SODIUM. 265 



saturating a cold solution of sodium carbonate with sulphur dioxide, 

 sodium bisulphite is formed, and separates in opaque crystals : 



Na-jCOg -f 2SO 2 + H 2 O = 2NaHSO 3 -f 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 : 

 2NaHSO 3 -(- Na. 2 CO 3 = 2NaaSO s + H 2 O + CO 2 . 



Sodium thiosulphate, Sodium hyposulphite, Sodii thiosul- 

 phas, Na 2 S 2 O 3 .5H 2 O = 246.46. Made by digesting a solution of 

 sodium sulphite with powdered sulphur, when combination slowly 

 takes place : 



Na 2 S0 3 + S : Na 2 SA- 



It is used under the name of " h^po " in photography to dissolve 

 chloride, bromide, or iodide of silver. 



Sodium phosphate, Sodii phosphas, Na 2 HPO 4 .12H 2 O 355.61, 

 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 (PO 4 ) 2 + 2H 2 S0 4 = 2CaSO 4 -f 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 2 CO 8 = CaHPO 4 -f H 2 O + CO 2 -f N^HPO,. 



The filtered and evaporated solution yields crystals of sodium 

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

 to phenol-phthalein. 



By exposure of the crystallized sodium phosphate to warm air its water of 

 crystallization is expelled and the dry salt is Exsiccated sodium phosphate, 

 Sodii phosphas exsiccatus. This salt, when mixed with the proper quantities 

 of sodium bicarbonate and tartaric and citric acids, is official under the name 

 of Effervescent sodium phosphate, Sodii phosphas effervescens. 



Experiment 24. 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. 



Sodium pyrophosphate, Sodii pyrophosphas, Na 4 P 2 O 7 .lOH 2 O = 

 443.O2. When exsiccated sodium phosphate is heated to a low red 



