$6 PRACTICAL AGRICULTURAL CHEMISTRY [159-164 



NH 4 OH ; hence this reagent should only be added in slight 

 excess. Al(OH) 3 is readily soluble in hydrochloric acid 

 (HC1) and in acetic acid (HA). 



159. Potassium Hydrate (KHO) or Sodium 

 Hydrate (NaHO) when added drop by drop gives the 

 same precipitate as NH 4 OH, but it is readily soluble in 

 excess of these reagents. By adding ammonium chloride 

 (NH 4 C1) in large excess to this solution, and boiling, the 

 precipitate will be thrown down again. 



160. Ammonium Sulphide, (NH 4 ) 2 S, gives the 

 same precipitate as NH 4 OH with evolution of sulphuretted- 

 hydrogen gas. This precipitate is insoluble in excess of 

 the reagent. 



IRON (Fe).— Use ferrous sulphate (FeS0 4 .7H 2 0) and 

 ferric chloride (Fe 2 Cl 6 ) solutions. 



161. Note Iron forms two classes of compounds, known respec- 

 tively as ferrous and ferric compounds. It is often necessary that the 

 analyst should ascertain whether one or both of these two classes are 

 present in a substance containing iron. Tests are here given for both 

 classes. 



FERROUS SALTS. — Use ferrous sulphate 

 (FeS0 4 . 7 H 2 0). 



162. Ammonium Hydrate (NH 4 OH) or potassium 

 hydrate (KHO) gives a dingy green precipitate of ferrous 

 hydrate, Fe(OH) 2 , which becomes brown on exposure to 

 the air. It is soluble in hydrochloric acid (HC1), and in- 

 soluble in excess of KHO. 



163. Ammonium Sulphide, (NH 4 ) 2 S, gives a black 

 precipitate of ferrous sulphide (FeS), soluble in hydro- 

 chloric acid (HC1). 



164. Potassium Ferrocyanide, K 4 Fe(CN) 6 , gives 

 a light- blue precipitate, becoming dark on exposure to the 

 air. 



