521 

 or 



(Anhydride) (Water) 



^ ~ (Hjdrate) 



or 



(Anhydride) 



~ (Hydrate) 



Through the slow setting in of the internal equilibrium in solution 

 milk-sugar hydrate presents the remarkable phenomenon that the 

 initial solubility is much smaller than the final solubility. 



As, when the total concentration is not too great, it may be 

 assumed as certain that the concentration of the hydrate in the 

 solution in equilibrium with the solid hydrate, remains permanently 

 the same, the constant of equilibrium will follow from the determina- 

 tion of initial and final solubilities, for then: 



final solubility — initial solubility (Anhydride) 

 initial solubility (Hydrate) 



In this way was found /v' = 1.44 at 15°^), hence a value which 

 is considerably smaller than that found from the reaction velocities, 

 from which it therefore follows that the total concentration for this 

 determination of K' was already too great. 



From initial and final rotation of hydrate and /^-anhydride also 

 I he constant K' can be determined. 



Hudson found [«]^^=:z86°,0 as initial rotation for the hydrate, 

 and [«]/^=:35°,4 for the ^-anhydride. On the other hand the 

 value 55°,3 had already been fixed for the final rotation by Schmoegkr. 



From this follows for 20° ^) : 



86,0—55.3 



K'=-^ = 1.54 



55.3 — 35.4 



a value which lies between the two others. 



In consequence of the slow setting in of the internal equilibrium 

 in the solution it was possible" to determine the initial solution heats 

 of the different modifications, and likewise the heats of the trans- 

 formation of the hydrate and of the <^^-anhydride in the /^-anhydride 

 in dissolved state. 



In this Hudson and F". C. Brown ') succeeded according to a 

 method of H. T. Brown and Pickering ^). 



1) J. Amer. Chem. Soc. 26 1074 (1904). 

 «) J. Amer. Chem. Soc. 30 1781 (1908). 

 3) J. Amer. Chem. Soc. 30 960 (1908). 

 *) Chem. Soc. 71, 782 (1897). 



Proceedings Royal Acad. Amsterdam. Vol. XX. 



34 



