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May 2015 | Resource Feature
The Indestructible Crucible
By Bob Worley
Chemists have been using porcelain crucibles for centuries. They are used in gravimetric analysis so that residues of inorganic salts in ashless filter paper can be heated to high temperatures. The goal of such experiments is usually to determine the mass of a leftover inorganic salt. Crucibles are used in schools, but just how suitable are they for students, who are not professional chemists? Thorough pyrolysis often requires hours of heating, and student boredom and finite class periods do not allow for this. Additionally, crucible tops are difficult to manipulate with traditional tongs, especially if the tongs are in poor condition (Figure 1). Crucibles can also crack, which introduces a safety risk because of the potential for spilling red-hot contents.
Combustion of magnesium in a crucible
On our helpline in the U.K., CLEAPSS received repeated reports of poor results when synthesizing magnesium oxide in the lab because students could not lift and replace the crucible lid fast enough. As a result, they lost the oxide smoke, or they dropped the lids and broke them during the experiment. Also, crucibles would frequently crack on heating and cooling. Stainless steel and nickel crucibles are the only alternatives available, but they are expensive. In the 1990s, Worcester Royal Porcelain shut down their crucible-making division. I phoned to ask about it and a company representative responded:
Not a week goes by without a school teacher or technician phoning us up to complain about the quality of porcelain crucibles because they break and they could not be cleaned to their original state. Don’t teachers know they are only supposed to be used once? They are used to remove samples of molten steel from a furnace for analysis, and now that steel industry has stopped in the U.K., we do not need to make them any longer.
The bottle cap alternative
Millions of magnetic crown-bottle caps used in the beverage industry are made from steel. When removing the cap from a bottle, be careful not to bend it too much so you can use it as a crucible alternative. You can also buy unused caps online. Before the cap can be used in the lab, the plastic insert must be taken out; it can be removed by heating the cap in a fume hood.
To react magnesium with oxygen, with magnesium inside this alternative device, tie together two bottles caps with nichrome wire. They fit snugly onto a pipe-clay triangle (Figure 2). Air can find its way to the hot magnesium via the serrated edges to react, so no further manipulation of the equipment is needed once it is on the triangle. This apparatus should be heated strongly for 10 minutes and allowed to cool.
The results are excellent compared to the traditional crucible method (see this CLEAPSS production). In fact, it is rare when using this method to achieve a reduced mass, which is common with traditional crucibles. Additionally, with this apparatus, the increase in mass corresponds closely to the correct formula, MgO.
What percentage of your students will become chemists? If a person really loves chemistry, he or she may become a chemist, though some students may be more likely to become doctors, where the wages are higher. Our first aim as chemistry educators should be to try to make young minds understand the subject. If we carry out laboratory activities that do not produce accurate results, what message does this send? If students oxidize magnesium and the mass reduces, contrary to expectations, this confuses students. We then run the risk of them doubting the subject, which is difficult enough as it is. If a bottle cap works better than a traditional crucible to illustrate our subject and gets the message across, then so be it.
Equipment and techniques have been handed down to us by top chemists, but they may not always be suitable for students to best understand chemistry. Many times the teacher has to think outside the box!
But first, have nice drink while you ponder this profound thought … and save the bottle cap!
"I am not doing it by that method"
These were the words from a teacher who saw that the results were convincing but still refused to use this apparatus with students. The reason? He was afraid that students would lose marks on standardized tests if they referenced the alternative apparatus. Because students didn’t use traditional equipment, the grader may not recognize this method and deduct points from students’ responses. Students would be at a disadvantage if a question was posed that uses a crucible because they would not necessarily equate the bottle cap setup as a crucible alternative. While chief examiners provide encouragement for alternative methods such as this, many teachers are still skeptical.