The Preparation of Ammonia
Ammonia is quite useful for a varity of chemical applications, but alas it is not something you can get at the store. The only commercial source of ammonia is on the farm where it is sprayed into the ground as fertilizer.
Industrially ammonia is prepared by the direct combination of hydrogen and nitrogen, themselves obtained from water, air, and natural gas heated to high temperatures and compressed at enormously high pressures. This process is not economical on the laboratory scale, although it can be done at atmospheric pressure with considerable waste of reactants.
It is more convenient to prepare ammonia in the laboratory starting with commercially available ammonium hydroxide, available in any supermarket. There are a few different methods, but they are quite similar. They involve decomposing any ammonium salt with a strong base to liberate ammonia.
Ammonia can either be prepared as a gas, or as a solution of ammonia in water. Depending on your needs the procedure can be modified to fit the application. It is not necessary to use the reagents described here as the presented reactions are quite versatile. Any ammonium salt such as the halide, sulfate, sulfide, nitrate, acetate, and phosphate will work about the same. Furthermore, any strong base such as the hydroxides of sodium, potassium, lithium, or calcium, as well as calcium oxide or carbonate will work.
For matters of economic convenience, using ammonium chloride as the ammonia source, and either calcium oxide (lime), calcium hydroxide (slaked lime), or calcium carbonate (limestone) as the base is best. Ammonium chloride is easily prepared from readily available materials, and slaked lime is quite common as a fertilizer. Please refer to the appropriate precursor sections for synthesis information on ammonium chloride and the calcium compounds.
It will be found necessary to have a suitable apparatus setup in place before commencing with ammonia generation. This also applies to having the intended reaction in place and underway, for gaseous ammonia, as it is not easily stored for future use. For preparing aqueous solutions all that is required is a means of bubbling the gas into water. For in situ generation of ammonia the ammonia generator can be affixed to the desired apparatus by an appropriate means.
In either case it is advantageous to assemble a flask to contain the reaction (500 mL should be fine for most applications), a one-hole stopper, a short length of glass tube, and a length of plastic or rubber tubing to convey the ammonia. Insert the short length of glass tubing into the stopper such that the tube is flush with the bottom of the stopper. If you intend to use the ammonia immediately, the tubing may be connected to your apparatus in the usual manner. For aqueous preparations the tubing may be immersed into water. A bubbler or fish aerator is advantageous for this, but certainly not necessary. See the section on ammonium hydroxide solutions below for more information.
For anhydrous ammonia it is necessary to affix a drying apparatus between the generator and the intended reaction. One can use either a drying tube filled with calcium chloride, hydroxide, or other suitable inert drying medium, or one can use a bottle filled with the same that has a two-hole stopper with a long length of glass tubing and a short length. In this manner the ammonia is forced to diffuse through the drying medium whereupon any water present is taken up.
The preparation of gaseous ammonia from solid reactants is quite simple. Heating a thoroughly homogenized mixture of an ammonium salt with a base is sufficient to release ammonia. The resulting gas is slightly moist as water is made in the reaction. Passing the gas through a suitable drying medium can easily dry it. Typically the base of choice is any of the calcium compounds with ammonium chloride.
Both the ammonium salt and the base are thoroughly powered separately and then well mixed to make a completely homogeneous mixture. It is important that the two chemicals be finely powered and well mixed because they must interact as much as possible for the reaction to work.
Using calcium oxide produces the least amount of water, about 1 mol for each mol of ammonium chloride reacted. Calcium hydroxide produces 2 moles of water in the same reaction. The use of calcium carbonate poses some special problems.
Unfortunately calcium carbonate is the most plentiful version of calcium compounds that can be obtained from retail sources and so is the most logical choice for improvised use. The reaction of calcium carbonate with ammonium chloride actually produces ammonium carbonate in the reaction. As ammonium carbonate is quite unstable under the elevated temperatures of the reaction it decomposes into ammonia. Further information on using calcium carbonate is presented later on.
Example: Separately prepare dry and finely powdered ammonium chloride and calcium hydroxide or calcium oxide. Weigh out a portion of ammonium chloride (ex. 50 g) and an equal mass of calcium oxide (50 g) or 1.5x of calcium hydroxide (75 g). The base exists in roughly twice the molar amount needed to complete the reaction. Mix the two powders as thoroughly as possible and place them in a suitable container as described in equipment setup above. Apply moderate heat to the flask throughout the reaction. Ammonia gas will be released according to the following equations:
2NH4Cl + CaO --> 2NH3 + CaCl2 + H2O; 2NH4Cl + Ca(OH)2 --> 2NH3 + CaCl2 + 2H2O
The exact same reaction setup as above can also be done with calcium carbonate with a few essential modifications. The major difference in using calcium carbonate as the base in the reaction is the production of significant quantities of ammonium carbonate. Ammonium carbonate is actually a mixture of ammonium bicarbonate, NH4HCO3, and ammonium carbamate, NH4NH2CO2, in roughly a 2:1 ratio. The presence of ammonium carbonate will clog an ordinary apparatus and will contaminate a reaction if the ammonia is used as is. A means of condensing the carbonate during ammonia production is essential. The ammonium carbonate can also be completely converted into ammonia with further effort.
The simplest means of condensing the carbonate is to affix a condenser or an empty container similar to the setup for a drying apparatus. An empty bottle immersed in cold water will be sufficient to remove most of the carbonate. This bottle should be closed with a two-hole stopper having a length of glass tubing in each hole. An improvised condenser would accomplish the same purpose a bit better. The wider openings of the condenser provide less chance of becoming clogged. The additional use of a drying tube should remove the last traces of errant carbonate.
Unfortunately the presence of condensed ammonium carbonate on any glass surface of the reaction vessel can lead to breakage. The difference between areas of expansion of coated and uncoated surfaces can lead to great stress on the glass. Substituting a metal container in this reaction will avoid any breakage problems, or disposable glass vessels could be used. As improvised condensers are often made of metal, they are ideal.
Since ammonium carbonate is also a useful source of ammonia it too can become a valuable source instead of a waste product. Adding the entire amount of ammonium carbonate to cold water will dissolve only the carbamate portion leaving ammonium bicarbonate crystals. On exposure to air the carbamate will eventually become ammonium bicarbonate. Bubbling air, or better still carbon dioxide, into this solution will accelerate the process. After a suitable amount of time the crystals of ammonium bicarbonate can be placed into a small amount of water, which is brought to a boil. The high temperatures will decompose the bicarbonate into ammonia, which will pass over along with water vapor. See the section on ammonium hydroxide solutions below for more information on preparing ammonia this way.
A weak ammonia solution, about double the concentration of that which can be purchased in the stores, can be made by conducting the reaction in the aqueous state. A saturated solution of ammonium chloride at room temperature holds about 0.5 moles per 100 mL. Adding this solution to an equal molar mass of base will produce an ammonia solution of about 8.5%. After allowing sufficient time for the reaction to be completed the ammonia solution can be boiled to expel the ammonia. This process can also be conducted with straight ammonium hydroxide although increasingly larger amounts of water vapor will be released at lower ammonia concentrations.
Using soluble alkali bases, such as sodium or potassium hydroxide, is best for this type of reaction. Calcium hydroxide will take longer as it is not very soluble in water. However, the presence of ammonium chloride will increase its solubility. Calcium oxide will react to form calcium hydroxide when in solution, which works the same way. Calcium carbonate is insoluble in water, so it is unacceptable for aqueous preparations.
Example: Dissolve 25 g of ammonium chloride into 100 mL of tepid water in a small flask or bottle. Add 35 g of calcium hydroxide or 19 g of sodium hydroxide to the solution. Cap the flask and set it aside for a few hours to allow time for the reaction to go to completion. The rate of reaction of calcium hydroxide may take longer, but it can be assumed to have gone to completion when most material has dissolved. This reaction will prepare ammonium hydroxide and either calcium chloride or sodium chloride depending on which starting material is used.
Ammonium Hydroxide Solutions
Ammonium hydroxide is the old name for solutions of ammonia dissolved in water. There is actually no such compound as ammonium hydroxide, but by convention that is what it is called. The best means of storing ammonia in an improvised lab is by dissolving it into water. Water can hold a considerable volume of ammonia at lower temperatures. At room temperature a 31% ammonia solution can be had, at 0 degrees C the concentration can be up to 47%. Commercially ammonia solutions are between 20-30%.
When heated these solutions readily give up ammonia gas. At higher concentrations more ammonia will be released with less heat. This is important as the amount of water vapor also rises with more heat. By gently heating a highly concentrated ammonium hydroxide solution a steady quantity of moist ammonia gas can be prepared. This gas can be dried quite easily by passing it through a drying agent. Lower concentrations of ammonium hydroxide are more difficult to dry because of the increased volume of water present. By varying the heat and ammonium hydroxide concentration a steady amount of ammonia gas can be generated as needed.
To prepare a highly concentrated ammonium hydroxide solution set up a series of bottles filled with water. Place a long and short length of glass tube into a two-hole stopper affixed to the bottles. The long end can have a bubbler attached to it to improve the solubility of ammonia. The short length of tube should not go under the surface of the water. The bottles should be connected with plastic or rubber tubing with the short tube of one bottle attached to the long tube of another. In this manner the ammonia gas will be bubbled into the first bottle, any excess will pass into the next bottle, and so on. This setup will help to minimize any waste of ammonia. At the bare minimum one can stick a length of tubing into a bucket of water.
The ammonia source can be either the solid or aqueous preparations above, or from a weaker ammonium hydroxide solution. As previously mentioned it is more difficult to do this with weak solutions such as grocery store ammonium hydroxide. The ammonia generator is connected to the first bottle in the series and the reaction is commenced.
The longer this reaction is conducted the more bottles that will be needed. Eventually the first bottle will become completely saturated, while the secondary bottles will have decreasing concentrations. Cooling the water bottles, or more appropriately only the first bottle can increase the maximum concentration.
The best drying agent for ammonia gas containing traces of water vapor is calcium oxide. Calcium oxide will remove considerable amounts of water while remaining unaffected by ammonia. Other suitable drying agents include potassium or sodium hydroxide, potassium carbonate, aluminum oxide, calcium sulfate, magnesium perchlorate, and magnesium oxide. The use of calcium chloride is not recommended as ammonia can react with it.