SCHULZE-TIEMANN METHOD 403 



centimeters of ferrous chlorid and two cubic centimeters of hydro- 

 chloric acid are usually added. 



352. Measurement of the Gas. The measurement of the gas 

 was for some time conducted by the use of concentrated potash 

 for absorbing the carbon dioxid, and ferrous chlorid for absorbing 

 the nitric oxid. The use of the ferrous chlorid, however, was 

 found to introduce a source of error. The treatment of the gas 

 with oxygen and pyrogallol over potash has, therefore, been sub- 

 stituted by Warington for absorption by ferrous chlorid. 



The chief source of error attending the oxygen process lies in 

 the small quantity of carbon monoxid produced during the 

 absorption with pyrogaliol, but this error becomes negligible if the 

 oxygen be onjy used in small excess. The amount of oxygen 

 employed can be regulated by the use of Bischof's gas delivery 

 tube. This may be made of a test-tube having a small perfora- 

 tion half an inch from the mouth. The tube is partly filled with 

 oxygen over mercury, and its mouth is then closed by a finely 

 perforated stopper made from a piece of wide tube and fitted 

 tightly into the test-tube by means of a covering of rubber. 

 When this tube is inclined, the side perforation being down- 

 wards, the oxygen is discharged in small bubbles from the per- 

 forated stopper, while mercury enters through the opening. 

 Using this tube, the supply of oxygen is perfectly under control 

 and can be stopped as soon as a fresh bubble ceases to produce 

 a red tinge on entering. Warington concludes his description 

 by stating that in the reaction proposed by Schloesing the 

 analyst has a means of determining a very small quantity of 

 nitric acid with considerable accuracy, even in the presence of 

 organic matter; but to accomplish this, the various simplifica- 

 tions consisting in the omission of the stream of carbon dioxid, 

 and the collection of the gas over caustic soda must be aban- 

 doned, and special precautions must be taken to exclude all 

 traces of oxygen from the apparatus. 



353. Schulze-Tiemann Method. The modification of Schulze- 

 Tiemann in the ferrous salt method consists chiefly in the omis- 

 sion of the use of carbon dioxid, and in the simplified form of 

 apparatus, which permits rapid work and gives, also, accord- 



