Bleach

Bleach, also known as bleach, whitener, or white vinegar, all refer to the same meaning, which is whitening and disinfection, mostly used for fabrics and clothing. Of course, bleach is the only term commonly used in Iran, while in most parts of the world, the term Bleach is used for whitener. The purchase of bleach is primarily done for disinfection and sterilization on an industrial and household scale. When it comes to buying bleach, we generally refer to the chemical formula sodium hypochlorite, NaOCl. This chemical compound melts at 18 degrees Celsius and boils at 101 degrees Celsius, and generally, if exposed to light and heat, it will lose its properties.

Bleaches have broad antibacterial (antimicrobial) properties and are very useful for disinfection and sterilization. For example, bleach is used to eliminate bacteria, viruses, and algae in swimming pool environments, and it is also used to remove weeds, eliminate mold, and whiten wood pulp. Some bleaches are oxidizers, and some are reducers, and almost all of them have the ability to combine with natural pigments and remove them from the contact environment. Contact with bleach on the skin and eyes is dangerous and will cause damage to them. Also, breathing them for a long time at low concentrations will cause symptoms of poisoning, and at high concentrations, they will cause damage to the respiratory system.

History of Bleach

The history of bleach dates back to years before the birth of Christ, although the liquid bleach known as bleach entered human life years after Christ, and even the word Bleach was recorded in human history twelve centuries after Christ. However, the bleach itself was discovered three thousand years before Christ, and its first process was executed two thousand years before that. The first bleaching process was recorded five thousand years before Christ by the Egyptians using sunlight. They washed their linen fabrics in water and spread them under the sun to become whiter (brighter).

Evidence shows that two thousand years after the Egyptians, people had enough knowledge to create a bleaching solution from wood ash, which, after mixing with water, turned into an alkaline solution capable of lightening colors and, in some cases, removing them. Interestingly, they would immerse clothes in the prepared alkaline solution for a long time to make them completely white. However, this process was very time-consuming and caused damage to the fabric's texture, and due to its high alkalinity, there was a need for careful handling when working with the solution.

The Dutch in the eleventh and twelfth centuries were known as masters of bleaching in European laundries. They added sour milk like a spice to the alkaline water, which made the clothes or fabric soft after bleaching and reduced its negative effects on the fabric's texture. Interestingly, they kept this practice a secret among themselves, and for years after doing this, the process remained a secret in the Netherlands.

Although this process was recorded in England in the year 1322, until the mid-eighteenth century, the Dutch were the undisputed kings of bleaching in the world. Most of the brown linen fabrics produced, especially Scottish fabrics, went directly to the Netherlands for bleaching, but this process was very lengthy; it took seven to eight months for the fabrics to be properly bleached, as in addition to the nearly two-month bleaching process, a large space (so much so that the city of Haarlem in western Netherlands became a center for this work) was also required for drying all the fabrics. For this reason, from the mid-seventeenth century, scientists began to think about replacing something with milk to speed up this process.

In the year 1746, John Roebuck, an English scientist, used diluted sulfuric acid instead of sour milk. This substitution proved to be very effective, reducing the time required for the bleaching process from six weeks to two months to just twenty-four hours and sometimes twelve hours, leading to tremendous economic growth in this trade.

In the year 1774, Swedish chemist Carl Wilhelm Scheele (discoverer of oxygen) discovered that chlorine gas, a phosphorescent (greenish-yellow) gas belonging to the halogen family and irritating, had the ability to eliminate plant colors. This discovery inspired Claude Louis Berthollet to discover bleach in the year 1785.

The process of discovering bleach took seven years, in initial experiments that had a high health risk, where chlorine gas was placed directly in a chamber exposed to fabrics or directly introduced into a solution, leading to numerous failures for Berthollet's experiments until in the year 1792 in the city of Javel near Paris, bleach (named after its production in this city) was obtained by mixing one part of potassium solution with eight parts of water. Berthollet then discovered sodium hypochlorite, and near World War I, the commercial production of bleach became possible.

However, the biggest moment in the bleaching industry occurred in the year 1799 by Charles Tennant from Glasgow with the introduction of calcium chloride (known as bleaching powder). He introduced calcium hypochlorite as a substitute for bleach.

Around the year 1820, French chemist Antoine Germain Labarac discovered the antibacterial and anti-pollution properties of sodium hypochlorite, leading to significant advancements in the disinfection of medical instruments and having a profound impact on public health, cleanliness in hospitals, surgical instruments, and slaughterhouses (meat production).

Although many advancements have been made since then in the field of bleaching, today bleach (5 percent instead of 17-18 percent at that time) plays a major role in the family and industrial life of people alongside many other bleaches, and although ingesting it is toxic (if ingested accidentally, drink milk immediately) and contact with skin, eyes, and the respiratory system causes damage, its presence has led to significant improvements in human health.

Properties of Bleach

Bleach or bleach is a colorless or greenish-yellow liquid with an irritating chlorine smell, with a molecular weight of 442.74 grams per mole. Its density is very close to water, equal to 1.11 grams per cubic centimeter. Bleach pentahydrate melts at 18 degrees Celsius and boils at 101 degrees Celsius at atmospheric pressure and will decompose. Bleach dissolves in water at zero degrees Celsius at a rate of 29.3 grams per hundred milliliters and has three heavy atoms with two covalent bonds.

Anhydrous hypochlorite is highly unstable in the presence of air unless combined with sodium hydroxide. This combination produces dangerous gases Na2O and HCl upon decomposition, which can be explosive. Typically, pentahydrate hypochlorite is used more due to its greater stability and non-explosive nature, and it is commercially distributed as dihydrate hypochlorite. The crystalline form of the chemical compound bleach is orthorhombic and decomposes and evaporates quickly at room temperature, but at 7 degrees, only 1 percent will decompose and evaporate after one year, and at 4 degrees Celsius, it will remain intact after two years.

How to Prepare Bleach

Currently, there are five methods for producing bleach, which we will examine:

1- Injection of Chlorine Gas into Caustic Soda

Potassium hypochlorite was produced by Claude Louis Berthollet in Javel by passing chlorine gas through a solution of alkaline water. This weak solution was converted into sodium hypochlorite by Anthony Labarac by replacing the potassium solution with caustic soda solution.

Cl2(g) + 2NaOH(aq) → NaCl(aq) + NaClO(aq) + H2O(aq)

In this reaction, chlorine is simultaneously oxidized and reduced, and during this process, pentahydrate hypochlorite will also be prepared using a NaOH solution of 45 to 48 percent.

2- From Calcium Hypochlorite

In this method, sodium hypochlorite is prepared from the reaction between sodium carbonate and bleaching powder (a mixture of calcium hypochlorite, calcium chloride, and calcium hydroxide). This method was used when the produced bleach was intended to be used as a disinfectant in hospitals after World War I.

3- Electrolysis of Salt Water

In the late nineteenth century, E. S. Smith devised a method for producing sodium hypochlorite using the electrolysis of salt water. In this method, sodium hydroxide and chlorine gas are first produced, and then the mixture is converted into bleach. This made the production of sodium hypochlorite much cheaper than before, and today its improved process, known as the Hougker process, is the only process used on an industrial scale for producing bleach. In this process, chlorine gas passes through cold dilute sodium hydroxide, producing NaOCl and NaCl, and chlorine gas is also produced in this method through electrolysis. The process temperature is maintained below 40 degrees Celsius to prevent the unwanted production of sodium chlorate, and the produced salt is also offered as a byproduct.

4- Combination of Hypochlorous Acid and Caustic Soda

This method was registered in 1966 for the production of dibasic sodium hypochlorite from hypochlorous acid and sodium hydroxide solution. However, in 1991, a method based on this method for producing pentahydrate sodium hypochlorite was proposed, which is not economically feasible for large-scale production.

5- Combination of Ozone and Salt

The combination of ozone and salt is an expensive but easy method for preparing bleach for research purposes. In this method, salt is reduced by ozone gas, producing sodium hypochlorite and releasing oxygen gas. This reaction occurs at room temperature and is also suitable for the oxidation of alcohols.

Applications of Bleach

In summary, bleach is used wherever disinfection or whitening is needed. Generally, the applications of bleach can be divided into four main categories: bleaching, cleaning, disinfecting, and sanitizing. Other applications include root canal treatment, wastewater treatment, neutralizing nerve gas, and reducing skin damage during eczema.

From A to Z of Bleach – History, Properties, Applications

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