Letter to the editor

  • There is more nitrogen in the air across the pond!

I was surprised by the article from Tim Allman in the March issue of Chem 13 News. I have never failed to detect magnesium nitride after heating magnesium in the air.

A piece of red litmus paper which appears to have turned blue placed over the top of a jar with pieces of white material on the jar’s bottom.I detect the ammonia from the hydrolysis by adding about 1 or 2 mL of warm water from the tap to the magnesium oxide in a vial. I then lay moist red litmus over the top turns blue (photo 1). The amount is extremely small though and I have never seen any work carried out on estimating the amount of nitride in the product.

The quantitative oxidation of magnesium oxide has always caused problems. A complaint I received at CLEAPSS1 was that crucibles break too easily when heated strongly. On hearing that Worcester Royal Porcelain was to stop making crucibles, I asked them for some advice. “We get complaints from teachers all the time! They are only supposed to be used once. We make them for the steel industry to take samples but as we have no steel industry to speak of, they are not worth making anymore” was the reply. It took some time for me to get over the shock that they were not made for chemistry teachers.

Two bottle blackened bottle caps, one with a strip of coiled magnesium ribbon and the other is empty. Also pictured is a wire with a loop made on one end – the loop is the size of a bottle cap.Another problem was the use of poor tongs where the teeth did not meet properly. This resulted in the lids not being removed carefully enough and dropped, allowing all the oxide to escape. Teachers in the UK regularly complained that they never got decent results from this unconvincing experiment. So how do we solve these issues? I took to drink!

I drink two bottles of beer and carefully save the bottle tops! Try not to distort the serrated edges too much when removing them from the bottle. Two bottle cap put together – one is the bottom, the other acts as a cover and a wire holding them together The bottle caps are shown on a clay triangle over a blue Bunsen burner flame.Now burn out the plastic insert in the fume cupboard. The two bottle tops are held together with nichrome wire as shown in the photos. Between 0.1 and 0.2 g of magnesium ribbon should be used, all weighings on a 2 decimal place balance. I was amazed to find that the designer of these bottles tops gave them the correct dimensions to fit a small pipe-clay triangle. I heat strongly for 10 minutes with a Bunsen burner and then allow them to cool.

See a two-minute YouTube video of this MgO experiment using the bottle-cap crucibles.

The bottle top can even be modified into an open crucible. I drill a hole through it and insert a small bolt. This enables me to lift the crucible. See a two-minute video of this constructed crucible being used to measure the percentage water in copper(II) sulphate crystals.

  1. www.cleapss.org.uk  (Consortium of Local Education Authorities for the Provision of Science Services based in the UK)

[Editor’s note: These little bottle-cap crucibles look wonderful — inexpensive and unbreakable. Take two-minutes to watch Bob’s YouTube videos as this is a great lab hint for high school teachers! ]