Effect of Salts on Hydration of Acetic Anhydride. 115 



in the solution in question. This showed that water of hydration has a 

 very different resonance from that of pure water. 



Having found this physical difference between combined and free 

 water, the problem was now to see if there was any chemical difference. 



Jones and Holmes 1 studied the action of strongly hydrated salts 

 and slightly hydrated salts on the saponification of methyl acetate and 

 of methyl formate in the following manner: He measured the velocity 

 of saponification of the ester by pure water, by a solution of slightly 

 hydrated salts containing the same amount of water as the pure water 

 used, and by solutions of strongly hydrated salts containing the same 

 amount of water as the free water employed, and as the water in the 

 slightly hydrated salts. Taking into account the hydrolysis of the 

 strongly hydrated salts he found that these salts saponified much more 

 rapidly than pure water itself. 



The reaction studied by Jones and Holmes was a very slow one and 

 indicated that combined water has greater activity than free water. 

 We wished to investigate the same problem, using a reaction that pro- 

 ceeded much more rapidly; therefore we chose the reaction involving 

 the conversion of acetic anhydride into acetic acid. 



EXPERIMENTAL. 



PURIFICATION OF MATERIAL. 



Very pure acetic anhydride was necessary for our work. The phys- 

 ical properties as described varied greatly. The boiling-points given 

 ranged anywhere from 135 to 140, at 760 mm. pressure. The densi- 

 ties given varied between 1.07 and 1.09. From this it can be seen that 

 it was impossible to test its purity by the ordinary simple means. 

 Acetic acid is the impurity most likely to be present in the anhydride, 

 and is very difficult to detect if only small amounts are present; 0.51 

 gram of pure acetic anhydride, when completely hydrated, is equiva- 

 lent to 100 c.c. N/10 solution of sodium hydroxide; while the same 

 weight of a mixture containing 1 per cent of acetic acid is equivalent to 

 99.85 c.c. This is within the experimental error. The method finally 

 used to purify the anhydride was to distill repeatedly, using a five-bulb 

 distilling head, discarding the first and last fractions. This gave a 

 very pure anyhdride with a constant boiling-point. 



The salts used in this work were the purest obtainable. They were 

 repeatedly recrystallized. 



APPARATUS AND SOLUTIONS. 



All of the glassware used was of Jena make. The constant tempera- 

 ture baths were those commonly used in this laboratory. All solutions 

 were made up gravimetrically, except those of the non-hydrated salts 



Chapter V. 



