EXAMINATION OP HYDROGEN DIOXID SOLUTIONS. 19 



tralize or is equivalent to 0.75 cc of tenth-normal potassium hydroxid. In actual 

 practice, however, we find by making numerous testa upon a given specimen of 

 hydrogen peroxid, to a portion of which we add three-sixteenths grain acetanilid per 

 ounce and to the second portion of which no acetanilid is added, that this three- 

 .ixttMMiths -niin of preservative will increase the seeming content \>y about !.:'> cc of 

 tenth-normal acid. We can not altogether account for this increase above theory, 

 but presume that it is due to some acid body produced by the decomposition of the 

 acetanilid, produced by the action of the hydrogen peroxid on the aeetanilio! in ilie 

 presence of the alkali in carrying out the evaporation. 



Turning now to the five samples reported, if we make a correction in each instance 

 for the amount of acetanilid reported by you, the equivalents would be as follows: 



cc N/io KOH. 



Sample No. 1 0. 6 



No. 2 0.7 



No. 3 0. 5 



No. 4 0.45 



No. 5 



Subtracting the above quantities of tenth-normal acid from the quantities you 



report, the free acid would appear as follows: 



cc. 



Sample No. 1 2. 65 



No. 2 3. 35 



No. 3 2.7 



No. 4 2.55 



No. 5 



Making the theoretical correction, each of these samples would fall below the require- 

 ments of the U. S. P. for the content of free acid with the exception of No. 3. 



We do not regard this matter of slight excess of free acid as being of very great 

 consequence, but think it best to call your attention to this error, inasmuch as we 

 presume you will go into this matter of hydrogen peroxid very thoroughly, and the 

 evidence which we offer is the result of a very great deal of experience. There is 

 really no reason for resorting to the indirect method as outlined in the Pharmacopoeia. 

 Equally good results and (in the case^of hydrogen peroxid containing acetanilid) 

 more correct results are obtained by direct titration. It is only necessary to add to 

 25 cc of the hydrogen peroxid about 5 or 6 minims of phenolphthalein test solution 

 (U. S. P.) and titrate directly with tenth-normal alkali to the development of a light- 

 pink color. Under the conditions this method is accurate to 0.1 to 0.2 cc. For practical 

 purposes the test in the Pharmacopoeia should be stated as follows: 



If to 25 cc of the solution of hydrogen peroxid 5 or 6 minims of phenolphthalein 

 test solution are added, not more than 2.5 cc tenth-normal KOH should be required 

 to produce a pink color (limit of free acid). 



Comment by authors. It is probably true that the presence of 

 acetanilid will increase the acidity of hydrogen peroxid, particularly 

 when determined according to the prescribed method, but the 

 manufacturer is aware of the facts and should, therefore, make due 

 provisions. If he desires to add acetanilid, and it is known to him 

 that the addition of this chemical will increase the acidity of the 

 product, he should make ample allowance to come within the maxi- 

 mum limit of the prescribed acidity. The subject of direct titration, 

 using phenolphthalein as indicator, is discussed on page 22. 



