114 § 81. BASES AND ACIDS WITH REAGEXTS. 



ties should be filled to the top, so that no carbonic acid 

 can be absorbed. 



Before using the solution, boil about 10 c.c. of it with 

 about 40 c.c. of water, or of a dilute solution of sodic hy- 

 drate if there is any reason to believe that carbonic acid 

 has been absorbed ; there should not be the least change 

 in the liquid when subjected to this trial. 



To perform the analysis, put 10 c.c. of the cuprlc solu- 

 tion in a porcelain dish, and add 40 c.c. of water, or of a 

 solution of sodic hydrate if it was found necessary to 

 add this in testing the solution ; heat the mixture until it 

 boils gently, and allow the sugar solution, wbich should 

 be colorless and not acid, and should not contain more 

 than ^1^ to ^|/|g of sugar, to drop in from a burette or a 

 pipette, graduated into ^1^^ c.c, so slowly that the boiling 

 will not be stopped. After the addition of the first few 

 drops, the fluid shows a greenish-brown tint ; as more sug- 

 ar solution is added, the precipitate becomes more copi- 

 ous, acquires a reddish tint, and subsides more speedily • 

 when it presents a deep red color, remove the lamp, allow 

 the precipitate to settle, and give the dish an inclined 

 position, so that the color of the supernatant liquid can 

 be more readily distinguished over the Avhite porcelain 

 surface ; if no blue or bluish-green color is seen, probably 

 enough of the solution of sugar has been added. To be 

 sure, test a small portion of the clear liquid Avith a drop 

 of the solution from the burette ; if a yellowish-red 

 precipitate appears on heating, which, at first, may 

 look like a cloud in the liquid, pour the contents of the 

 tube back into the dish again, and continue to add the 

 sugar solution until the reaction is completed. Then 

 the solution in the dish should contain neither copper nor 

 sugar, nor a brown product resulting from the decompo- 

 sition of the latter ; filter off a portion of it while still 

 hot ; the filtered liquid should have no brown tinge ; one 

 portion, heated with a drop of the standard cupric solu- 



