242 ALKALIMETRY. 



to dissolve, and so much water added that the whole 

 solution may measure 1 litre or 1000 cubic centime- 

 tres, at 17.5 C. 



One hundred cub. cents, of this test-acid will then 

 exactly neutralize T V of an equivalent proportion of 

 either alkali. It is therefore necessary to weigh out 

 y'g of an equivalent proportion (in grammes) of the 

 anhydrous alkali to be tested, that is, 6*92 grms. of 

 potashes, or 5'32 grms. of soda-ash. In order to obtain 

 perfectly accurate results, the process is conducted as 

 follows : The solution of alkali to be tested, is intro- 

 duced into a flask colored, with tincture of litmus, 

 and the test-acid poured into it from a burette, until 

 the color changes from blue to violet, and the effer- 

 vescence is very feeble. The solution is now heated to 

 ebullition, and more acid added until the color has 

 become decidedly red. 5 or 10 cub. cents, of the test- 

 acid are then added in excess ; the alkali will be now 

 supersaturated. By boiling, agitating, and finally suck- 

 ing out with a glass tube, the last traces of carbonic 

 acid are removed r -now required to determine 

 exactly how far tne neutralization of the alkali has 

 been exceeded ; for this purpose a standard solution 

 of caustic soda is employed, of such strength that it 

 is exactly neutralized by an equal volume of the test- 

 acid ;* this solution is added from a burette graduated 

 to T*$ cub. cent., when the red color rapidly changes 

 to violet, and then suddenly to pure blue. The num- 

 ber of cubic centimetres of soda-solution employed, is 

 then deducted from the volume of test-acid previously 

 added ; the remainder gives the percentage of pure 

 alkaline carbonate. 



* This solution of soda must be perfectly free from carbonic 

 acid. In order to preserve it in that state, the bottle is closed 

 with a cork, through which passes an ordinary chloride-of-calcium- 

 tube, open at both ends, and filled with a mixture of Glauber's 

 salt and quicklime in powder. 



