Gas Bags
The use of gas bags for the storage and handling of
gases
is well over 200 years old. Henry Cavendish described the
use
of a “bladder” for gas manipulation in his 1766 publication On
Factitious
Airs (Figure 1) and Carl Scheele used the a large gas bag when he
produced
“fire air” (oxygen) in 1772.(A Short
History
of Chemistry, J. R. Partington, 3rd edition, (1957)) (Figure 2)
Figure 1. Gas bladder used by Henry Cavendish
Figure 2. Gas bladder used
by
Carl Scheele
A century later, gas bags
were
still common. In the 1872 text by Storer and Lindsay,(An Elementary Manual of Chemistry, F. H. Storer
and
W. B. Lindsay, American Book Company, 1872.) a gas bag was used to make a
mound
of soapy bubbles containing a hydrogen-oxygen mixture (Figure 3).
Figure 3. A 19th century gas bag experiment
And now, another century later, gas bags are still used, although
plastic
has replaced animal organs. Courneya and McDonald described the use of
a
plastic bag to store gases in 1978.(Courneya,
D and McDonald, H; The Science Teacher 45(6), 43, September 1978.
The
article is reprinted in Irwin Talesnick’s Idea Bank Collation, A
Handbook
for Science Teachers, Volume 1; 1984, Part Number CB 066, S17 Science
Supplies
and Services. Link to the S17 website is provided at our microscale gas
website.)
A modification of this can
be purchased from S17 Science Supplies and Services.(Part Number EQ 059, S17 Science Supplies and
Services.)
In this chapter we describe the use of 1 L (1 qt)
household
food storage bags such as Ziploc‚ bags for transferring gases and
temporarily
storing gases. The gas bag can be used to fill syringes for use
by
students. One gas bag will fill at least twenty 60 mL
syringes.
The technique is exceedingly convenient in situations such as
obtaining oxygen
from
a mechanical or welding shop
obtaining helium from a
grocery
or flower shop
collecting natural gas
which
has enough pressure to inflate a bag but not enough to push a plunger
outward
preparing large quantities
of
a gas such as carbon dioxide, hydrogen or oxygen (inside the 1 L bag)
for
use in multiple experiments
discharging a gas such as
CH4,
H2 or CO at a constant, controlled rate
in
order to sustain a small flame.
Basic materials needed
60 mL plastic
syringe
1 L (qt) freezer-quality
food
storage bag, for example, Ziploc
tubing, 1/8 inch (3.175
mm)
ID, 15 cm length
plastic hemostat or pinch
clamp
dish soap
Suitability
The use of gas bags is intended for use by
teachers.
Gas bags can save time and provide a useful method for delivering
larger
quantities of gas at a constant flow rate.
Construction of the gas bag from a food storage bag
Use a pencil or similar round, sharp object to poke
a
hole through the plastic bag in a position similar to that shown in
Figure
4. Moisten one end of the piece of tubing with dish soap in order
to
facilitate pushing it through the hole in the bag. The gas bag is
now
ready for testing. Fill a large pail (10 L) with water.
Inflate
the gas bag with air. Hold the point of connection between the gas bag and the tubing under the
water
and check for leaks (or place some water in the bag and let it pool
near
the tubing and then check for leaks on the outside of the bag).
We have
found that there are seldom leaks. The seal, however, often leaks
slowly, but in most cases slow leaks are of little consequence.
Figure 4. A 1 L Gas Bag
Possible uses for the gas bag
1. The gas bag is a time-saving device when used as a large reservoir
to
fill syringes for use in microscale gas experiments by students.
2. Use the gas bag to transfer oxygen from a mechanical or welding shop
to
the lab. Connect the tubing from the gas directly to the cylinder
of
compressed oxygen. Squeeze all of the air out of the bag before
connecting
to the cylinder. After the bag is full, clamp the tubing shut
with
the hemostat or pinch clamp. Avoid overfilling the gas bag.
3. Use the gas bag to transfer helium for from a grocery or flower
shop.
Same instructions as above, however it is also possible to buy a helium
balloon
from a store (not overfilled) and transfer the gas from the balloon to
the
bag: Near the mouthpiece, but 3 cm away from the knot, twist the
balloon
to make a new temporary seal. Cut a small slit (2 mm) with a
scissors
between the twist and the knot. Work the tubing into the hole
(use
soap, as before). Open the hemostat and slowly release the twist
seal.
Gas will start filling the gas bag. (We have used helium in the
molar
mass experiment (Chapter 6), but better results are obtained with gases
having
large molar masses.)
4. Use the gas bag to collect natural gas or propane. These gases
have
enough pressure to inflate a gas bag but not enough to push a plunger
outward.
Simply connect the tubing to the gas jet. For propane from a
propane
torch tank, remove the nozzle from the propane torch and slip rubber
tubing
of suitable diameter over the brass fitting on the torch. The
tubing
can be “reduced” in diameter by slipping it over a smaller diameter
piece
of tubing connected to the gas bag. Use tape to achieve an air-tight
seal
if necessary. Gas bags filled with flammable gases should be in
the
custody of the teacher at all times.
5. Use the gas bag to prepare large quantities of a gas such as carbon
dioxide,
oxygen or hydrogen.
o Carbon dioxide: Place 3 g NaHCO3 inside
a
gas bag. Squeeze out most of the air, zip the bag shut and remove
the
remaining air by withdrawing it via the tubing using a 60 mL
syringe.
Use a 60 mL syringe to transfer 50 mL of vinegar to the bag. The
reaction
will commence upon contact between the two reagents. After the
reaction
is complete, samples of CO2 can be
withdrawn
for various experiments.
o Hydrogen is prepared in a similar fashion using 1 g powdered
magnesium
(inside the bag) and 50 mL 1.2 M HCl(aq) admitted via the tubing.
The
reaction becomes quite warm and is complete within one minute.
Other
forms of magnesium (turnings, ribbon) also can be used.
o Oxygen is prepared using 0.5 potassium iodide (inside the bag) and 60
mL
3% H2O2
(aq)
admitted via the tubing. The reaction is considerably slower; it
takes
about 5 minutes.
v Use the gas bag to discharge a gas such as CH4,
H2 or CO at a constant, controlled rate
in
order to sustain a small flame. Assemble the apparatus as shown in
Figure
5. Use a 15 cm length of tubing to connect the gas bag to a glass
pipet.
(The tubing will form a snug fit inside the pipet.) Keep the gas bag
away
from flames. Open the pinch clamp and ignite the gas issuing from
the
pipet. Gently press down on the gas bag to control and sustain the
flame.
To stop the combustion, pinch the tubing shut.
Figure 5. The gas bag being used to discharge a flammable gas at
a
constant rate.
Figure 6. The gas bag used to generate strange musical sounds.
Combustion of hydrogen in oxygen demonstration. A flask as a
musical
instrument?
Generate oxygen and hydrogen in separate gas bags
and
label them. Fill a 250 mL Erlenmeyer flask with oxygen from the
gas
bag and stopper it until needed. Refer to Figure 6 for a pictorial
representation
of the apparatus and procedure: Connect the hydrogen gas bag’s
tubing
to a 5 mm OD, 15 cm piece of glass (a pipet will not work) held in
position
with the aid of a ring stand and clamp as per the figure. Keep
the
gas bags away from flames. Remove the clamp/hemostat and
immediately
ignite the hydrogen issuing from the glass tubing. If necessary,
gently
press down on the gas bag to control and sustain the flame. Slip
the
flask of oxygen over the burning hydrogen and move the flask over the
flaming
glass tube. As the glass tube/flame moves deeper into the flask, the
pitch
will drop. Move the flask “on and off” to cause the pitch to
vary.
The flask will become hot. To stop the combustion, remove the
flask
and pinch the tubing shut. Note the water vapor in the flask!
View a QuickTime video of this musical demonstration.