-518 PRINCIPLES OF CHEMISTRY 



likely because (1) there exists another salt, Na. 2 CO 3 ,2NaHCO 3 ,2H. 2 (> 

 (sodium sesquicarbonate), obtained by cooling a boiling solution of 

 sodium bicarbonate, or by mixing this salt with the normal salt ; 

 but the formula of this salt cannot be derived from that of normal 

 carbonic acid, as the formula of the bicarbonate can ; 20 (2) water 

 of crystallisation does not enter into the composition of the crystals 

 of the acid salt, so that on its formation (occurring only at low tem- 

 peratures, as in the formation of crystalline compounds with water) 

 the water of crystallisation of the normal salt separates and the water 

 is, as it were, replaced by the elements of carbonic acid. In any case 

 the acid sodium carbonate is an unstable salt. Not only when heated 

 alone, but even on being slightly heated in solution, and also at the 

 ordinary temperature in damp air, it loses carbonic anhydride and forms 

 the normal salt. And at the same time it is easy to obtain it in a pure 

 crystalline form, if a strong solution of sodium carbonate be cooled and 

 a stream of carbonic anhydride gas passed through it. The acid salt 

 is less soluble in water than the normal, 21 and therefore a strong 

 solution of the latter gives crystals of the acid salt if carbonic 

 anhydride be passed through it. The acid salt may be yet more 

 conveniently formed from effloresced crystals of sodium carbonate,. 



20 At the same time the sesqui-salt has all the properties of a definite compound ; it. 

 crystallises in transparent crystals, has a constant composition, its solubility (at in 100 

 of water, 12'6 of anhydrous salt) differs from the solubility of the normal and acid salts, 

 and shows changes in composition ; it is found in nature, and is known by the names of 

 trona and urao. The observations of Watts and Richards showed (1886) that on pouring 

 a strong solution of the acid salt into a solution of the normal salt saturated by heating, 

 crystals of the salt NaHCO 3 ,Na.,CO5,2HoO may be easily obtained, as long as the tempe- 

 rature is above 85. The natural urao (Boussingault) has, according to Laurent, the same- 

 composition. This salt is very stable in air, and may be used for purifying sodium carbo- 

 nate on the large scale. From the theoretical side such compounds have been little studied,, 

 yet are particularly interesting because, in all probability, they correspond with ortho- 

 carbonic acid C(OH)}, and at the same time correspond with double salts like astrachanite 

 (Chapter XIV.). 



n 100 parts of water at dissolves 7 parts of the acid salt, which corresponds with 

 4 '8 parts of the anhydrous normal salt, but at 100 parts of water dissolves 7 parts of 

 the latter. The solubility of the acid salt varies with considerable regularity ; 100 parts 

 of water dissolves at 15 9 parts of the salt, at 80 C 11 parts. 



The ammonium, and more especially the calcium, salt, is much more soluble in water- 

 The ammonia process (see ante, p. 516) is founded upon this. Ammonium bicarbonate 

 (acid carbonate) at has a solubility of 12 parts in 100 water, at 80 of 27 parts. The 

 solubility, therefore, increases very rapidly with the temperature. But its saturated solu- 

 tion is more stable than a solution of sodium bicarbonate. In fact, saturated solutions of 

 these salts have the vapour tension of a mixture of carbonic anhydride and water namely 

 at 15 and at 50, for the sodium salt 120 and 750 millimetres, for the ammonium salt 120 

 and 563 millimetres. These data are of great importance in understanding the pheno- 

 mena connected with the ammonia process. They demonstrate that with an increased 

 pressure the formation of the sodium salt ought to increase if there be an excess of ammo- 

 nium salt. 



