288 METALS AND THEIR COMBINATIONS. 



active bases, and is dissolved by them to form compounds called aluminates. 

 This means that A1(OH) 3 has two modes of ionization, namely, 

 A1(OH) 3 ^ Al f 3(OH)'; 

 Al(OH), ; A10/" + 3H-. 



The first mode takes place mainly, and in the presence of acids, salts are 

 formed thus : 



Al- -f- 3(OH)' -f 3H- + 3C1' = Al- + 3C1' + 3H 2 O. 



The second mode of ionization takes place to a less extent, but in the presence 

 of excess of alkalies action results thus : 



A10 S "' -f 3H- + SNa- + 3(OH)' = A1O 3 ' " -f 3Na' + 3H,O. 

 These reactions are generally written thus : 



A1(OH) 3 + 3HC1 = A1C1 3 + 3H 2 O; 

 A1(OH) 3 -f 3NaOH = Na 3 AlO 3 + 3H 2 O. 



The compounds of the form Na 3 AlO 3 , called aluminates, are largely hydrolyzed 

 by water into NaOH and A1(OH) 3 . Hence, an excess of alkali is required to 

 dissolve the aluminum hydroxide. Ammonium hydroxide is too weak a base 

 to unite with it. 



Aluminum hydroxide, in common with many other substances, as hydroxide 

 of iron, chromium, tin, tannic acid, etc., has the power of uniting with dyes and 

 forming colored compounds which adhere firmly to cotton and linen fabrics. 

 Such substances are called mordants (meaning biting), and without their use it 

 is impossible to dye cotton and linen permanently with most dyes. The insoluble 

 compounds of dyes with mordants are called lakes. When aluminum hy- 

 droxide is to be the mordant, the fabric is immersed in a hot solution of alum, 

 aluminum sulphate or acetate, or sodium aluminate, by which some aluminum 

 hydroxide, formed by hydrolysis of the compounds, is taken up by the fibres 

 of the fabric. The latter is then boiled in water containing the dye, which 

 unites with the mordant in the fibres, to form an insoluble permanent color. 



Experiment 29. Dissolve 10 grammes of sodium carbonate in 100 c.c. of 

 water, heat it to boiling, and add to it, with constant stirring, a hot solution, 

 made by dissolving 10 grammes of alum in 100 c.c. of water. Wash the pre- 

 cipitate first by decantation, and then upon a filter, until the washings are not 

 rendered turbid by barium chloride. Dry a portion of the precipitate at a low 

 temperature, and use as aluminum hydroxide. Mix a small quantity of the 

 wet precipitate with a decoction of logwood (made by boiling about 0.2 

 grammes of logwood with 50 c.c. of water), agitate for a few minutes, and 

 filter. Notice that the red color of the solution has entirely disappeared, or 

 nearly so, in consequence of the combination of the aluminum hydroxide 

 and coloring matter. 



Aluminum sulphate, Alumini sulphas, A1 2 (SO 4 ) 3 .16H 2 O = 

 625.93. A white crystalline powder, soluble in about its weight 

 _of water, obtained by dissolving the oxide or hydroxide in sulphuric 

 acid and evaporating the solution to dryness over a water-bath. 



2A1(OH) 3 + 3H 2 S0 4 == A1. 2 (S0 4 ), + 6H 2 O. 



