Antimicrobial agents: hypochlorite. Part 1


The active principle of the so-called chlorine bleach is not chlorine, but hypochlorite — a substance to which all known microbes are sensitive.

But, despite the effectiveness of this substance, a number of organizations are actively demanding its ban.

Chlorine is a toxic yellow-green gas that can kill living organisms in just a few seconds. At a concentration of 3 parts per million parts of air, it irritates the eyes and mucous membranes of the respiratory tract, inhalation of its vapors at a concentration of 50 parts per million is dangerous even for a short time, inhalation of air with a chlorine content of 500 parts per million will cause death in less than one minute.

Much less dangerous is chlorine dissolved in water, with which it reacts to form a mixture of hydrochloric (HCl) and hypochlorous (HOCl) acids. Better than water, chlorine dissolves in a solution of sodium hydroxide, also known as caustic soda, its formula is NaOH), which, when reacted with alkali, forms sodium hypochlorite (NaOCl), which has powerful bactericidal properties, often called household bleach or chlorine bleach, which is not entirely correct, since it does not contain chlorine in the form of a gas.

History of hypochlorite

Chlorine has an interesting history. In 1774, in the Swedish city of Uppsala, it was first received by the thirty-two-year-old chemist of German origin Karl Wilhelm Scheele (1742-1786). He did this by heating hydrochloric acid with the mineral manganese dioxide (MnO2). At the same time, the chemist recorded the release of a yellow-green gas, with an unpleasant odor, soluble in water with the formation of an acidic solution, which stained litmus paper and discolored the leaves and flowers of plants. Scheele called the gas dephlogisticated hydrochloric acid, a name he retained for more than thirty years, until, in 1807, the twenty-nine-year-old English chemist Humphry Davy examined it and discovered that this compound was not an acid at all, but a new chemical element.

By that time, a use had already been found for chlorine. In 1786, James Watt from Birmingham, England, demonstrated the bleaching effect of chlorine dissolved in water, but such a solution could not be used in practice. However, at that time they used a different method of bleaching fabrics — they were simply laid out in the sun for several weeks. In 1787, the French chemist Claude Louis Berthollet (1784-1822) passed chlorine through caustic potash (potassium hydroxide, KOH), resulting in a solution of potassium hypochlorite (KOCl). He named this substance Eau de Javelle after a small village near Paris that specialized in bleaching fabrics. Soon, factories began to produce a new bleaching agent, which also interested paper manufacturers.

In 1799, by passing chlorine through slaked lime (calcium hydroxide, Ca (OH) 2), calcium hypochlorite, now known as white lime, was first obtained. Its advantage was ease of transport, the substance could be easily transported to the place where it was needed, and dissolved on the spot, obtaining a bleaching solution of hypochlorite.

In 1820, the French chemist Antoine-Germain Labarraque (1777-1850) launched the production of Eau de Labarraque, a process based on passing chlorine through an aqueous solution of sodium hydrochloride to form sodium hypochlorite. This substance turned out to be much cheaper than Eau de Javelle, and it is still available under the name «chlorine bleach». Although now it can be sold under many other commercial names, some of which have already become part of our language. So, in the UK people call bleach «Domestos» and in the United States «Chlorox», bottles with these words can be found in many kitchens, bathrooms and toilets.

Initially, hypochlorites were considered as a convenient form of transportation of chlorine, because when their solutions were acidified with hydrochloric acid, gas was immediately released from them, and its mass was one third of the mass of the original salt. Today it is more economical to transport chlorine in liquid form under pressure in special tanks.

To date, millions of tons of hypochlorite are produced annually, the exact figure is difficult to determine, since often the production of the substance takes place in the same place as consumption, for example, at water treatment plants. Household bleach production consumes about a quarter of the world’s total chlorine, despite protests from environmentalists against chlorine bleach manufacturers. The use of bleach in oil production, where hypochlorite is used to sterilize sea water that is pumped into oil wells and pushes oil out, has been unexpected. If the water is not sterilized, this leads to the growth of fungi living in the mud, such fungi clog the well and make it difficult to extract oil.

In some countries bleaches are especially popular. For every inhabitant of Spain, an average of twelve liter bottles of bleach is consumed per year. Whereas in other countries chlorine bleach, on the contrary, enjoys low popularity, in Germany, for example, the average citizen uses the same amount of bleach throughout his life. In the UK, the average resident buys two liter bottles a year. Bleach cannot be stored for long periods of time as it slowly breaks down into inactive agents such as sodium chlorate (NaC1O3) and oxygen (O2) when exposed to sunlight, which is why bleaches are usually sold in opaque containers.

While some people find the smell of bleach unpleasant, many perceive it as a fresh scent, and in the early nineteenth century it was used to get rid of the stinking miasma. In 1827, Thomas Alcock published his essay entitled An Essay on the Uses of Chlorites, Oxides, Sodium, and Lime, recommending chlorine bleach and bleach for the deodorization of particularly malodorous places such as hospitals, pigsties, and toilets. Two years earlier, in 1825, Labarrac conducted experiments on himself on washing wounds with sodium hypochlorite, based on the results he recommended using this substance to treat many inflammatory diseases, but his technique did not take root.

In May 1847, the Hungarian surgeon Ignaz Semmelweis (1818-1865) used bleach to disinfect the hands of medical students studying at the Laying Clinic (maternity clinic) in Vienna, where the doctor himself worked. At the same time, he noticed a decrease in the frequency of cases of puerperal fever, the cause of which was a vaginal infection with subsequent blood poisoning and death, more than thirty percent of mothers from those wards where students underwent practice died from this disease. In neighboring wards, where experienced midwives worked, the mortality rate was significantly lower. The reason for this was that medical students often came to the maternity hospital immediately after the mortuary, and Semmelweis suggested that they were bringing some infectious agent with them. Thanks to his insistence that every student wash their hands with hypochlorite, the death rate from puerperal fever dropped to 1%. (Today this disease occurs in only 0.0001% of women in labor.)

But this was only the beginning of the success story of hypochlorite, later in the nineteenth century, the problem of infection contained in the water was solved. In 1881, the German microbiologist Robert Koch (1843-1910) showed how effective chlorine bleach could be in the fight against pathogens that contaminate water, and already in 1892 this substance was used in Hamburg to disinfect drinking water, which helped to end the city’s cholera epidemics. . Five years later, sodium hypochlorite was added to the tap water in Maidstone to deal with outbreaks of enteric fever (typhoid fever). And in 1905, an epidemic of this disease began in Lincoln and was also successfully suppressed with the help of water chlorination.

It became clear that waterborne infection could be fought by this method, so more and more chlorination stations were set up in cities to purify drinking water, and now chlorination remains the gold standard of disinfection throughout the world. The use of hypochlorite to disinfect water has brought enormous benefits to mankind, in 1998 in Life magazine this technology was included in the hundred most important achievements of the millennium. Hypochlorite has one huge advantage: it is so cheap that even the poorest nations can afford it.

Source: based on the book by John Emsley «On the benefits and harms of the products we love to buy»

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