MICROCHEMICAL REACTIONS OF THE COMMON ACIDS 415 



9. Always test the precipitate obtained bv NH 4 OH for Al, Mg, Fe, Mn, Cr, Sn, (Si), 

 (B'i), (Sb), (Hg). 



Take a portion, treat with HNO3 and evaporate to dryness. Repeat several times to 

 convert any Sn into the oxide. Dissolve any insoluble material in HNO3 + HCi and test 

 for Sn and Sb with CsCl. 



Test another portion with HNO3 and a fragment of KCIO3 and heat to boiling. Mn will 

 be precipitated as Mn02, Cr will be oxidized to a chromate. Decant or filter. Test residue 

 for Mn and solution for CrO,). 



10. To a drop of H2O or HNO3 solution add HCI: — Ag, Pb, Hg. Treat any amor- 

 phous precipitate with NH4OH: — Ag, Hg. 



11. To a drop of solution add dilute H2SO4. A crystalline precipitate indicates Ca, Ag, 

 Hg; under certain conditions Sb, Bi may yield crystalline precipitates as may also a num- 

 ber of difficultly soluble stable salts. An amorphous precipitate indicates Sr, Ba, Pb. 

 (Rarely all three of these elements yield crystalline precipitates.) 



12. Evaporate to dryness a drop of the solution of the substance after acidifying with 

 HNO3. Dissolve the residue in a drop of water and add a fragment of potassium mercuric 

 thiocyanate reagent. 



A crystalline precipitate Zn, Cu, Cd, Co, Ag, Pb, Au, (Ni), (Mn). 



Amorphous precipitate, Pb, Ag. 



Red or pink color, Fe. 



(If the oxidation of Fe or Mn salts has not been complete, an amorphous or crystalline 

 precipitate may be obtained, see page 356. Around the edges of the drops, crystals of the 

 components of the reagent always eventually separate.) 



13. To a moderately concentrated solution add a fragment of KI: — Pb, Hg, Ag, (Cu). 



14. To a very concentrated drop of the substance to be tested (which must not contain 

 free HNO3) add a drop of dilute HCI. Introduce zinc sulphide fibers, warm and examine. 

 Evaporate to dryness, add NH4OH. Examine the fibers again and introduce one or two 

 new fibers. 



In acid solution the fiber is: 



Lemon yellow — As, Cd. 



Reddish yellow — Sb, Bi. 



Straw vellow — Sn. 



Brownish vellow — Ag, Bi, Cu, Hg, Pb, Sb (Co, Fe, Mn, Ni). 



Black — Ag, Bi, Cu, Hg, Pb. 

 In acid solution no color, but in alkaline solution the fiber may turn: 



Brownish — Co, Fe, Mn, Ni. (These elements rarely give good reactions with the 

 fibers.) 



15. To a moderately concentrated drop containing a trace of HNO3 add CsCl and KI 

 (see page 400); crystalline precipitates are obtainable with Sn, Sb, Bi, Pb, Hg. Amor- 

 phous precipitates with Ag, Hg. Iodine often separates, especially in the presence of Cu. 



16. In the above outline no satisfactory indication for the presence of the following have 

 been obtained: Na, NH4, Ni, Si, Bo, As. Hence apply suitable identity tests for these 

 elements. 



17. Remember that carbonates and hydroxides must be converted into chlorides or 

 nitrates (or sulphates) before being tested for bases. 



18. The material is insoluble in water and in acids: Crush to the finest possible powder 

 in a tiny agate mortar. Fuse with Na 2 C03, with K2CO3, or with HKSO4, using the smallest 

 possible amount of reagent. Test in the usual way for Si with NH 4 F and NaCl and for 

 the bases after treatment with acid and removal of th«. SiC>2. 



In some cases silicates may be decomposed by heating in a tinv platinum cup with NH4F 

 and H 2 S0 4 . 



19. Remember to test for NH4 and for Mg. In the presence of ammonium salts Mg is 

 not precipitated by NH4OH, or if a precipitate is formed it slowly disappears. 



THE COMMON ACIDS. 



In the elementary course whose outline is covered by this 

 textbook the identification of the acid radicals in simple salts 

 or simple mixtures alone is undertaken. With materials of this 

 nature the qualitative analysis is comparatively easy and no 

 elaborate directions or schemes of procedure are necessary. 

 Most of the tests for the acids have already been studied and 



