The Thermal Method utilizes thermal techniques whereby the gas samples are collected in a syringe connected to test tube in which one or more chemicals are heated together.  The general idea for this method was first proposed by LeBlanc in the 18th century and described in Wurtz's 1875 Dictionary of Chemistry.  We have modified the method to utilize clean, dry 60-mL syringes. The Thermal Method is used to generates the gases:
 

• hydrogen chloride, HCl
• carbon monoxide, CO
• ethene, C2H4
• methane, CH4
• nitrous oxide, N2O

     The In-Syringe Method was originally described in our first article on microscale gas chemistry that appeared in the October, 1996 issue of Chem13 News (Number 251.) The Thermal Method did not appear until Part 11 of the series (Chem13 News, Number 266, April, 1998) when it was first called for to generate a gas.

 A list of equipment needed to generate gases by the Thermal method is given in the Table.
 

• safety glasses
• balance capable of measuring to 0.01 g
• several 60-mL plastic syringes with a LuerLOK fitting
• latex LuerLOK syringe cap fittings
• plastic vial caps that fit within the barrel of the syringe (variety of sizes)
• latex tubing, 1/8-inch (3.175 mm) ID, 15 cm lengths
• two 18 x 150 mm test tubes
• two-hole #1 stopper fitted with two short lengths (2 cm) of glass tubing
• two pieces of latex tubing, 1/8-inch (3.175 mm) ID, 5 cm lengths
• pinch clamp or hemostat
• ring stand and three suitable clamps to hold test tube and syringes
• small Bunsen burner
• matches or a lighter
• ‘permanent’ marker pen

• silicone oil (Fisher Catalog number S159-500)  In our opinion, this works the best.
• silicone spray (available from hardware stores) Easy to use, but messy, also, contains a hydrocarbon propellant that may damage rubber after extensive use.
• KY Jelly (sold in the family planning section at drug stores).  This water-based product gives excellent results and will not damage rubber.  We recommend that you transfer the gel to some small bottles and label it ‘Syringe Gel.’  We apply it with a cotton swab. Keep the bottle covered when not in use or it will dry out.

     A number of gases cannot be obtained by the ambient temperatures of a syringe — and heat cannot be applied to a plastic syringe.  Thus, we have had to develop the Thermal Method described here.  This method utilizes a clean, dry 60-mL syringe connected by latex tubing to a test tube containing the reaction mixture.  The reaction is performed by heating two reagents together in a test tube.
 

The Thermal Method utilizes two clean, dry 60-mL syringes connected by latex tubing to a 120 x 15 mm test tube fitted with a suitable (#0) two-hole stopper.  Short lengths of glass tubing are inserted through the rubber stopper.  (CAUTION!  Soak the rubber stopper in alcohol and lubricate the glass tubing with alcohol before inserting the tubing through the stopper.  Hold glass tubing with a thick towel while inserting!  Avoid puncture wounds!  Do not force the glass!)  Syringe plungers should move easily in barrels.  This can be facilitated by applying a thin film of silicone oil, vegetable oil, or glycerin to the plunger's rubber seal.  The assembled apparatus is shown in Figure 1.  Also needed is a small flame source such as a long-nosed butane lighter, an alcohol lamp or a Bunsen burner.  Each gas has different requirements with respect to heating.  The left syringe, labeled 'pure gas collection' is used to collect relatively pure gas and the syringe labeled 'Waste' is used to collect impure samples of gas and unwanted air.  A pinch clamp or hemostat is used to pinch closed one of the latex tubes.

Figure 1

    Start by pinching closed the gas collection syringe. Place the appropriate amount of each reagent in the test tube.  Insert the stopper firmly in order to form an air-tight seal.  Hold the heat source with one hand while manipulating the tubing clamp or hemostat with the other.  This maneuver works well with two individuals working together.  Caution: Do not crimp the latex tubing!  The 3-step maneuver is:

• Step 1. Heat the mixture by playing a gentle flame over the end of the test tube.  Gas soon will be produced and the plunger of the Waste syringe should begin to move.  It may be necessary to assist the sliding movement of the plunger up the barrel of the syringe during the reaction.  Continue to heat while gently assisting the plunger's movement.  Collect 20 - 25 mL of gas in the waste syringe.  This gas is mostly air originally present in the test tube.
• Step 2. After 20-mL of waste gas has been collected in the Waste syringe, switch the location of the pinch clamp to the other latex tube so that relatively pure product gas can be accumulated in the gas collection syringe.  Continue to collect pure gas until at least 50-mL has been collected.
• Step 3. Switch the pinch clamp back to the tubing connected to the pure gas collection syringe and remove the heat source.  WARNING: Never simultaneously pinch both latex tubes!  Replace the latex tube from the gas collection syringe with a latex syringe cap.  The syringe is > 95% pure and ready for experiments.  Allow the apparatus to cool.  The plunger in the Waste syringe may move outward at first because gas generation may continue for several seconds after the test tube is removed from the flame.  The plunger may move inward as the apparatus cools.

Note.
    It is possible to generate multiple syringefuls of pure gas by scaling up the amount of reagents used.

Disposal of reagents.
    Follow the specific procedure provided in each chapter to treat the chemical reagents used in each gas preparation.

Washing of Gases is Unnecessary.
    Because the reagents are never present in the syringe, it is unnecessary to wash the gas samples collected by the Thermal Methods.
 
 

    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.