DELICACY OF THE METHOD 455 



of the receivers are united by disconnecting the apparatus from 

 the carbon dioxid generator and passing water through the 

 pipette. The introduction of the manganous chlorid into the 

 mixture does not interfere with the titration of the iodin. This 

 is accomplished in the usual way with sodium thiosulfate using 

 starch as an indicator. The quantity of material used contains 

 about the amount of nitric acid that is found in two-tenths of a 

 gram of potassium nitrate. This method, so similar to the pre- 

 ceding, is somewhat less complex, and, to that extent, preferable 

 to it. 



ESTIMATION OF NITRIC ACID BY COLORIMETRIC 

 COMPARISON 



393. Delicacy of the Method. The remarkable delicacy of those 

 methods of chemical analysis which depend on the production 

 of a pronounced color, which can be compared with that produced 

 by a known quantity of a given substance, has been long illus- 

 trated by the nesslerizing process for the estimation of ammo- 

 nia. By such methods minute amounts of substances can be 

 quantitatively determined with great accuracy, when they would 

 escape all effort for their estimation by gravimetric methods. 

 Processes based on this principle are, therefore, peculiarly appli- 

 cable to the detection and estimation of oxidized nitrogen in 

 waters, fertilizer and soil extracts, whether they be present as 

 nitric, nitrous, or ammoniacal compounds. The very delicacy of 

 the process is the chief objection to its use since, except in the 

 most experienced hands, and without the observance of all of the 

 conditions of manipulation, errors may vitiate the results. 



In the following paragraphs will be given with sufficient detail 

 for the needs of the analyst, the principles and practice of the 

 colorimetric comparison methods which have been approved as 

 best by the experience of analysts. These methods are appli- 

 cable especially to cases in which only minute quantities of the 

 substances looked for are present, and where celerity of deter- 

 mination is especially desirable. They are, therefore, of espe- 

 cial value in the analysis of rain, drainage, and irrigation waters, 

 and of soil and fertilizer extracts poor in oxidized nitrogen. 



