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THE LAST WORD

Juiced up 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.

n Is the bar counter quite old? Zinc used to be extracted from its ore, an impure zinc sulphide known as sphalerite or zinc blende, 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.

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. John RowlandDerby, UK

n 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 WinokurLeeds, UK

Answer in depth When will the Challenger Deep cease to be the deepest ocean trench? And how will this occur?

n The Mariana trench, whose deepest point below sea level is Challenger Deep (10,898 metres), is the result of the planet’s largest tectonic plate, the Pacific plate, slipping under its neighbour the Mariana plate. This process of one plate sliding under another is called subduction.

Ocean trenches are formed over millions of years, and the simple answer to the above question is: nobody knows. Challenger Deep may always be the deepest point beneath sea level because of the relative masses of the plates that form it.

Interestingly, Challenger Deep is not the point on the surface that

is nearest to the centre of the planet. Because Earth is flattened at the poles, that point on the seabed is likely to be in the Arctic Ocean.Tony HolkhamBoncath, Pembrokeshire, UK

n Exactly when the forces that created Challenger Deep will moderate we cannot say. What is clear is that as soon as they stop, the hole will begin to fill with biological detritus, chemical concretions and the products resulting from erosion of the surrounding topography. Once that begins, the process should be pretty quick and it might be difficult to locate the trench 1 million years or so after its generating forces have abated. Structures such as this tend to be geologically short-lived. By way of comparison, a 1-million-year-old lake is a pretty old one – it will usually have silted up, dried up or eroded long before that.

The reason the African Great Lakes are still there is that the Great Rift Valley is still rifting – they are, in fact, an ocean in embryo. It is a similar story with oceanic deeps; remove the forces that cause them, and they pass away like dimples in foam.Jon RichfieldSomerset West, South Africa

This week’s questionPAss The PorTWhy is gout worse at night?Paul GrimesBelfast, UK

“herbert Armstrong bought arsenic to kill dandelions, but found it equally effective on his wife”

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