possible, should be without a lip. The larger beaker, B, should hold at least 1 liter. Provide three flat corks of equal thickness to serve as supports for A; a stirrer C made from a piece of light-weight rod and bent into a ring at the end; and a thermometer D graduated to one tenth of a degree. Cut a cardboard cover E, circular in form, to fit over A and inside B; also a cover F for B, which may be left square. Punch a hole in the middle of each cover, through which the thermometer may be pushed, but small enough to hold the latter in place. A second hole must also be punched in each cover to provide for the handle of the stirrer. B-- In a graduated cylinder, measure as accurately as possible 150 cc. of a normal solution of NaOH, and pour it into A; measure also 150 cc. of a normal solution of HCl, and pour it into a second beaker. Read the temperature of the two solutions at frequent intervals until they have come to exactly the same constant point, warming the cooler one, if necessary, with the hand. Make a note of this temperature, reading it as closely as possible. Arrange the thermometer and the stirrer through the holes in the covers so that all may be put in place together; then pour the acid into the alkali and at once adjust the covers and the thermometer. Maintain a slow but steady motion with the stirrer, watching the rise in temperature very closely. Make a note of the highest temperature reached. FIG. 51 The specific heat of the dilute solution may with little error be taken as equal to that of water (unity), so that the rise in temperature, multiplied by the volume of the solution (in this case 300 cc.), will give the apparent heat of the reaction. Some heat has been absorbed by the calorimeter beaker, the thermometer, and the stirrer, and this must be determined and added to the apparent heat. If the materials are of the character specified, experiment has shown that the heat absorbed will be about 12 cal., and this value may be assumed with little error. The constant may be calculated by determining the weight of the portion of the beaker and stirrer in contact with the water, multiplying this by 0.19 (the specific heat of glass), and to this adding the volume of the submerged part of the thermometer, multiplied by 0.49. Having in this way found the heat of neutralization of 150 cc. of normal NaOH, calculate the heat of neutralization of 1 gram-molecular weight of the base (1000 cc.). The value determined by accurate experiment is 13,700 cal. 132. Heat of solution. Accurately measure 300 cc. of pure water into the calorimeter beaker A (Fig. 51). Grind about 15 g. of potassium nitrate to a very fine powder in a mortar, and place it in a test tube. Weigh the tube and nitrate very accurately and immerse the lower end of the tube in the water in the calorimeter until the nitrate has come to the temperature of the water (about fifteen minutes). Make a note of the temperature. Remove the tube with as little loss of water as possible, roughly dry it with a towel, and at once pour most of the nitrate into the water. Quickly replace the covers, as in § 131, stir vigorously, and note the lowest temperature recorded. Then weigh the tube and the remaining nitrate, deducting this weight from the original one to get the weight of the nitrate added. From the data so secured, together with the calorimeter constant, calculate the heat of solution of 1 gram-molecular weight of potassium nitrate. Berthelot found this to be -8300 cal. CHAPTER XXIII CARBOHYDRATES; ALCOHOLS; SOAPS 133. Preparation of Fehling's solution. The most common test for sugars is their reaction with the so-called “Fehling's solution." This solution is prepared as follows: Dissolve 3.5 g. of copper sulfate crystals in water and dilute to 50 cc. Pour the solution into a bottle and label it "Solution A." Dissolve 5 g. of sodium hydroxide and 17.5 g. of sodium-potassium tartrate (Rochelle salts) in about 40 cc. of water and dilute to 50 cc. Pour this into a bottle and label it "Solution B." 134. Action of Fehling's solution on dextrose. Pour into a test tube about 3 cc. of each of the above solutions marked "A" and "B." When thoroughly mixed, the resulting solution should be deep blue but perfectly clear. Heat nearly to boiling, add a few drops of a solution of commercial glucose, and continue the heating. The copper compound in the solution is reduced to cuprous oxide by the dextrose present, and this separates in the form of a red or yellow solid. Levulose will act in the same way. Test samples of candy, honey, and molasses for the presence of these sugars. 135. Action of Fehling's solution on cane sugar. In a similar way try the action of cane sugar on the solution. Note that the pure sugar does not reduce the alkaline copper compound. Now dissolve about 1 g. of the sugar in 10 cc. of water. Add 4 or 5 drops of concentrated hydrochloric acid and slowly heat to boiling. Set aside for about five minutes, then cool and neutralize the acid in the solution by adding a concentrated solution of sodium carbonate until the resulting mixture is just alkaline to litmus paper. Test this with Fehling's solution according to above directions. Account for the result. |