Microscale Gas Chemistry:

     We describe here the use of a microwave oven as an interesting alternative for generating some of the gases previously described in this series.  The microwave oven provides distinct advantages over traditional laboratory methods.  Gases can be generated faster, the use of open flames is eliminated, and the quantities of reagents can be reduced.  In this article we will only describe the generation of various gases in the microwave oven.

General Safety Precautions.
    Always wear safety glasses.  Gases in syringes may be under pressure and could spray liquid chemicals.  Follow the instructions and only use the quantities suggested.

Toxicity.
    Four of the six gases described here, ammonia, carbon monoxide, sulfur dioxide, and hydrogen chloride, have relatively high toxicities.  Of these four, only CO has no odor; it is also the least toxic considering the small quantity produced.  Do not discharge these gases into the room.  Oxygen and methane are non-toxic in the quantities used.  Methane is flammable in air and forms explosive mixtures with air.  Use appropriate caution when working with sulfuric acid.
 

Suitability.
    All of these experiments are suited for use as classroom demonstrations.  Individuals attempting these experiments should be experienced with the simpler syringe/gas techniques.  Advanced students or students with special laboratory skills could be allowed to perform these experiments under close supervision by the instructor.

Syringe Lubrication.
    We recommend lubricating the black rubber diaphragm of the plunger with silicone spray (available from hardware stores) or medium-grade silicone oil (Fisher Catalog Number S159-500; $34/500 mL.)

Equipment. (This equipment can be ordered from a variety of vendors including Educational Innovations, Flinn Scientific (US sales only), Micro Mole, and Fisher Scientific.  Part numbers and links to their websites are provided.)

• microwave oven
• several 60-mL plastic syringes with a LuerLOK fitting
• several syringe caps, Latex LuerLOK syringe cap fittings
• Latex tubing: 1/8-inch (3.175 mm) ID, 15-cm length and 4-cm length
• gallon (4-L) sealable storage bags (such as Ziploc or equivalent)
• Silicone spray (available from hardware stores), silicone oil or equivalent lubricant.
• glass Pasteur pipet
• Additional equipment necessary for generating CH4(g) and HCl(g): 250-mL beaker (or similar), medium test tube (18 X 150 mm), small test tube (13 X 100 mm); each test tube should be fitted with a one-hole rubber stopper and each stopper should be fitted with a 4-cm length of glass tubing (must be a air-tught fit — use alcohol to lubricate rubber stopper and glass before inserting glass into stopper)

Preparation of Gases.
     Ammonia, oxygen, carbon monoxide, sulfur dioxide, methane and hydrogen chloride are conveniently generated in a microwave oven.  Details for each gas are given below and summarized in the Table.  When using a microwave oven to generate gases, the progress of the reaction should be checked periodically during the allotted time.  Each microwave oven is different, so the times given may vary.

In-Syringe Method.
     The preparation of NH₃, O₂, CO, and SO₂ are performed by this method.  In each case, the appropriate amount of reagent(s) is/are added to the syringe, and the syringe is capped.  As a precautionary measure to protect the microwave oven, the syringe is placed inside a 4-L sealable plastic bag.  Position the syringe diagonally inside the bag so that the plunger can move outward unimpeded by the bag.
 

Preparation of Gaseous Ammonia.
     Gaseous ammonia is normally generated by the In-Syringe Method, by placing a syringe containing 3-mL concentrated ammonium hydroxide in a hot water bath (60 - 70 ^oC) for 20 - 60 seconds.  (See details) The reaction is:

     The same reaction can be performed in a microwave oven (on high setting, 500 W) in 30 - 40 seconds.  Draw 3-mL concentrated ammonium hydroxide into a syringe and cap the syringe.  Place the syringe diagonally in a sealable 4-L plastic bag and set the bag in the microwave oven.  Start the microwave oven and check the extent of gas production every 10 seconds.  After 50 - 60 mL NH₃(g) has been collected, remove the syringe from the bag and remove the Latex syringe cap while the cap is directed upwards.  Rotate the syringe 180^o and discharge the warm ammonia solution at close range above the surface of a large (> 250 mL) container of water (to prevent excessive ammonia odor.)  Recap the syringe.    CAUTION! The liquid will vigorously spray out of the syringe.

    We have described seven experiments that can be performed with gaseous ammonia. (See details)

Syringe positioned in bag so that plunger can move outward.

Syringe in bag in microwave oven.

Gas produced and collected!

Preparation of Oxygen.
     The generation of oxygen is typically slow when done by the In-Syringe Method (See details).  Oxygen is generated by the decomposition of 6% hydrogen peroxide with a small quantity of potassium iodide serving as a catalyst.  Using a microwave oven, the reaction takes only a few seconds compared to over a minute by our original method.

     Lubricate the plunger's rubber diaphragm with silicone spray or oil.  Drop a few crystals of solid KI into the syringe barrel and install the plunger fully into the barrel.  Draw 5-mL 6% H₂O₂(aq) into the syringe and immediately cap the syringe with the Latex syringe cap.  Place the syringe inside a sealable storage bag and seal the bag.  The syringe must be positioned so that the plunger can move outward unobstructed by the bag.  Heat the syringe/bag assembly in the microwave oven on the highest (500 W) setting for 7 - 10 seconds.  Remove the bag and note the volume of gas produced.  Additional time in the microwave may be necessary.

    After the plunger has reached the desired mark (usually 50-mL), position the syringe with the syringe cap upward and carefully remove the syringe cap.  Turn the syringe 180^o and discharge the excess liquid reagent into a plastic cup filled with water.  Wash the gas in order to remove traces of aqueous chemicals from the inside surfaces of the syringe before the gases are used in experiments.  To do this, draw 5-mL water into the syringe without discharging any gas, cap the syringe and shake the syringe to splash the rinsewater on every inside surface.  Remove the cap (cap end up!) and discharge the water but not any of the gas.  Repeat if necessary.

     We have published eight experiments that can be performed with oxygen.  (See details).
 
 

Preparation of Carbon Monoxide.
     The ease of generating carbon monoxide is substantially improved by the use of a microwave oven.  Normally prepared by the Thermal Method, 8 - 10 drops of each formic acid and concentrated sulfuric acid are heated with a small flame in a test tube connected to syringes in order to generate CO(g)  (See details) according to the reaction:

Microwave oven method.
    The same general approach as described for oxygen is employed.  Premix 8 - 10 drops of each HCOOH and H₂SO₄(conc) in a small plastic weighing boat and draw the mixture into a syringe.  Cap the syringe and place in a sealable plastic bag.  Heat in the microwave oven for 3 - 5 seconds or until at least 50-mL CO(g) has been generated.  Wash the gas as described for oxygen.

     We have described eleven experiments that can be performed with gaseous carbon monoxide. (See details)
 
 

Preparation of Sulfur Dioxide.
    Like with oxygen, the generation of sulfur dioxide is especially slow.  This is due to the fact that SO₂(g) is highly soluble in water.  Using the In-Syringe Method, we called for the use of 2-g sodium bisulfite (placed in an over-sized vial cap) and 5-mL 6 M HCl in order to generate 50-mL SO₂(g)  (See details) according to the reaction:

     When using a microwave oven, we recommend using only 0.25 g NaHSO₃(s) and 3-mL 6 M HCl(aq).  Lower the solid reagent into the syringe barrel by flotation and install the plunger as shown in Figure 1.  Draw the HCl(aq) into the syringe, cap and mix the solid and liquid by shaking the syringe.  Generation of SO₂(g) will be slow.
 
 

     Place the syringe inside a sealable storage bag and seal the bag.  The syringe must be positioned so that the plunger can move outward unobstructed by the bag.  Heat the syringe/bag assembly in a microwave oven on the high setting for 5 - 7 seconds.  Note the volume of gas produced.  Additional time in the microwave may be necessary.  Sulfur dioxide is exceedingly soluble in water so the plunger may start moving inward soon after removal from the microwave oven.  To minimize this, remove the liquid reagents as soon as possible: With the syringe in the cap-up position, remove the syringe cap.  Rotate the syringe and discharge the excess liquid reagents into a plastic cup filled with water.

     Sulfur dioxide cannot be washed because it is water-soluble.  Instead, transfer the gas sample to a clean, dry syringe by connecting the two syringes with a short length of latex tubing.

    We have described eight experiments that can be performed with gaseous sulfur dioxide. (See details)
 
 
 

Preparation of Methane.
     Gaseous methane (CH₄) is generated by heating anhydrous sodium acetate and sodium hydroxide together in a test tube by the Thermal Method (See details) according to the reaction:

     The microwave method is similar except that a microwave oven is used instead of the Bunsen burner flame.  Place 0.50 g NaOH(s) and 0.50 g  NaC₂H₃O₂(s) in a medium test tube and fit with the one-hole stopper/glass connector tube.  Connect the test tube assembly to a clean, lubricated syringe with a 15-cm length of Latex tubing and set the entire assembly in a 250-mL beaker as shown in Figure 2.  Heat in a microwave oven using the 400-watt setting for 2 - 3 minutes.  Check the progress periodically.  After 50-mL HCl(g) have been collected, remove the bag from the microwave oven.  CH₄(g) generated in this way is ready to use.
 
 

     We have described twelve experiments that can be performed with methane. (See details)
 
 
 
 

Methane and hydrogen chloride are prepared using this apparatus.  Clockwise from right: (1) Beaker is used to keep hot glass away from the plastic syringe; (2) Apparatus in plastic bag ready to go into microwave oven; and (3) Assembled apparatus in sealed bag in microwave oven.

Preparation of Hydrogen Chloride.
     Gaseous hydrogen chloride (HCl) is generated by heating 2.0 g anhydrous sodium hydrogen sulfate (sodium bisulfate) NaHSO₄ (or 2.3 g sodium hydrogen sulfate (sodium bisulfate) monohydrate NaHSO₄^.H₂O)  and 1.0 g sodium chloride, NaCl together in a test tube by the Thermal Method (See details) according to the reaction:

     Our original article called for heating the mixture with a Bunsen burner flame.  Here we report that similar results can be obtained with a microwave oven.  The general procedure described for methane is followed, except that a smaller test tube is used.  It typically takes 2 - 3 minutes in a microwave oven running on the highest setting to generate 50-mL HCl(g).  Check the progress of the reaction every 30 seconds.

     We have described eight experiments that can be performed with gaseous hydrogen chloride. (See details)
 
 

More on Microwave Ovens.
     The microwave oven has been used throughout science in a myriad of ways throughout its 50-year history.  Check these out:
 

   At the end of the experiments, wipe excess lubricant off of rubber diaphragm. Clean all syringe parts (including the diaphragm), caps and tubing with soap and water.  Use plenty of soap to remove oil from the rubber seal.  This extends the life of the plunger.  It may be necessary to use a 3-cm diameter brush to clean the inside of the barrel.  Rinse all parts with distilled water.  Be careful with the small parts because they can easily be lost down the drain. Important: Store plunger out of barrel.