THE RECOGNITION OF SUBSTANCES, II, ETC. 539 



green precipitate of Cr(OH) 3 indicates chromium, a reddish-brown 

 precipitate of Fe(OH) 3 indicates iron. The presence of NH 4 C1 is 

 necessary to reduce the concentration of OH~ furnished by the 

 NH 4 OH below the point at which other less insoluble hydroxides 

 (such as Mn(OH) 2 ) would be precipitated. To tell whether iron 

 was originally present in the ferrous or the ferric condition, the 

 ferricyanide test (p. 498) should be applied. 



Group Hlb. If, still, no precipitate is obtained, hydrogen 

 sulphide is led into the alkaline solution. Sulphides which are 

 insoluble in water, but soluble in active acids (see pp. 460-4), 

 now appear. They are CoS and NiS (both black), MnS (flesh- 

 colored) and ZnS (white). To distinguish between Co and Ni, 

 add NaOH to the original solution. Cobalt gives a blue precipi- 

 tate of a basic salt, changing to pink Co(OH) 2 on boiling; nickel 

 gives a light-green precipitate of Ni(OH) 2 . 



Group IV. If negative results are still obtained, add 

 (NH 4 )2C0 3 . A white precipitate is given by three of the remain- 

 ing metals whose carbonates are insoluble, calcium, strontium 

 and barium. Distinction between these three may be made by 

 the flame test. A small portion of the precipitate is taken up on 

 a platinum wire and held in the Bunsen flame. A brick-red 

 coloration signifies Ca, a crimson-red Sr, a green Ba. 



Group V. The only other common positive radicals are 

 Mg, NH 4 , K and Na. On addition of ammonium phosphate to 

 the solution from Group IV, magnesium, if present, is precipitated 

 in the form NH 4 MgP0 4 (white). An ammonium salt may be 

 recognized by boiling some of the original solution with NaOH, 

 when ammonia is evolved. Potassium salts confer a violet color- 

 ation to the Bunsen flame; sodium salts a bright yellow. 



Confirmatory Tests. From the context in earlier chapters, 

 the student will be able to pick out for each particular metal other 



