The residual sulfuric acid is titrated with a set alkali in excess, ammonia being preferred.
The solution of ammonia is made by diluting 200 cubic centimeters of commercial aqua ammonia with 800 of water. Its strength is determined by adding a little copper sulfate solution as indicator and then the set solution of sulfuric acid until the blue color disappears. The copper sulfate secured from the cuprous sulfate as described above is cooled, and a quantity of the ammonia, equal to twenty-five cubic centimeters of the set sulfuric acid, added. The excess of the ammonia is then determined by titration with the sulfuric acid, the disappearance of the blue color being the indication of the end of the reaction. The number of cubic centimeters of the set sulfuric acid required to saturate the ammonia represents the equivalent of cuprous oxid originally present. One cubic centimeter of normal sulfuric acid is equivalent to 0.0317 gram of metallic copper.
To determine the weight of invert sugar oxidized, multiply the weight of copper, calculated as above described, by the factor 0.3546.[102] For a general application of this method of analysis the relative quantities of copper reduced by different quantities of sugar must be taken into consideration.
While, as has already been stated, the copper carbonate process has heretofore been applied chiefly to the detection of invert sugar, it has merits which justify the expectation that it may some time supplant the fehling liquor both for volumetric and gravimetric work. Large volumes of the reagent can be prepared at once and without danger of subsequent change. The action of the reagent on the hexobioses and trioses is far less vigorous than that of the alkaline copper tartrate, and the end reactions for volumetric work are, at least, as easily determined in the one case as the other.
134. Method Depending on Titration of Excess of Copper.—Instead of measuring the quantity of copper reduced, either by its disappearance or by reducing the cuprous oxid to a metallic state, Politis has proposed a method of analysis depending on the titration of the residual copper.[103] The reagents employed are:
(1) A copper solution containing 24.95 grams of crystallized copper sulfate, 140 grams of sodium and potassium tartrate, and twenty-five grams of sodium hydroxid in one liter:
(2) A solution of sodium thiosulfate containing 24.8 grams of the salt in one liter:
(3) A solution of potassium iodid containing 12.7 grams of iodin in one liter.
The reaction is represented by the formula
2CuCl₂ + 4KI = Cu₂I₂ + 4KCl + I₂.