EMPIRICAL STANDARD SOLUTION 525 



proportions just before use, because the mixture is not very stable and 

 decomposes. After the two fluids are mixed the copper is present 

 in the form of a hydrate of the metal, and this hydrate in hot alkaline 

 solution is reduced by the sugars named into, first, a cupric oxide, and 

 then a cuprous oxide. The former is a yellow, the latter a red-brown 

 copper salt. The details of this process of reduction are not yet 

 perfectly clear, but it is established beyond doubt that 10 milligrams 

 of glucose, 16 of maltose, or 13.5 of lactose will bring about the 

 complete reduction of all of the copper hydrate contained in 2 c.c. 

 of the mixture of solutions No. 1 and No. 2; that is, of the copper 

 salt originally contained in 1 c.c. of solution No. 1. When the 

 reduction of the copper hydrate occurs in the hot alkaline solution a 

 yellowish or orange precipitate is generally first formed, and finally a 

 red-brown precipitate at the bottom of the beaker; the supernatant 

 fluid after the fluid has become cool is perfectly colorless (all blue 

 color has disappeared). As it would be impossible to recognize at the 

 right moment the end point of the reaction, i. e., the complete reduction 

 of the copper hydrate) an indicator must be used. As such a saturated 

 watery solution of ferrocyanide of potash strongly acidulated with 

 glacial acetic acid is employed in the following manner: Small 

 pieces of filter paper are moistened with the indicator and from time 

 to time a drop of the boiling copper solution (to which the sugar- 

 containing fluid from the burette is being added) is allowed to fall on 

 them. As long as unreduced copper hydrate remains in the boiling 

 solution it forms a red cyanide of copper with the acid ferrocyanide 

 solution. When the reduction, however, is completed no more 

 cyanide of copper is formed and no more sugar containing fluid 

 should be added from the burette. 



Steps in the Determination. 1. Mix equal amounts of solutions 

 from bottle No. 1 and No. 2. This mixture is the standard solution 

 now ready for use. 



2. Take 20 c.c. of the mixture, place into a beaker, dilute with 

 30 c.c. of distilled water, and heat over a small flame, or, still better, 

 in a water bath. 



3. The sugar containing fluid must first be filtered, and if it contain 

 much sugar (which should be ascertained by a preliminary qualitative 

 test) it should be diluted with distilled water, so that it probably 

 contains between J and 1 per cent, sugar approximately. 



4. Heat the standard solution in the beaker to boiling and then add 

 the sugar solution gradually from the burette. Continue heating and 

 adding, and from time to time test the boiling solution with the indi- 

 cator. When cyanide of copper ceases to be formed on the moistened 

 filter paper, stop adding sugar solution from the burette. If the test 

 has been made properly the fluid in the beaker should be colorless 

 after cooling, and if mixed with the indicator should not form a red 

 precipitate. Nor should it look yellow, for this would indicate that 

 an excess of sugar solution had been added. 



