the ionization data at 18, 100, and 



*Thermochemische Untersuchungen, 1, 100-102 (1882). 

 fDrude's Ann. Phys., 18, 793 (1905). 



