Laboratory Safety Demonstration

Transcription

1 Laboratory Safety Demonstration This short demonstration is a complement to the new variant of the firefighting course, which takes place in the Hönggerberg area now and therefore takes only two hours instead of a full afternoon. An additional hour is used to show the student major safety issues in laboratory work. We consider 1. Weighing and handling of powders, general cleanliness 2. Mixing of reagents 3. Malicious reaction behavior 4. Safe and stable apparatus setup Materials needed: - Spatula, some crystalline reagents (e.g. CuSO 4 5H 2 O, Al 2 O 3 ) - 3 Beakers of 50 ml - Glass rod - Weighing paper - Balance - 20 ml 2 M Citric acid - 20 ml 1 M Na 2 CO 3 - Pasteur pipettes and small balloons - 2 medium test tubes - KSCN - Conc. HNO 3 - Water bottle - distillation adapter with Liebig condenser (14.5 joints) ml and 50 ml round flasks wit 14.5 necks joint reflux condenser joint clamps - Stand and stand clamps - Electric heating plate with aluminum pot (better not filled with bath) - Height adjustable platform or simple box to support the heater Demonstrations: 1. How to use a spatula properly to avoid contamination on and around balances Inappropriate grip at the remote end of the spatula. The guidance is poor and the long lever arm favors uncontrolled swings. The powder mass on the spatula is not too large. 1

2 Correct grip on the spatula with index finger in position to knock gently to drop powder in small amounts. Powder mass on the spatula is appropriate. However, spatula too far above receiving container. High risk of spilling powder when dropped. Correct grip and appropriate low position above receiver, but spatula is overloaded. High risk of spilling powder on balance. Correct weighing: short grip on spatula, index as knocker, low position above receiver and no overload on spatula. 2

3 Please, please, never do this. Chemicals have to be carried around in bottles or glass or ceramics containers (beakers, flasks, crucibles, etc.) 2. Mixing of Reagents Careless mixing of reagents can sometimes lead to unpleasant surprises. For a demonstration, place 10 ml of 2 M citric acid in a 50 ml beaker and 10 ml of 1 M Na 2 CO 3 in a second one. Pour the carbonate solution at once into the citric acid. Repeat the mixing, but use a Pasteur pipette to add the carbonate dropwise to the citric acid. An alternative method is the addition by letting the carbonate solution flow slowly along a glass rod into the acid. This technique should also be applied in filtration to prevent liquid spills. 3. Malicious reaction behavior The students should be aware that some reactions do not proceed smoothly, after an induction period they suddenly become violent. Our manual does not contain such variants, but they occur, according to Murphy s Law, whenever they can happen and at the worst possible moment. Prepare two medium test tubes with 0.1 g and 0.5 g solid KSCN. Mount them with clamps on a stand in a fume hood or at least in a well ventilated area. Make sure that the students keep at least 1 m distance. Add about 1 ml of conc. HNO 3 to the 0.1 g KSCN and step back at least 1 m. Observe! (you should always be observant as a scientist) A deep red color develops in about 10 seconds, and small gas bubbles appear. Suddenly, a rapid eruption of NO 2 occurs. If you do the experiment outside a fume hood, everybody should move away from the trajectory of the gas cloud. Repeat the procedure with the 0.5 g KSCN sample. This demonstration emphasizes two rules of experimenting: a) You might see an interesting phenomenon when doing a new experiment, but should not trustfully approach it until you have become familiar with it. b) Never use high doses of reagents in potentially harmful or completely unexplored reactions. The effect of the 0.5 g dose is clearly more spectacular than with 0.1 g. 3

4 4. Safe and convenient apparatus setup Primary goals in the setup of glassware apparatus are: - Stability - Stress-free assembly of glass parts - Convenient handling - Control of energy flow - Absence of leaks Stability is achieved by using a sturdy stand and placing the stand clamps in locations where slipping is prevented. Stress-free assembly is obtained by using as few stand clamps as possible, only one in ideal cases. This also improves handling, since the fewer the clamps, the more mobile the parts. Control of energy flow means that heating and cooling are installed properly, and that the heat source can be removed quickly in case of exothermic events. The absence of leaks is important in the coolant connections. Water injection into the common oil bath of the heat source poses a considerable danger. This reflux prototype is flawed by the upper clamp. Since the heater stands on the table, the only escape is upward, which is difficult to handle with two clamps. If the clamps are not properly aligned, they cause stress on the joint. 4

5 This arrangement is stress-free, but the body of the cooler is smooth and the assembly could slip down deeper into the bath. Removal from the heat source is possible by lifting the single clamp. This is the simplest safe setup. A single clamp holds the neck of the flask which cannot slip because it has a border. The cooler is held in place by gravity but can easily be removed. The whole assembly can be lifted from the heat source by moving the single clamp. 5

6 If the glass assembly cannot be moved upward, the heat source must be placed on a removable socket, best on an elevating platform ( Laborboy ). A simple box may do, but one has to be able to hold the heater with the bath while pushing the box away. This can be performed by holding the heater on both sides with thumbs and indexes while grabbing and pushing the box with the remaining fingers. Beware not to burn your thumbs or to spill the oil. If the box is heavy, no way. This distillation setup can be disassembled quickly, and also be removed from the heat source. However, the mount far from the center of gravity suffers considerable torque and may fail by a rotation. 6

7 This distillation setup is more balanced and has the same advantages as the previous. The danger of slipping of the cooler through the clamp is lower than in the reflux assembly, because the mount is not vertical. Please build some of these assemblies in front of your student groups and discuss advantages and disadvantages with them. You may add your own concepts and explain the students why they are safe and what their special features are. October 2010 Reinhard Kissner 7