018 PRINCIPLES OF CHEMISTRY 



sulphate be mixed with a solution of aluminium, magnesium, or ferrous 

 sulphate, it gives crystals of a double salt when evaporated. Sulphuric 

 acid itself forms a compound with sodium sulphate, which is exactly 

 like these double salts. It is formed with great ease when sodium 

 sulphate is dissolved in sulphuric acid and the solution evaporated. 

 On evaporation, crystals of the acid salt separate, Na 2 S0 4 + H 2 SO 4 

 = 2NaHSO 4 . This separates from hot solutions, whilst the crystallo- 

 hydrate, NaHS0 4 ,H 2 O, l separates from cold solutions. The crystals 

 when exposed to damp air decompose into H 2 S0 4 , which deliquesces, 

 and Na 2 SO 4 (Graham, Rose) ; alcjohol also extracts sulphuric acid from 

 the acid salt. This shows the feeble force which holds the sulphuric 

 acid to the sodium sulphate. 11 Both acid sodium sulphate and all 

 mixtures of the normal salt and sulphuric' acid lose water when heated, 

 and are converted into sodium pyrosulphate, Na 2 S 2 O 7 , at a low red 

 heat. 11 bi * This anhydrous salt, at a bright red heat, parts with the 

 elements of sulphuric anhydride, the normal sodium sulphate remaining 

 behind Na 2 S 2 O 7 = Na 2 SO 4 + SO 3 . From this it is seen that the 

 normal salt is able to combine with water, with other sulphates, and 

 with sulphuric anhydride or acid, &c. 



Sodium sulphate may by double decomposition be converted into 

 a sodium salt of any other acid, by means of heat and taking advantage 

 of the volatility, or by means of solution and taking advantage of the 

 different degree of solubility of the different salts. Thus, for instance, 

 owing to the insolubility of barium sulphate, sodium hydroxide or 

 caustic soda may be prepared from sodium sulphate, if barium hydroxide 

 be added to its solution, Na 2 S0 4 -* Ba(HO) 2 = BaSO 4 + 2NaHO. 

 And by taking any salt of barium, BaX 2 , the corresponding salt of 

 sodium may be obtained, Na 2 S0 4 + BaX 2 = BaSO 4 * + 2NaX. Barium 



10 The very large and well-formed crystals of this salt resemble the -hydrate 

 H 2 SO 4 ,H 2 O, or SO(OH) 4 . In general the replacement of hydrogen by sodium modifies 

 many of the properties of acids less than its replacement by other metals. This most 

 probably depends on the volumes being nearly equal. 



11 In solution (Berthelot) the acid salt in all probability decomposes most in the 

 greatest mass of water. The specific gravity (according to the determinations of 

 Marignac) of solutions at 15/4 = 9,992 + 71-92p + Q-231p 2 (see Note 7). From these 

 figures, and from the specific gravities of sulphuric acid, it is evident that on mixing 

 solutions of this acid and sodium sulphate expansion will always take place; for 

 instance, H 2 SO 4 +25H 2 O with Na 2 SO 4 + 25H 2 O increases from 483 volumes to 486. In- 

 addition to which, in weak solutions heat is absorbed, as shown in Chapter X., Note 27. 

 Nevertheless, even more acid salts may be formed and obtained in a crystalline form. 

 For instance, on cooling a solution of 1 part of sodium sulphate in 7 parts of sulphuric 

 acid, crystals of the composition NaHSO 4 ,H 2 SO 4 are separated (Schultz, 1868). Thia 

 compound fuses at about 100 ; the ordinary acid salt, NaHSO 4 , at 149. 



11 bb On decreasing the pressure, sodium hydrogen sulphate, NaHSO 4 , dissociates 

 much more easily than at the ordinary pressure ; it loses water and forms the pyrosul- 

 phate, Na2S 2 O 7 ; this reaction is utilised in chemical works. 



