The Preparation of Ammonium Hydroxide for Laboratorv Use J
KENNETH A. KOBE a n d TED S. MARKOV University of Washington, Seattle, Washington
A
MMONIUM hydroxide is used in large quantities ber of small holes in the body. Because of these holes for a variety of chemical reactions. The purchase a large number of small bubbles were formed which inof carboys of the solution is expensive for experi- creased the gas surface and the rate of absorption. ments using large quantities of this reagent. There- When the ammonia in the cylinder is exhausted, solufore, an investigation was initiated to determine the tion will syphon slowly back from the first carboy into best method of preparing this reagent in the laboratory the trap (C). from liquid ammonia. Ammonium hydroxide solution contains approximately seventy per cent of water, the freight on which, together with the heavy carboy and crate, is the largest item of cost. Liquid ammonia in iron cylinders may be purchased in every industrial center. METHODS OF ABSORPTION
The tower system for absorption of ammonia in a countercurrent flowing stream of water comes to mind first. Several disadvantages are apparent, such as careful control necessary to produce the desired concentration, large capacity, and necessity for removal of heat of solution. In fact, the latter factor is the most important consideration in all systems preparing large quantities of ammonium hydroxide in a short period of time, and cooling must be carried out. The reaction may be considered according to Le Chatelier's principle NHdd
+ HnO (1) ZZ? NHaOH(Aq.1 + A
thus the exothermic reaction raises the temperature and shifts the equilibrium to the left, decreasing the concentration of ammonium hydroxide, unless the heat of reaction is removed. Increased pressure is seen to increase the concentrations of ammonium hydroxide obtained. A simple absorption system utilizing these principles was devised. (See the figure.) Three large carboys were connected in series. A fifty-pound ammonia cylinder ( A ) was connected to the first carboy (D) through a one-gallon jug (C) whicb acted as a trap to prevent solution from being sucked into the cylinder. The line B was a three-eighths-inch rubber pressure tube, but all lines thereafter (F, I) were three-eighths-inch glass tubing with rubber tubing a t the joints. The carboys were fitted with two-inch rubber stoppers (E) in which three boles were cut. One was for the tube F, bringing the ammonia into the solution, the second for the tube I to pass unabsorbed ammonia into the second carboy, and the third for the three-sixteenthinch sample tube H from which samples of the solution in the carboy could be drawn. In the third carboy the stopper had a small safety tube (J)%r the passage of unabsorbed gas to the atmosphere. At the end of each inlet tube was a 75-ml. flask (G)which had a num-
_I In o~eration,sixtv-eicht D O U ~ of ~ Swater are ~ o u r e d into each carboy add tge stoppers fastened do& with wire or tape. The valve of the ammonia cylinder is opened wide and absorption allowed to continue until the ammonium hydroxide in the first cylinder has reached the desired concentration. The first carboy is then removed and a new one added after the third carboy, the second carboy now being the first carboy. This procedure can be followed until the desired amount of ammonium hydroxide has been prepared. The concentration is determined by titrating the solution with normal acid, or by hydrometer reading of density. RESULTS
It was found that the 6rst carboy reached the desired normality in about ninety hours. It had a normality of 15.5, the second had reached 8.8, and the third 3.6. The absorption efficiency found by weighing the cylinder and comparing with the carboys was over ninety-five per cent. The maximum concentration obtained was 17 normal. Commercial ammonium bydroxide showed slight variation from 15 normal, and solutions as uniform could be made in this absorption apparatus. DISCUSSION
Obviously, atmospheric cooling has been utilized to remove the beat of solution. Using sixty-eight pounds of water per carboy, which will give ninety-five pounds of 15 normal ammonium hydroxide it may be calcu-
lated' that 23,100 B.t.u. are given off per carboy during the absorption. As absorption takes place, the temperature rises, and less ammonia is absorbed, so that the second carboy has more ammonia to absorb. As the Grst carboy cools, absorption can increase, but the increasing concentrations of ammonium hydroxide again decrease the rate. The hydrostatic bead of solution in the second and third carboys produce a back pressure on the first carboy that increases the concentration. Winter months or rainy weather are better for absorptien than hot summer days because of the atmospheric cooling required. No heat was applied
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' Brcnowsm AND ROSSINI,"Thermochemistry of chemical substances." Rheinhold Publishing Company, New York City. 1936.
to the ammonia cylinder, which drops considerably in temperature as evaporation of liquid ammonia occurs. Heating of the cylinder would give an increased rate of production, due to the more rapid supplying of ammonia to the solution. The economies effected by this procedure depend upon local costs, particularly freight costs from the manufacturer. In the soft water region, as the Pacific slope, the water contains but little dissolved material, much of which precipitates out during the absorption of the ammonia. Distilled water for the preparation of a pure ammonium hydroxide may be figured a t a cost of two-thirds of a cent per ga1lon.l 'Barnstead Still and Sterilizer Company, Boston. Massachusetts. Catalog B 1930.
