I work in a restaurant that has a bar with a zinc surface. We use lemon juice to clean the bar top, which leads me to two questions: why is lemon juice so good at cleaning zinc? And why does lemon juice on zinc smell so awful? We have to clean the bar in the early morning to avoid putting customers off.
• Is the bar counter quite old? Zinc used to be extracted from its ore, an impure zinc sulphide known as , by heating it with carbon. The metal arising from this 19th-century process, last used in the 1950s, was contaminated with unconverted zinc sulphide and traces of heavy elements close to zinc in the periodic table, including arsenic.
When pure zinc is treated with acid such as citric acid from lemons, hydrogen gas is generated, and the oxidised or stained surface layer is dissolved, leaving a bright, clean finish. But the zinc sulphide in old zinc sheet will give off the toxic gas hydrogen sulphide, notorious for its rotten-egg odour. In addition to this, any arsenic will be converted to another poisonous gas, arsine, which smells of garlic.
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Although the concentrations of these gases will be low, they are best avoided, so give metal polish a try.
The zinc-acid reaction was used to detect traces of arsenic in one of the first forensic tests, developed by James Marsh in 1836. This involved heating up material, such as exhumed body parts, with zinc and hydrochloric acid. The resulting hydrogen was burned in a gas jet aimed at a cooled metal or a glass surface, where it would leave a shiny black deposit if arsenic was present. But care had to be taken because impure zinc would give false positives.
The Marsh test was cranky and time consuming, and the 1922 case against English solicitor Herbert Armstrong was delayed because of disputes over who should foot the bill for the lab work. Armstrong had bought arsenic trioxide to kill dandelions in his lawn, but found it equally effective on his wife.
“Herbert Armstrong bought arsenic to kill dandelions, but found it equally effective on his wife”
John Rowland, Derby, UK
• Zinc is reactive, and the dull and dirty film you wipe off is a layer of zinc carbonate and oxide complex that forms on contact with carbon dioxide in the air. But this film does protect the underlying metal from further reactions. However, zinc carbonate reacts with acids to form a soluble salt and odourless hydrogen.
Lemon juice is particularly effective because of the low pH (or high acidity) of its citric acid. In a bar, it is the cleaner of choice because there is always a handy supply and because of its gastronomic safety and fragrance.
The bar will be subject to food and drink spills, residues of which lodge in minute surface scratches and pits. Microbes metabolise this bounty anaerobically to produce hydrogen sulphide – the characteristic smell of rotten eggs. This is mopped up by the oxide in the surface film in a reaction that converts it to zinc sulphide. But on cleaning with lemon juice, the sulphide reacts with the citric acid to yield hydrogen sulphide again: it is this that makes the zinc stink. Scrupulous hygiene doesn’t really help because even 0.47 parts per billion will turn the nose up.
Len Winokur, Leeds, UK