34.64 grams copper sulfate; 187.0 potassium-sodium tartrate; 78.0 sodium hydroxid made up to one liter. Ten cubic centimeters equal 0.050 gram dextrose. Ten cubic centimeters equal 0.0475 gram cane sugar.
117. Volumetric Method used in this Laboratory.—The alkaline copper solution preferred in this laboratory has the composition proposed by Violette. The copper sulfate and alkaline tartrate solutions are kept in separate vessels and mixed in proper proportions immediately before use, and diluted with about three volumes of water. The reduction is accomplished in a long test tube at least twenty-five centimeters in length, and from thirty-five to forty millimeters in diameter.
The sugar solution employed should contain approximately one per cent of reducing sugar. If it should have a greater content it should be reduced with water to approximately the one named. If it have a less content, it should be evaporated in a vacuum at a low temperature until it reaches the strength mentioned above. A preliminary test will indicate almost the exact quantity of the sugar solution to be added to secure a complete reduction of the copper. This having been determined the whole quantity should be added at once to the boiling copper solution, the test tube held in the open flame of a lamp giving a large circular flame and the contents of the tube kept in brisk ebullition for just two minutes. The lamp is withdrawn and the precipitated suboxid allowed to settle. If a distinct blue color remain an additional quantity of the sugar solution is added and again boiled for two minutes. When the blue coloration is no longer distinct, the presence or absence of copper is determined by aspirating a drop or two of the hot solution with the apparatus described below. This clear filtered liquor is then brought into contact with a drop of potassium ferrocyanid solution acidulated with acetic. The production of a brown precipitate or color indicates that some copper is still present, in which case an additional quantity of the sugar solution is added and the operation continued as described above until after the last addition of sugar solution no coloration is produced.
118. The Filtering Tube.—The filtering tube used in the above operation is made of a long piece of narrow glass tubing with thick walls. The length of the tube should be from forty to forty-five centimeters. One end of the tube being softened in the flame is pressed against a block of wood so as to form a flange. Over this flange is tied a piece of fine linen.[82]
Instead of using a linen diaphragm the tube is greatly improved, as suggested by Knorr, by sealing into the end of the tube while hot a perforated platinum disk. Before using, the tube is dipped into a vessel containing some suspended asbestos felt and by aspiration a thin felt of asbestos is formed over the outer surface of the platinum disk. By inverting the tube the water which has entered during aspiration is removed. The tube thus prepared is dipped into the boiling solution in the test tube above described and aspiration continued until a drop of the liquor has entered the tube. It is then removed from the boiling solution, the asbestos felt wiped off with a clean towel, and the drop of liquor in the tube blown through the openings in the platinum disk and brought into contact with a drop of potassium ferrocyanid in the usual way. In this way a drop of the liquor is secured without any danger of a reoxidation of the copper which may sometimes take place on cooling.
Figure 42. Apparatus for the Volumetric Estimation
of Reducing Sugars.
The careful analyst by working in this way with the volumetric method is able to secure highly accurate results. The apparatus used is shown in the accompanying illustration.
119. Suppression of the Error Caused by the Action of the Alkali on Reducing Sugars.—Three methods are proposed by Gaud for correcting or suppressing the error due to the action of the alkali upon reducing sugars. In the first place, the common method followed may be employed, depending upon the use of an alkaline copper solution of known composition and the employment of a reducing sugar solution of a strength varying between one-half and one per cent. The error which is introduced into such a reaction is a constant one and the solution having been tested once for all against pure sugar is capable of giving fairly accurate results.
In the second place, a table may be constructed in which the error is determined for sugar solutions for varying strengths, viz., from one-tenth of one per cent to ten per cent. If y represent the error and x the exact percentage of reducing sugar present then the correction may be made by the following formula;