Molecule of the Month: A pinch of salt is a matter of life and death: Potassium chloride is essential to all living things, says John Emsley. But, as recent events have demonstrated, a large dose can be fatal

MANY people were surprised last month when Dr Nigel Cox was found guilty of murdering his terminally ill patient, 70-year-old Mrs Lillian Boyes. Even more surprising was that he dispatched her with an injection of potassium chloride solution, a simple salt essential to all living things and sold in supermarkets as a substitute for common salt, sodium chloride. We have about twice as much potassium in our bodies as sodium. The average adult weighing 70kg (11st) contains about 120g (4oz) of potassium. Without either salt we cannot live, and an injection of a lot of potassium chloride kills quickly.

Almost all food contains potassium, exceptions being vegetable oils, butter and margarine. Some foods are particularly rich in potassium: seeds and nuts may have up to 1 per cent by weight, compared with the normal range of 0.1- 0.4 per cent. Common foods with more than 0.5 per cent potassium are kippers, peanuts, raisins, potatoes, bacon and mushrooms. Foods with more than 1 per cent include All-Bran (1.1 per cent), butter beans (1.7 per cent), dried apricots (1.9 per cent), Marmite (2.7 per cent) and instant coffee (4.0 per cent).

The report of the Committee on Medical Aspects of Food Policy, published by Her Majesty's Stationery Office last year, recommended a daily intake of 3.5g of potassium for adults - much more than the 1.5g a day it recommended for sodium. Those wishing to cut down on sodium by reducing their intake of common salt can buy a costly substitute that is a mixture of sodium and potassium chlorides.

Potassium is found in all parts of the body. Red blood cells have most potassium, followed by muscles and brain tissue. It is found mainly in the fluid between the cells of the body as the electrolyte ion K+, but occurs in its highest concentrations inside cells. Its most important role is in operating the nervous system. The sending of signals to our vital organs is done by moving sodium and potassium out of nerve cells.

Professor Oliver Dolly, of the department of biochemistry at Imperial College, London, investigates the channels in cell walls that allow only potassium to pass through. His research involves the venom of the black mamba snake, whose bite injects a poison that disrupts the movement of potassium in its victim's body. The venom has a toxin that blocks the potassium channels, causing convulsions and death. Professor Dolly uses the toxin to probe the layout of potassium channels in brain tissue by injecting a radioactive form and then mapping where it has collected. His research shows that the highest concentration of potassium channels is in the hippocampus region of the brain, important in learning.

Professor Dolly is working on ways in which similar molecules might target the the potassium channels as a way of controlling their function, and so stimulate or calm activity in the brain. The behaviour of potassium explains how an injection of potassium chloride can be fatal. Too much potassium outside the nerve cells prevents potassium ions moving from the inside to the outside of a nerve cell, and the signal fades to zero. All body functions are affected, especially the heart, which stops beating.

Some conditions, such as starvation or kidney malfunction, and certain diuretics, cause potassium deficiency. Humans need a constant throughput of this essential element to make lean tissue and keep the kidneys working. If we are not getting enough we experience muscular weakness, and this, too, has an effect on the heart muscle, causing irregular beats and even cardiac arrest. Chronic deficiency leads to depression and confusion.

Medical treatment requires potassium supplements, and some medicines - diuretics, for example - may include extra potassium. However, ill health due to potassium deficiency is rare, because it is almost impossible to avoid potassium in the food we eat, particularly vegetables and fruit. Some people may even get too much, such as heavy beer drinkers, and their craving for salted snacks may be the body's way of maintaining its electrolyte balance.

All plants absorb a lot of potassium from the soil, and it was the ash from wood fires, or potash, that gave this element its name. The chemical symbol, K, is from the Roman name for potash, kalium.

Potassium chloride is one of the few minerals in which the UK is self-sufficient. Professor Peter Scott of the Camborne School of Mines reports that world production of potassium ore is about 40 million metric tons, mainly from mines in Russia Germany, Canada Chile and from Dead Sea brines. The Chilean deposit is potassium nitrate (saltpetre) but the other sources are potassium chloride mixed with salts such as sodium chloride and magnesium chloride.

Britain's only potassium chloride mine is at Boulby, where Cleveland Potash produces almost a million tons a year of the pinkish ore sylvinite. The pit is more than a kilometre deep and its 500 miners work with rock that reaches temperatures of 40 C.

Dr Nevill Rice, senior lecturer in mineral engineering at Leeds University, is researching better ways of extracting potassium chloride from sylvinite. The process of extraction involves crumbing the ore and then separating the potassium chloride from the other minerals present by flotation on a saturated brine, which must be of exactly the right composition to ensure that no potassium chloride dissolves. Potassium chloride mining in the UK is a pounds 50m a year industry, with the bulk going into making fertilisers, the rest into chemicals such as potassium hydroxide - used to make liquid soaps and detergents - and potassium carbonate, which goes into special glass for television sets.

A large dose of several grams of potassium chloride will paralyse the central nervous system, cause convulsions, diarrhoea, kidney failure and rapid death from heart failure. But what Dr Cox did illegally in Britain is done legally in the United States - as a method of capital punishment. Condemned people who agree to donate their organs for transplants may be executed by 'non-toxic lethal injection' of potassium chloride. Unlike poison gas or the electric chair, it leaves all organs intact.

The author is science writer in residence in the department of chemistry at Imperial College, London.