Bleach

Bleach, also known as bleach, bleaching agent or Whitex, all refer to the same meaning, which is whitening and disinfecting, mostly used for fabrics and clothing. Of course, bleach is the only term commonly used in Iran, whereas in most parts of the world, the term Bleach is used for a whitening agent. Purchasing bleach is mainly done for disinfection and sterilization on both industrial and household scales. When we talk about purchasing bleach, we usually refer to the chemical formula sodium hypochlorite, NaOCl. This chemical compound melts at 18 degrees Celsius and boils at 101 degrees Celsius and generally loses its properties when exposed to light and heat.

Bleaches have broad antibacterial (microbial) properties and are very useful for disinfecting and sterilizing. For example, bleach is used to eliminate bacteria, viruses, and algae in swimming pool environments, and is also used to remove weeds, eliminate mold, and whiten wood pulp. Some bleaches are oxidizers, while others are reducers, and almost all of them have the capability to combine with natural pigments and remove them from contact environments. Contact of the bleach with skin and eyes is hazardous and will cause damage. Additionally, long-term inhalation of it in low concentrations will cause poisoning symptoms and in high concentrations may damage the respiratory system.

History of Bleach

The history of bleach dates back to years before Christ, although the bleach solution emerged many years after Christ entered human life and even the term Bleach was recorded twelve centuries after Christ in human history, but the bleach itself was discovered 3000 years before Christ, and the first process was executed 2000 years before that. The first bleaching process was recorded 5000 years before Christ by the Egyptians, who 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 when mixed with water turned into an alkaline solution capable of lightening colors and in some cases removing them. Interestingly, they soaked clothes for a long time in the prepared alkaline solution until they were completely white. However, this process was very time-consuming and damaged the fabric's fibers, and due to its high alkalinity, it required great care when handling the solution.

In the 11th and 12th centuries, the Dutch were known as masters of bleaching in Europe's laundries. They added sour milk like a spice to alkaline water, which helped to soften the clothes or fabric after bleaching and reduced its negative effects on the fabric's structure. Interestingly, they kept this practice a secret among themselves, and for many years after implementing it, the process remained a secret in the Netherlands.

Although this process was recorded in England in 1322, the Dutch remained the uncontested kings of bleaching in the world until the mid-18th century. Most brown linen fabrics produced, especially Scottish fabrics, went directly to the Netherlands for bleaching, but this process took a long time; it took seven to eight months for the fabrics to be properly bleached, as in addition to the roughly two-month bleaching process, a large space (so much that the city of Haarlem in western Holland became a center for this work) was also needed for drying all the fabrics. For this reason, by the mid-17th century, scientists began to think of replacing something instead of milk to speed up this process.

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

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

The process of discovering bleach took seven years. In the initial experiments, which posed a high health risk, chlorine gas was either placed directly in a chamber with fabrics or introduced into the solution, leading to numerous failures in Berthollet's experiments until in 1792, in the city of Javel near Paris, bleach (named due to its production in this city) was obtained by mixing one part of potash solution with eight parts of water. After this, Berthollet discovered sodium hypochlorite, and near the time of World War I, commercial production of bleach became possible.

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

Around the year 1820, French chemist Antoine-Germain Labarrak discovered the antibacterial and anti-pollution properties of sodium hypochlorite, leading to a significant advancement in disinfecting medical equipment and generally having a profound impact on public health, the cleanliness of hospital environments, surgical room tools, and slaughterhouses (meat production).

Although many advancements in bleaching have been made since then, today, bleach (5% instead of the 17-18% of that time) plays a major role in the family and industrial lives of people alongside many other bleaches, and although its ingestion is poisonous (if accidentally ingested, drink milk immediately) and contact with skin and 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 a pungent chlorine odor and a molecular weight of 442.74 grams per mole. Its density is very close to that of water, at 1.11 grams per cubic centimeter. Bleach pentahydrate melts at 18 degrees Celsius and boils and decomposes at 101 degrees Celsius under atmospheric pressure. Bleach dissolves 29.3 grams in every 100 milliliters of water at zero degrees Celsius, 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 generates dangerous gases Na2O and HCl during decomposition, which can be explosive. Generally, pentahydrate hypochlorite is used more due to its greater stability and lower explosiveness and is commercially distributed as a decahydrate hypochlorite. The crystalline form of bleach's chemical compound is orthorhombic and decomposes and evaporates rapidly at room temperature, but at 7 degrees, just 1% will decompose and evaporate after one year, and at 4 degrees Celsius it can remain intact for two years.

How to Prepare Bleach

Currently, there are five methods for producing bleach that 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 an alkaline potash solution. This dilute solution was converted into sodium hypochlorite by Antoine Labarrak, replacing the potash 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 45 to 48 percent NaOH solution.

2- From Calcium Hypochlorite

In this method, sodium carbonate reacts with bleaching powder (a mixture of calcium hypochlorite, calcium chloride, and calcium hydroxide) to produce sodium hypochlorite. This method was used when the resulting bleach was intended to be used as a disinfectant in hospitals after World War I.

3- Electrolysis of Saltwater

In the late 19th century, E.S. Smith invented a method for producing sodium hypochlorite using the electrolysis of saltwater. In this method, sodium hydroxide and chlorine gas are first produced, which is then mixed to form bleach. This made the production of sodium hypochlorite much cheaper than before, and today its improved process is known as the Hougker process, the only process used at an industrial scale for bleach production. In this process, chlorine gas passes through cold diluted sodium hydroxide, producing NaOCl and NaCl, and the chlorine gas itself is also produced 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 marketed as a byproduct.

4- Combination of Hypochlorous Acid and Caustic Soda

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

5- Combination of Ozone and Salt

The combination of ozone and salt is an expensive but simple 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: whitening agent, cleaner, disinfectant, and sanitizer. Other applications include dental root canal treatment, wastewater treatment, neutralizing nerve gas, and reducing skin damage in eczema.

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

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