(A) Apparatus.—The reduction is carried out in a 15 by 23 cm. battery jar (Note 1), which is surrounded by a vessel suitable for a cooling bath. The bottom of the battery jar is covered with mercury, which serves as the cathode of the cell. The anode is a heavy lead coil separated from the catholyte by being suspended in a porous cup. This cup is set upon a support which holds it just above the cathode surface and does not appreciably diminish the latter (Note 2). The cathode is connected with the circuit by means of a glass tube partially filled with mercury and carrying a small piece of platinum wire sealed through the lower end. An efficient mechanical stirrer is provided in the catholyte. The current used in these experiments was 110 d.c., with a field rheostat, having a resistance of 8 ohms and a capacity of 20 amperes, for controlling the current. Any source of current supplying 30–110 volts and 15–20 amperes and any rheostat capable of carrying this current may be used. An ammeter reading to at least 15 amperes is connected in the circuit. Several reduction cells of the size described above may be run at one time, if they are connected in series. Figure 25 shows the general set-up.

(B) Reduction of Nitrourea.—After the apparatus is assembled, 600 cc. of 20 per cent sulfuric acid (Note 3) is placed in the battery jar, and the lead anode in the porous cup is also covered with acid of the same strength. The cell is surrounded by an ice-salt bath, the stirrer is started, and, while the solution is cooling, 50 g. (0.48 mole) of nitrourea (Note 4) is added to the catholyte. A thermometer is placed in the catholyte, and when the temperature drops to +5° the current is turned on as follows: with the rheostat set for maximum resistance, the current is switched on, then the rheostat is gradually adjusted until the current flowing through the cell is about 0.06 ampere per square centimeter of cathode surface (Note 5). The cell must be efficiently cooled with ice and salt so as to keep the temperature at all times below 10° (Note 6). The reduction requires five to six hours. The nitrourea is quite insoluble but gradually dissolves as it is reduced. The foam and clumps of solid are worked down into the liquid occasionally. When almost all the nitrourea is in solution, some of the catholyte should be drawn up with a pipette and used to wash down the nitrourea that adheres to the walls of the vessel. The reduction is continued for about ten to twenty minutes after the nitrourea has dissolved. When the reduction is complete, as is indicated by a marked evolution of hydrogen, the current is shut off, and the porous cup is washed down with a little distilled water and removed.

The solution of semicarbazide sulfate is removed from the cell and filtered (Note 7). The filtered solution is concentrated under reduced pressure on a water bath (Note 8) to a volume of 125–150 cc. Meanwhile, considerable semicarbazide sulfate will have crystallized. The mixture is cooled thoroughly in ice and the crystals are collected on a hardened filter paper, or better, on a filtros plate (see Note 5, on p. 11), and washed several times with absolute alcohol to remove sulfuric acid. The crystals are dried on clay plate or paper. The yield is 50–57 g. (61–69 per cent of the theoretical amount) of a product melting at 144–145° with decomposition.