Monthly Archives: October 2014

Autumn leaves – the chemistry behind the colour

As Autumn pushes onwards the leaves on the trees have lost their green colour and have allowed the vibrant hues of autumn to show through. Although this change may initially seem a simple one, the vivid colours are a result of a range of chemical compounds.

A green leaf is green because of the presence of a pigment known as chlorophyll, which is inside an organelle called a chloroplast. When they are abundant in the leaf’s cells, as they are during the growing season, the chlorophylls’ green colour dominates and masks out the colours of any other pigments that may be present in the leaf. Thus the leaves of summer are characteristically green.

As summer fades, so too does the amount of light, and thus chlorophyll production slows  The existing chlorophyll decomposes. As a result of this, other compounds present in the leaves can come to the fore, and affect the perceived colouration as shown in the infographic below featured on the CompoundInterest website.  Click on the link below for a larger picture.

Autumn Leaves-click to enlarge
Carotenoids are present in leaves the whole year round, but their orange-yellow colours are usually masked by green chlorophyll.  Carotenoids provide colourations of yellow, brown, orange, and the many hues in between.

The reds, the purples, and their blended combinations that decorate autumn foliage come from another group of pigments in the cells called anthocyanins. Unlike the carotenoids, these pigments are not present in the leaf throughout the growing season, but are actively produced towards the end of summer.   They develop in late summer in the sap of the cells of the leaf, and this development is the result of complex interactions of many influences — both inside and outside the plant. Their formation depends on the breakdown of sugars in the presence of bright light as the level of phosphate in the leaf is reduced.

The brown colour of leaves is not the result of a pigment, but rather cell walls, which may be evident when no colouring pigment is visible.

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On this day…

After the end of World War II on October 24, 1946 and a good while before the Sputnik satellite opened the space age, a group of soldiers and scientists in the New Mexico desert saw something new and wonderful—the first pictures of Earth as seen from space.

The White Sands rocket (official name V-2 No. 13) was the first man-made object to take a photograph of the Earth from outer space.   Launched from the White Sands Missile Range in White Sands, New Mexico, the rocket reached a maximum altitude of 107.5 miles (173 km), well above the commonly accepted boundary of space at 100 kilometres.
The famous photograph was taken from an altitude of 65 miles (104 km) with an attached 35 mm black-and-white camera.

Snapping a new frame every second and a half, the rocket-borne camera climbed straight up, then fell back to Earth minutes later, slamming into the ground at 500 feet per second. The camera itself was smashed, but the film, protected in a steel cassette, was unharmed.

It was one of many firsts for the V-2 research program of the late 1940s, during which the Army fired dozens of captured German missiles brought to White Sands in 300 railroad cars at the end of the war. While the missileers used the V-2s to refine their own rocket designs, scientists were invited to pack instruments inside the nosecone to study temperatures, pressures, magnetic fields and other physical characteristics of the unexplored upper atmosphere.

Earth from Space in colour

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pH Indicators

A pH indicator is a colour changing (or halochromic) chemical compound that is added in small amounts to a solution so that the pH (acidity or basicity) of the solution can be determined visually.

A pH indicator is a chemical detector for hydronium ions (H3O+) or hydrogen ions (H+).  Normally, the indicator causes the colour of the solution to change depending on the pH.

pH (potential of hydrogen) is a scale of acidity from 0 to 14. It tells how acidic or alkaline a substance is. More acidic solutions have lower pH. More alkaline solutions have higher pH. Substances which are not acidic or alkaline (neutral) usually have a pH of 7. Acids have a pH less than 7. Alkalis have a pH greater than 7.

pH indicator solutions are themselves weak acids or bases.  As one chemical is added it changes the arrangement of the electrons in the molecule causing it to absorb different wavelengths of light and therefore appear different in colour.

Different chemicals can be used for different pH ranges as shown in the diagram below.


pH indicators are frequently employed in titrations in analytical chemistry and biology to determine the extent of a chemical reaction. Because of the subjective choice (determination) of colour, pH indicators are susceptible to imprecise readings. For applications requiring precise measurement of pH, a pH meter is frequently used.

Many plants or plant parts contain chemicals from the naturally-coloured anthocyanin family of compounds. They are red in acidic solutions and blue in basic. Anthocyanins can be extracted with water or other solvents from a multitude of coloured plants or plant parts, including from leaves (red cabbage); flowers (geranium, poppy, or rose petals); berries (blueberries, blackcurrant); and stems (rhubarb). Extracting anthocyanins from household plants, especially red cabbage, to form a crude pH indicator is a popular introductory chemistry demonstration.
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On this day……Aspirin

On the 10th October 1897 German chemist Felix Hoffmann discovered an improved way of synthesizing acetylsalicylic acid or ‘aspirin’.

Around c400 BC Hippocrates in Greece gives women willow leaf tea to relieve the pain of childbirth.  In 1763 Reverend Edward Stone of Chipping Norton near Oxford gives dried willow bark to 50 parishioners suffering rheumatic fever and describes his findings in a letter to the Royal Society of London.  In 1823 the active ingredient is extracted from willow and named salicin.  Salicylic acid is made from salicin by French scientists in 1853 butis found to irritate the gut.  In 1893 German scientists find that adding an acetyl group to salicylic acid reduces its irritant properties and in 1897 in Germany, Bayer’s Felix Hoffmann develops and patents a process for synthesising acetyl salicylic acid or aspirin. First clinical trials begin.

Aspirin is often used as an analgesic to relieve minor aches and pains, as an antipyretic to reduce fever, and as an anti-inflammatory medication.

Aspirin is now accepted as an important weapon in the prevention of heart disease. A single dose of 300 mg is now recommended for patients in the acute stages of a heart attack followed by a daily dose of 75-100 mg. A similar low dose treatment regime is recommended for patients with angina, a history of heart problems or who have undergone coronary by pass surgery.

Aspirin is also used in other medical situations:-

  • Strokes – to reduce the risk
  • Pregnancy Complications – Pre-eclampsia and foetal growth retardation, both caused by blockages of the blood vessels of the placenta, are two of the commonest complications of pregnancy – aspirin helps to reduce this risk.
  • Colon cancer – In a long term study of 90,000 US nurses between 1976 and 1995, those who took 4-6 tablets of aspirin a week had a reduced incidence of colorectal cancer. The benefits were greatest in those who had taken the drugs the longest.
  • Diabetes  Blindness, coronary artery disease, stroke and kidney failure are all common complications of diabetes resulting from impaired blood circulation. The benefits of taking one aspirin a day are now so widely accepted that it is considered unethical to perform placebo controlled trials to prove the case.
  • Dementia (including Alzheimer’s disease)- There is some evidence that aspirin may help prevent both the condition resulting from impaired blood flow and the most serious form of dementia, Alzheimer’s disease.
The most common use is as a painkiller for headaches or fevers.

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Zinc is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element of group 12 of the periodic table. It’s the 24th most abundant element in the Earth’s crust and has five stable isotopes. The most common zinc ore is sphalerite (zinc blende), a zinc sulfide mineral. The largest mineable amounts are found in Australia, Asia, and the United States.

Brass, which is an alloy of copper and zinc, has been used since at least the 10th century BC.

Zinc is an essential mineral of “exceptional biologic and public health importance”.  Zinc deficiency affects about two billion people in the developing world and is associated with many diseases.  In children it causes growth retardation, delayed sexual maturation, infection susceptibility, and diarrhoea, contributing to the death of about 800,000 children worldwide per year.

The metal is most commonly used as an anti-corrosion agent.  Galvanization, which is the coating of iron or steel to protect the metals against corrosion, is the most familiar form of using zinc in this way.  Zinc is more reactive than iron or steel and thus will attract almost all local oxidation until it completely corrodes away.  A protective surface layer of oxide and carbonate forms as the zinc corrodes.  This protection lasts even after the zinc layer is scratched but degrades through time as the zinc corrodes away.  The zinc is applied electrochemically or as molten zinc by hot-dip galvanizing or spraying. Galvanization is used on chain-link fencing, guard rails, suspension bridges, light posts, metal roofs, heat exchangers, and car bodies.

Zinc Oxide used in paint pigments

Zinc is useful for the human body and helps speed up the healing process after an injury.  It is also suspected of being beneficial to the body’s immune system. Indeed, zinc deficiency may have effects on virtually all parts of the human immune system.

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