378 CARBONATES. 



or difficultly soluble, the proportion of residuum, therefore, indicates 

 the amount of impurities. If the impurities are soluble, as muriate 

 of soda, then the sulphuric acid becomes a test ; 355 grains of this 

 acid of the sp. gr. 1.141, (which is the best for this purpose,) satu- 

 rate 100 grains of carbonate ofpotassa. Dissolve this in water, add 

 the diluted acid by degrees, till the alkali is neutralized, and weigh 

 the remaining acid ; then as 355 : 100 :: the acid expended to the 

 proportion of alkali.* 



BI-CARBONATE OF POTASSA. 



1. PREPARATION. 



(a.) In Nooth's, or a similar machine, pass carbonic acid gas, 

 to saturation, through a solution ofpotassa, or of the carbonate in 5 

 parts of water. The bi-carbonate crystallizes as the process goes 

 forward, or afterwards by gentle evaporation. 



(6.) Or, we may take If part of carbonate of ammonia, and 4 

 of the carbonate of potassa, and dissolve in 4 of water ; distil with a 

 gentle heat in a retort ; ammonia is found in the water of the re- 

 ceiver, and bi-carbonate of potassa in the retort, without any loss of 

 materials.f 



(c.) By exposing potassa, or its carbonate, in the vats of the brew- 

 er, or of the distiller, we can obtain the crystallized bi-carbonate. 



2. PROPERTIES. 



(a.) Crystallizes in tables, or quadrilateral prisms, and is termi- 

 nated by pyramids. 



(&.) Taste, slightly alkaline, but not caustic ; mild in the stomach, 

 not deliquescent. 



(c.) Sometimes efflorescent.^ Sp. gr. 2.012. 



(d.) Soluble at 60, in about 4 parts of cold water, and in about 

 5 at 212. The strongest permanent solution at common tempe- 

 rature, has the sp. gr, 1.54, and contains 48.81, of carbonate. 



(e.) Boiling hot water expels bubbles of gas, amounting to ,-\ of 

 its weight. A boiling heat is therefore sufficient partly to decompose 

 the salt. 



(/.) Decrepitates and melts with a gentle heat, loses its water, 

 and a red heat expels just half its carbonic acid, leaving it a pure 

 carbonate. 



3. PROPORTION OF PRINCIPLES. It contains twice as much car- 

 bonic acid as the carbonate ; proved by the quantity of gas given 

 out from each by the action of acids. 



* Henry, 10th Ed. Vol. 1, p. 544. 



t To 1 Ib. of sub-carbonate of potash, in solution, add 2 or 3 oz. of carbonate of am- 

 monia, and distil. Dr. Hope. 



\ During the saturation of common pot or pearlash, wilh carbonic acid, silica is 

 always deposited. 



Four. Vol. IV, p. 41. 



