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A coil of hot copper wire acts as the catalyst in this oxidation of ammonia. The ammonia is produced by warming a concentrated aqueous solution. The reaction between the ammonia and oxygen from the air takes place on the hot copper. It produces nitrogen monoxide, and so much heat that the copper wire melts and falls into the ammonia solution. The reaction will continue for a long time without having to re-heat the copper wire.
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Nitrogen monoxide is a clear, colorless and toxic gas. It reacts rapidly on contact with oxygen in the atmosphere, to form brown nitrogen dioxide.
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Methyl red acid-base indicator is yellow in neutral water. As nitrogen dioxide is bubbled through the water it reacts to form nitric acid. This turns the indicator red.
