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. The analytical process is carried out as follows: In a 100 cubic centimeter flask are boiled fifty cubic centimeters of the copper solution, ten cubic centimeters of about one-tenth per cent reducing sugar solution are added, the boiling continued for five minutes, the flask filled to the mark with boiling water and its contents filtered. Fifty cubic centimeters of the hot filtrate are cooled, slightly acidified, potassium iodid solution added in slight excess; and the iodin set free determined by titration with sodium thiosulfate. The quantity of iodin obtained corresponds to the unreduced copper remaining after treatment with the reducing sugar. The number of cubic centimeters of thiosulfate used subtracted from twenty-five will give the number of cubic centimeters of the copper solution which would be reduced by five cubic centimeters of the sugar solution used. _Example._—In the proportions given above it was found that eleven cubic centimeters of thiosulfate were required to saturate the iodin set free. Then 25 - 11 = 14 cubic centimeters of copper solution reduced by five cubic centimeters of the sugar solution. Since one cubic centimeter of the copper solution is reduced by 0.0036 gram of dextrose the total dextrose in the five cubic centimeters = 0.0036 × 5 = 0.0180 gram.