杏吧原创

Old odour

Last night my father pulled out all his old copies of New 杏吧原创 to show us that it really has been in existence since the 1950s. But we both noticed that the older the magazine, the more musty it smelt. What is it that creates the distinctive smell of old magazines and books and is there any way to counteract it?

鈥� The distinctive smell that is produced by old magazines and books comes from the waste products of fungi. Species including Mucor, Rhizous, Aspergillus and Penicillium grow on any product containing organic material, such as paper or wood, providing the conditions are sufficiently warm and humid.

These fungi create a characteristic musty odour (as well as causing discoloration or deterioration of the product they are living on) that is caused by waste products 鈥� the so-called microbial volatile organic compounds, or MVOCs.

The fungi produce different MVOCs depending on the species that is present, environment and stage in their life cycle. Some more common MVOCs include 1-octen-3-ol, 3-methyl-1-butanol, 2-hexanone and 3-methylfuran. The species of fungus can be identified from the MVOC being produced. Many MVOCs are toxic, allergenic or carcinogenic and some are blamed for aggravating a number of respiratory diseases such as asthma.

To prevent such smells occurring in the future, your magazines should be stored in conditions where fungi cannot easily grow. Cold, dry rooms help. This has been discussed on this page before (see 鈥淧reservation order鈥�, Last Word, 16 February). Additionally, storing magazines in plastic bags will help reduce the spread of fungal spores.

Rick Eraho

Cleckheaton, West Yorkshire

鈥� In addition to a number of high-tech solutions to the smell and decay problem, the following substances, when wrapped inside a sealed plastic bag with the magazines that you wish to preserve, also absorb smell and moisture: baking soda, unwrapped soap, ground coffee and cat litter.

Chris Jack

St Albans, Hertfordshire

Charging period

How can the 鈥渕emory effect鈥� which affects the charge level in nickel-cadmium batteries be explained in chemical terms?

ChaaaAArge!

Your recent question on electric toothbrushes led to an argument among my friends. Opinion is divided on whether you should replace the toothbrush on its recharger after each use or whether you should empty its power source to the point where its performance is notably reduced. To prolong the battery鈥檚 life, is it better to recharge it constantly or first let it run down?

鈥� These two questions (one still unanswered since being posed in 1994) are best answered together.

Many of our correspondents believe that if the nickel-cadmium, or NiCad, batteries that are usually used to power electric toothbrushes are regularly recharged when they have only partially run down, then they are liable to a 鈥渕emory effect鈥�. That is, the battery will only run down again to the level from which recharging began, and battery capacity will therefore be reduced.

Experiments show, however, that memory effects are very hard to create. Most apparent memory effects are probably due to a loss of capacity resulting from regular overcharging, as explained in the answers below 鈥� Ed

鈥� It is best for the batteries to be recharged after each use. But this gives only a marginal improvement in battery life, and most rechargers are not sophisticated enough to stop when the batteries are full. The result is that there is often significant overcharging, which damages the batteries.

An easy way to stop this overcharging is to use the toothbrush until it slows and then to charge it for the time taken for about a full charge, which is usually overnight, or until you next clean your teeth.

Malin Dixon

Nuneaton

鈥� Batteries are systems designed to release electrical energy that has been stored chemically. When a NiCad battery releases electricity, nickel (IV) hydroxide is reduced to nickel (II) hydroxide at the battery鈥檚 cathode, 鈥渆ating鈥� two electrons in the process. At the same time, metallic cadmium is oxidised to cadmium (II) hydroxide and releases two electrons at the anode. When the battery is charged, the reverse process occurs, depositing cadmium at the cathode.

A memory-like effect can occur when, over time, the cadmium slowly forms large crystals instead of the microcrystals required for a fast reaction. Also, overcharging or leaving your battery in the charger for a long time produces a different form of crystalline nickel (IV) hydroxide.

So basically, the effect comes from a different crystalline form of cadmium or nickel hydroxide in either or both electrodes of the cell. This results in a slower reaction rate in releasing the electrical current, leading to a lower voltage.

F. Denonne

University of Bordeaux Pessac, France

鈥� The build-up of cadmium crystals in the batteries can be clearly observed by electron microscopy. It is often possible to recover much of the battery capacity by running the battery down completely and then recharging it again, although great care should be taken to observe any safety instructions provided by the manufacturer regarding recharging. Readers should note that another apparent memory effect can result for devices with multiple batteries as described below 鈥� Ed

鈥� The 鈥渕emory effect鈥� sometimes present in NiCad batteries is largely a result of the fact that the individual cells are not identical, so after several charge-discharge cycles, some of the cells will be well-charged while others less so.

In an extreme case, some cells will not only be flat but may be exhibiting reverse polarity, which drastically reduces the voltage available from the battery. For particularly demanding applications, manufacturers try to match the characteristics of cells in a battery pack.

Brian Reed

Maidenhead, Berkshire

This week鈥檚 questions

Less cargo?

What happens to a snail鈥檚 anatomy when it appears to shrink and retreat back into its shell? It looks like it turns itself inside out like a sock.

Eugenie Kirk

Suhr, Switzerland

Jennifer Eccles

Why do some people develop freckles? What causes the freckles to appear, why do they often disappear in adulthood, and why are some people and skin types more susceptible than others?

M. Benn

Solihull, West Midlands