MERCURY AND SULFURIC ACID METHOD 413 



in h. The nitric oxid evolved is measured in the usual way and 

 calculated to nitric acid, one cubic centimeter of nitrogen dioxid 

 being equal to 2.417 milligrams of nitric acid. 



357. Merits of the Ferrous Chlorid Process. The possibility 

 of an accurate determination of nitrates, by decomposition with 

 a ferrous salt in presence of an excess of hydrochloric acid, has 

 been established by many years of experience and by the testi- 

 mony of many analysts. The method is applicable especially 

 where the quantity of nitrate is not too small and when organic 

 matter is present. In the case of minute quantities of nitrate, 

 however, the process is inapplicable and must give way to some 

 of the colorimetric methods to be hereafter described. 



In respect of the apparatus, modern practice has led to the 

 preference of that form which does not require the use of carbon 

 dioxid for displacing the air. Steam appears to be quite as ef- 

 fective as carbon dioxid and is much more easily employed. That 

 form of apparatus should be used which is the simplest in con- 

 struction and has the least cubical content. 



The measurement of the evolved gas is most simply made by 

 collecting over lye in an azotometer, reading the volume, noting 

 the reading of the barometer and thermometer and then reducing 

 to standard conditions of pressure and temperature by the cus- 

 tomary calculations. Where a very strong lye is used the ten- 

 sion of the aqueous vapor may be neglected. While every analyst 

 should have a thorough knowledge of the ferrous chlorid method 

 and the principles on which it is based, it can not be compared 

 in simplicity to the later methods with pure nitrates, which are 

 based on the conversion of the nitric acid into ammonia by the 

 action of nascent hydrogen. In accuracy, moreover, it does not 

 appear to have any marked advantage over the reduction methods. 



358. Mercury and Sulfuric Acid Method. This simple and ac- 

 curate method of determining nitric acid in the absence of organic 

 matter is known as the Crum-Franklarid process. 48 



The method rests on the principle of converting nitric acid into 



46 Philosophical Magazine, 1847, [3], 30 : 426. 

 Journal of the Chemical Society, 1868, 21 : 101. 

 Sutton, Volumetric Analysis, gth Edition, 1907 : 443, 



