502 



IOWA ACADEMY OF SCIENCE Vol. XXV, 1918 



A large volume of standard hydrochloric acid was made by 

 the method of Hulett and Bonner so as to contain .003647 gram 

 of HCl to 1 c.e. This acid was further titrated with benzoic acid 

 specially purified for such purposes by the Bureau of Standards, 

 which gave .003648 gram HCl to 1 c.e. This concentration was 

 confirmed by means of silver. A solution of sodium hydroxide 

 was prepared as described in the first paper and was used as 

 the intermediary between the acid potassium phthalate and the 

 hydrochloric acid. The indicator was phenolphthalein, and as 

 already pointed out its error in marking the neutral point was 

 cancelled by the method of titration. 



Titrations were made of samples of the acid phthalate which 

 had been three, four and five times re-crystallized from hot 

 water. The objects were to determine the number of crystalli- 

 zations necessary to secure a sufficiently pure product, and to 

 determine the constancy of the acid salt as an individual com 

 pound. It is well knowai that successive crystallization of some 

 so-called acid salts does not secure a constant product. The 

 results are shown in the following talble: in which "Third," 

 'Fourth" and "Fifth" show the number of re-cr3^stallizations 

 of the potassium acid phthalate, and following each the values 

 given for the standard hydrochloric acid. 



The results seem to show that there is nothing to be gained 

 beyond three re-crystallizations. Other titrations seem to show 

 that even two re-crystallizations, that is three crystallizations in 

 all will give a salt pure enough for ordinary purposes. There 

 seems to be no ground to doubt the constancy of the composition 

 of the acid salt. 



Working with no special care our results showed that about 

 50 per cent of the theoretical yield of the a.eid salt may be 



