480 AGRICULTURAL ANALYSIS 



part, amounting to a gallon for each inch of precipitation, is 

 placed in a carboy; at the end of each month the contents of 

 the carboy are mixed, and a sample removed for analysis. In 

 the carboy, receiving the rain for nitric acid estimation, a little 

 mercuric chlorid is placed each month with the view of pre- 

 venting any change of ammonia into nitric acid. It may be 

 doubted, however, if this precaution is necessary, as the rain 

 water thus collected always contains a very appreciable amount 

 of lead; and experiments have shown that on the whole rain 

 water more frequently gains than loses ammonia by keeping. 



Preparation of the Sample. The method first employed by 

 Warington was to concentrate 10 pounds of the rain water in a 

 retort, a little magnesia being used to decompose any ammonium 

 nitrite or nitrate present. Concentration by evaporation in the 

 open air, and especially over gas, results in a distinct addition 

 to the nitrites present. When concentrated to a small bulk, the 

 water is filtered and evaporated to dryness in a very small beaker. 

 The nitrogen, as nitrates and nitrites, is then determined by 

 the methods already described. 



DETERMINATION OF FREE AND ALBUMINOID AMMONIA 



IN RAIN AND DRAINAGE WATERS AND 



SOIL AND FERTILIZER EXTRACTS 



412. Nessler Process. The quantities of free ammonia in 

 rain and most drainage waters are minute, but may reach con- 

 siderable magnitude in some sewages. By reason of these mi- 

 nute proportions, gravi- and volumetric methods are not suita- 

 ble for its quantitative determination. Recourse is, therefore, 

 had to the delicate colorimetric reaction first proposed by Ness- 

 ler. This reaction is based on the yellowish brown coloration 

 produced by ammonia in a solution of mercuric iodid in potas- 

 sium iodid. The coloration is due to the formation of oxydi- 

 mercuric ammonium iodid, NH 2 Hg 2 OI, and the reaction takes 

 place between the molecule of free ammonia and the mercuric 

 iodid dissolved in the alkaline potassium iodid as represented by 

 the following equation: 



/Hg-O-Hg I /Hg, 



O< + 2H 3 N = 2O< >NH 2 I + H 2 O. 



\Hg-0-Hg-I \Hg/ 



