144 METALS AND THEIR COMBINATIONS. 



Experiment 18. Dissolve about 10 grammes of crystallized sodium carbonate 

 in 10 c.c. of hot water, add to this solution dilute sulphuric acid until all effer- 

 vescence ceases and the reaction on litmus-paper is exactly neutral. Evaporate 

 to about 20 c.c., and set aside for crystallization. Explain the action taking 

 place, and state how much H 2 S0 4 , and how much of the diluted sulphuric 

 acid, U. S. P., are needed for the decomposition of 10 grammes of crystallized 

 sodium carbonate. 



Sodium sulphite, Sodii sulphis, Na 2 SO 3 .7H 2 O = 252. Sodium 

 bisulphite, Sodii bisulphis, NaHSO 3 = 1O4. By saturating a cold 

 solution of sodium carbonate with sulphur dioxide, sodium bisulphite 

 is formed, and separates in opaque crystals : 



Na^COg -f 2SO 2 + H 2 O = 2NaHSO 3 + CO 2 . 



If to the sodium bisulphite thus obtained a quantity of sodium car- 

 bonate be added, equal to that first employed, the normal salt is formed : 

 2NaHS0 3 + Na^COs = 2^803 + H 2 O -f CO 2 



Sodium thlosulphate, Sodium hyposulphite, Sodii hyposul- 

 phis, Na 2 S 2 O 3 .5H 2 O 248. made by digesting a solution of sodium 

 sulphite with powdered sulphur, when combination slowly takes place : 

 Na,S0 3 + S = Na&O,. 



It is used under the name of "hypo" in photography to dissolve 

 Chloride, bromide, or iodide of silver. 



Disodium hydrogen phosphate, Sodii phosphas, Na 2 HPO 4 . 

 12H 2 O = 358 (Sodium phosphate) is made from calcium phosphate by 

 the action of sulphuric acid, which removes two-thirds of the calcium, 

 forming calcium sulphate, while acid phosphate of calcium is formed 

 and remains in solution : 



Ca 3 (POJ 2 + 2H 2 S0 4 = 2CaS0 4 + CaH 4 (PO 4 ) 2 . 



The solution is filtered and sodium carbonate added, when calcium 

 phosphate is precipitated, phosphate of sodium, carbon dioxide, and 

 water being formed : 



CaH 4 (P0 4 ) 2 + Na^COg == CaHPO 4 + H 2 O + CO 2 + Na 2 HPO 4 . 



The filtered and evaporated solution yields crystals of sodium 

 phosphate, which have a slightly alkaline reaction to litmus, but not 

 to phenol-phtalein. 



Experiment 19. Mix thoroughly 30 grammes of bone-ash with 10 c.c. of 

 sulphuric acid, let stand for some hours, add 20 c.c. of water, and again set 

 aside for some hours. Mix with 40 c.c. of water, heat to the boiling-point, and 

 filter. The residue on the filter is chiefly calcium sulphate. To the hot filtrate 

 of calcium acid phosphate add concentrated solution of sodium carbonate until 

 a precipitate ceases to form and the liquid is faintly alkaline, filter, evaporate, 

 and let crystallize. 



