=257. Removal of Dextrose by Copper Acetate.=—Maercker first called attention to the fact that Barfoed’s reagent (one part copper acetate in fifteen parts of water, and 200 cubic centimeters of this solution mixed with five cubic centimeters of thirty-eight per cent. acetic acid) reacts readily with dextrose, while it is indifferent to maltose and dextrins. Sieben’s method of removing dextrose is based on this fact.[214] It is found that under certain conditions pure maltose does not reduce either the acidified or neutral solution of copper acetate, while dextrose or a mixture of dextrose and maltose does so readily. It is also shown that the fermentation residue under suitable conditions acts like maltose. Maltose solutions reduce the reagent after boiling four minutes while at 40°-45° they have no effect even after standing four days. The amount of copper deposited by dextrose, under the latter conditions, is found to depend to a certain extent on the amount of free acetic acid present, and as the solutions of copper acetate always contain varying quantities of acetic acid which cannot be removed without decomposition and precipitation of basic salt, the use of an absolutely neutral solution is impracticable. The reagent prepared according to Barfoed’s directions is almost saturated, but a half normal solution is preferable. Sieben proposes two solutions: I, containing 15.86 grams copper and 0.56 gram acetic anhydrid per liter; II, containing 15.86 grams copper and three grams acetic anhydrid per liter. The reduction of the dextrose is secured by placing 100 cubic centimeters of the solution in a bottle, adding the sugar solution, stoppering and keeping in a water-bath at 40°-45° two or three days. An aliquot portion is then drawn off and the residual copper precipitated by boiling with forty-five cubic centimeters of the alkali solution of the fehling reagent and forty cubic centimeters of one per cent dextrose solution, filtered and weighed as usual. The results show that either solution can be used, and that standing for two days at 45° is sufficient. One hundred cubic centimeters of the copper solution are mixed with ten cubic centimeters of the sugar solution containing from two-tenths to five-tenths gram of dextrose, as this dilution gives the best results. No reduction is found to have taken place when solutions containing five-tenths gram maltose or five-tenths gram fermentation residue are used. The data can not be compiled in the form of a table similar to Allihn’s, as it is impossible to obtain a solution of uniform acidity each time, and the solution will have to be standardized by means of a known pure dextrose solution and the result obtained with the unknown sugar solution properly diluted compared with this. This method of Sieben’s has never been practiced to any extent in analytical separations and can not, therefore, be strongly recommended without additional experience.