Phelps — Titrimetric Estimation of Nitric Acid. 443 



permanganate. In the experiments where the dry salt was 

 used, the air was expelled from the apparatus by boiling 10^°^^ 

 of water to small volume in the flask, arranged as previously 

 described, the ferrous sulphate solution introduced and concen- 

 trated by boiling to a volume of about 20'^'^^ so that the amount 

 of acid used may not be so large ; then, the nitrate, dissolved 

 in a small amount of water, was allowed to flow in and was 

 finally washed in, as before, with an amount of concentrated 

 hydrochloric acid, which approximates in volume that of the 

 liquid contents of the flask. 



Experiments numbered I-YIII, inclusive, of the following 

 table show the results of a series of experiments made as 

 described above. 







Oxygen value 



Oxygen value 







KN03 



of ferrous salt 



of ferrous salt 



Error on 





taken. 



taken. 



found. 



oxygen. 





grm. 



grm. 



grm. 



grm. 



I 



0-0500 



0-01828 



0-00621 



0-00015 + 



II 



0-0500 



0-01865 



0-00681 



0-00003 — 



III 



0-0500 



0-01954 



0-00768 



o-ooooo± 



IV 



0-1000 



0-02881 



0-00507 



0-00001 + 



Y 



0-1000 



0-02822 



0-00441 



0-00008 + 



VI 



0-2500 



0-06453 



0-00512 



0-00009 + 



VII 



0-5000 



0-13394 



0-01524 



0-00005 + 



VIII 



0-5000 



0-12210 



0-00340 



0-00005 + 



IX 



0-0500 



0-01720 



0-00747 



00214 — 



X 



0-0500 



0-01550 



0-00389 



0-00026 — 



XI 



0-0525 



0-01550 



000318 



0-00014 — 



XII 



0-1000 



0-02765 



0-00432 



0-00040- 



Experiments numbered IX and X were made to determine 

 whether the long boiling in the previous experiments is actually 

 necessary ; for in the work by Fresenius as well as that of Eder, 

 directions are given merely to boil the hydrochloric acid 

 solution of the iron salt with the nitrate until the color of 

 the dark compound of the nitrogen dioxide with the ferrous 

 salt wholly vanishes and is replaced by the clear color of the 

 ferric salt, which means an active boiling of not longer than 

 five minutes in the experiments recorded in the table. Experi- 

 ment IX was made exactly like the similar ones above it in 

 the table, except that the boiling was interrupted after the 

 dark colored ferrous compound with the nitrogen dioxide was 

 completely broken up ; and experiment X similarly, except that 

 the boiling was continued for five minutes after the complete 

 disappearance of the dark color. Obviously the period of boil- 

 ing allowed by Fresenius and Eder is not enough, but since 

 the main error incidental to these processes is an error of excess 

 due to oxygen present, as already stated, it will readily appear 



