杏吧原创

Waxing lyrical

What affects the different shadings of earwax? Sometimes mine is a light honey colour, other days it is very dark orange/brown. And why does its consistency change?

鈥� About 2000 sebum-producing sebaceous glands and specialised sweat-like apocrine glands in the outer third of the ear canal produce mildly acidic secretions, called cerumen. Earwax is a mixture of cerumen, skin cells and hair fragments from the ear canal, plus bacteria and other substances caught in this waxy matrix.

Earwax normally moves out of the ear, and while there is some controversy over whether cerumen is bactericidal, the waxy substance is an effective material for catching dust, small particles, bacteria and fungi that enter the ear. Earwax also lubricates the ear canal and has had a number of other uses because of its properties. It has even been used as lip balm.

There are two types of cerumen, dry and wet, the latter being the genetically dominant form. Both are controlled by a single autosomal gene. This has been used by anthropologists to trace migrations, because earwax in people of Mongolian Asian ancestry is usually the less common, recessive, dry type.

Cerumen is composed of glycerides, lipids 鈥� including squalene, cholesterol and long-chain fatty acids 鈥� waxy esters, aromatic hydrocarbons, amino acids and sugars such as galactose. Earwax also contains a complex of biochemicals from skin and hair, including significant amounts of collagen and keratin, and dead bacteria and fungi. Differences in the composition of these substances is one of the reasons that earwax builds up more in some people than in others.

Earwax colour is a result of the light-absorbing properties of its chemical constituents. Wet and dry cerumen differ in lipid content: lipids account for about 50 per cent in the former and 20 per cent in the latter. So while dry earwax is somewhat crusty and typically greyish in colour, relatively clean, fresh, wet earwax is typically a light-brownish honey colour.

But the constituents also change with time. The colour darkens because much of the long-chain fatty-acid content is unsaturated and slowly oxidises on exposure to air. This yields a darker brown colour. Eventual inclusion of dirt, dead cells, bacterial products and hair fragments can turn earwax to an almost dark brown or black colour in ears that are not cleaned frequently.

The initial colour also varies due to idiosyncratic differences in the type and amount of glandular secretions that generate it. This balance can change in response to stress in the same way that the composition of sweat secretions does. Such changes reflect differences in the proportion of secretions from the sebaceous and apocrine glands, as well as variations in the concentration of the components.

Finally, as we age, even wet cerumen secretions become less liquid.

Mark Dubin

University of Colorado, Boulder, US

Evil ice

I have inherited an Australian edition of The Windsor Recipe Book. I cannot find a date on it but I suspect it is probably more than 70 years old, as it is priced 1 shilling. It contains wonderful recipes for wide-ranging items such as how to make nitroglycerine and fireworks, how to treat warts and wrinkles, how to destroy a range of vermin, how to bore holes in glass using camphor dissolved in turpentine, and how to increase the weight of gold. Perhaps the most intriguing recipe was one describing how to make ice. This suggested putting water in a container which itself was placed inside a larger one. In the space between the two you pour 鈥渁 mixture of 8 parts nitric acid, 18 parts phosphate of soda, and 12 parts of nitrate of ammonia.鈥� Would this really work? And if so, how? Where could this ice-making technique be used, and how would you safely dispose of the chemical cocktail?

鈥� Treat those recipes with caution. For example the freezing mix requires diluted acid, although it does not say so. Similar vagueness in detail could be fatal in, say, nitroglycerine recipes.

Such freezing mixtures exploit the nature of the process of dissolving: the molecules or ions of the solvent and the solute each need to be pulled apart, which takes energy, causing cooling. The subsequent solvation (drawing together of the particles of solvent and solute) releases energy, causing heating, especially when the ions are small and highly charged.

You can create overall heating or cooling depending on what you use and the amounts. If heating dominates, such as when sodium hydroxide dissolves, things get hot. When you dissolve sodium chloride, dissolution cooling approximately balances solvation heating, so the temperature hardly changes. Dissolving ammonium nitrate or urea causes cooling.

Your recipe solvates lots of large ions of low charge, but it can be tuned to work better with more expensive chemicals. Before freezers, dry ice and liquid nitrogen were common. Freezing mixtures were useful in the lab, and are still valuable in remote regions.

Jon Richfield

Somerset West, South Africa

鈥� Ammonium nitrate (nitrate of ammonia) is endothermic when dissolved in an aqueous solution. This means it abstracts heat from its environment when it dissolves. The concentration of nitric acid is not stipulated, but I assume that it is an aqueous acid solution of some concentration. The 鈥渉eat stealing鈥� of the dissolving ammonium nitrate will cool the water in the adjacent vessel.

The sodium phosphate (phosphate of soda) part of the question was, however, a puzzle. So I got hold of some and dissolved it in water and also in a water/nitric acid solution. Sure enough, heat was taken up just as with the ammonium nitrate. Because both water-dissolving salts abstract heat, it is very likely that water in close proximity to the dissolving chemicals could be chilled below its freezing point by the action of the mixture.

Dan Stotland

Patent Attorney

Montreal, Canada

This week鈥檚 question

Sweet heat

While looking through old recipe books for new things to try on a glut of summer fruit that I am trying to preserve, I was intrigued by descriptions of the different properties of sugar at different temperatures. Between 105 and 107 掳C the sugar syrup will form a fine, thin thread suitable for decorative use when cooled. Between 115 and 121 掳C the syrup will form a hard ball useful for making grained products such as fudge. At 138 掳C the syrup becomes brittle when cold, which is good for toffee-making. What chemical and physical properties change in the sugar and water molecules to make these different products?

Mog Bremner

Hughes, ACT, Australia