§ 47. AMMONIUM. 55 



A still more sensitive test is that known as Nessler's. 

 When a mixture of solution of mercuric iodide in potassic 

 iodide, and potassic hydrate, is added to a solution con- 

 taining ammonium, a light or reddish-brown precipitate 

 is obtained, NHg J. To make this test still more delicate, 

 as in the case of an exceedingly dilute solution of the am- 

 moniacal salt, add 25 c.c. of baric hydrate to a litre of 

 the water to be examined, distil off ^ 1^ of the mixture, 

 and test the distillate with the Nessler solution. 



If the solution is not too dilute, a good reaction is ob- 

 tained on holding a drop of the Nessler solution, sus- 

 pended on the end of a glass rod, in the test-tube just 

 above a mixture of the substance tested and baric hy- 

 drate ; if ammonium is present, the drop is colored red- 

 dish-brown. 



To' make a litre of the- solution for this test, and a solu- 

 tion that can also be used for quantitative purposes, dis- 

 solve 62.5 grms. of potassic iodide in 250 c.c. of water, 

 and add to this a concentrated solution of mercuric chlo- 

 ride, until the precipitated mercuric iodide ceases to be 

 dissolved on agitation ; then dissolve 150 grammes of 

 caustic potassa in its own weight of water, and add it 

 gradually to the iodized mercurial solution, and finally 

 the necessary amount of water to make one litre ; let the 

 mixture stand 8-10 days, decant the clear and nearly 

 colorless liquid, and keep it in well stoppered bottles in a 

 dark place. 



Qnantitatiyc estimation. — a. Ammonium may be de- 

 termined in the form of the ammonic platinic chloride, 

 (NHJ^PtClg, when all metals except sodium (and calcium 

 and magnesium, Stohmann^ are absent. The course to 

 be followed is precisely the same as that described for the 

 determination of potassium in the corresponding manner 

 (§ 44, c). The double chloride contains 7.64" |„ of ammonia 

 (NH3), or 8.07" I, of ammonium. 



