PLASTIC BOTTLES IN THE LABORATORY
Glassware is breakable, expensive and has to be obtained from laboratory suppliers. Many items of laboratory glassware can be replaced by plastic items, providing that heat is not involved. Also in some cases organic solvents may dissolve some types of plastic.
What are the advantages of plastic labware? Plastic items are unbreakable in normal use and are much safer from this aspect alone. Many items of use in the laboratory can be obtained from local suppliers or even scavenged from throw-away items, and are inexpensive.
Storage of laboratory solutions often presents a problem and glass stoppers invariably get stuck on standing. Most bottles in school laboratories are also too small to store large quantities of solution e.g. enough for a class or to store left-over solutions. The 2 litre or 4 litre plastic squash bottles are ideal for storing most aqueous solutions in the laboratory. They are light, unbreakable and have handles, making them easy to pour, and are free – ask your pupils to bring in empty bottles and you’ll be overwhelmed.
The bottles are blow moulded in one piece and have no seams to leak, and are probably made from high density polythene (HDPE). This is chemically inert and is actually better for storing many solutions than glass.
A recent note in Chem13 News (February, 1980, no.111) suggested using the new 2 litre soft drink bottles (Coca-cola etc.) for the same purposes. These bottles are taller and not as stable, have no handles and have a seam at the bottom which can leak. The bottles available in Ireland with fruit squashes (Robinsons, St.Bernards etc.) are much more suitable. Cut in half the bottles can be used as beakers, stands for round-bottomed flasks and pouring funnels.
The article in Chem13News also suggested that the bottles could be used to determine the molecular masses of gases very quickly and easily. I have tried this and it works well.
CAN YOU THINK OF ANY MORE USES FOR THESE PLASTIC BOTTLES?
MOLECULAR MASSES USING PLASTIC BOTTLES
The experiment consists of weighing a given volume of air (in a 2 L plastic bottle) and then weighing the same bottle full of air. From room temperature and pressure the density of the air can be obtained from a Book of Physiochemical Data. Hence the mass of the unknown gas with a volume of 2 L at room temperature and pressure can be found and this is converted to the volume this mass of gas would have at S.T.P. 1 mole of an ideal gas at S.T.P. has a volume of 22·4 L and hence we can find the molecular mass of our gas.
Sample result using Kosangas
Weight of bottle + lid + air = 75·46g
At 20ºC and 1 atm, density of dry air is 1·205 x 10-3 g cm-3 (from tables)
2 L of air thus weigh 1·205 x 10-3 x 2000 = 2·41g
Bottle + lid thus weighs 75·46 – 2·41 = 73·05g
Bottle was flushed well with gas from the tap using rubber tubing going to the bottom of the bottle.
Weight of bottle + lid + Kosangas = 76.68g at 20ºC
N.B. Kosangas is obviously heavier than air.
2 L of Kosangas at 20ºC thus weigh 76·68 – 73·05 = 3·63g
What volume would this weight of gas occupy at S.T.P. i.e. 0ºC and 1 atm pressure?
Volume at S.T.P. = 2 x 273/293 = 1·86L
22·4 L of gas at S.T.P. contains 1 mole
Thus molecular mass of Kosangas = 3·63 x 22·4/1·86 = 43·7 g mol-1
N.B. I didn’t know what the atmospheric pressure was and I should have corrected for this.
Kosangas is quite clearly propane, molecular mass = 44 g mol-1, not methane or butane.
This experiment is quick, easy to perform and fairly accurate – enough to distinguish between common hydrocarbons. Camping gas cylinders (blue) are a convenient source of butane. Oxygen, ethyne (acetylene), carbon dioxide, nitrogen and sulphur dioxide are fairly easily obtained in cylinders, or gases can be prepared and dried in the laboratory.
SAFETY GLASSES: EXCESSIVE OR ESSENTIAL?
It has become widely accepted in most countries that proper eye-protection is necessary for school students doing experimental work, particularly using chemicals. In the U.S.A. and the U.K. this is now a legal requirement. Very few Irish schools where practical work is done appear to use safety glasses as a routine procedure for all pupils doing practicals. The use of safety glasses or goggles makes practical work safer and would have prevented many serious accidents. It is worth remembering that skin tissue regenerates, but eye tissue does not. The financial investment to equip every pupil with eye protection is well worth it if one eye accident is prevented.
WATER POLARITY AND MICROWAVES
What do microwave ovens have to do with the polarity of water molecules? The answer lies in the oscillating electric field set up by the microwave field. Polar molecules, such as water which is present in all foods, have a positive and a negative end. This dipole tries to line up with the electric field (see Figure 1(a)). Unfortunately the microwave field changes polarity 5 billion times per sec at the frequency usually employed (2450 MHz). The water molecules reverse their direction to try and follow this change (Figure 1(b)) and end up oscillating madly 5 billion times per sec, generating heat by friction all through the food. The microwave energy is only absorbed by the cooking food and cooks uniformly all through the food, taking a few minutes instead of hours to cook a joint, for example, and consuming much less energy. The microwave oven must of course be shielded to prevent leakage of radiation into the kitchen since we also contain water!
The property of microwaves to interact with polar molecules is also used in microwave spectroscopy. The microwave region of the electromagnetic spectrum (Figure 2) occurs between the infrared and the radio frequency regions. Molecules possess rotational energy and a set of rotational energy levels whose spacings correspond to energies in the microwave radiation of the correct frequency, absorbing energy and increasing its rotational energy. The spectrum can be interpreted to yield very precise molecular parameters and the resolution is high enough to distinguish isotopic species. Every polar molecule has a characteristic microwave spectrum which can be used as a molecular fingerprint, though non-polar molecules cannot absorb microwave radiation. The ability of microwaves to travel long distances and their highly specific molecular fingerprints are the reasons why microwave spectroscopy has been used to identify molecules in interstellar space. Over 40 different molecular species (some of them isotopic isomers) have so far been identified in this way out among the stars.