An overall view of sodium hypochlorite and its importance.

An Everyday Chemical – Sodium Hypochlorite Bleach

By Catherine Graham (word count 1144)

Introduction

An overall view of sodium hypochlorite and its importance

Sodium hypochlorite is best known as the active ingredient in household bleach. The French chemist Berthollet discovered sodium hypochlorite in 1787. Its ability to effectively whiten textiles was quickly discovered and put to commercial use with great success. By the end of the nineteenth century Louis Pasteur had discovered the effectiveness of sodium hypochlorite against disease causing bacteria and it soon became widely used as a disinfectant.

Details of the raw materials needed to make hypochlorite bleach and where they come from

Sodium hypochlorite is produced by reacting chlorine with sodium hydroxide (caustic soda). These raw materials are produced from the electrolysis of brine (salt water). Therefore, a manufacturing plant requires only common salt (NaCl), which is mined here in the UK, and water.

The electrolysis process causes the solution to break down into sodium hydroxide (caustic soda), chlorine and hydrogen:-

2NaCl + 2H2O → 2NaOH + Cl2 + H2

Brief outline of the process used to manufacture sodium hypochlorite

By reacting sodium hydroxide with chlorine, sodium hypochlorite, sodium chloride and water is produced:-

Cl2 + 2NaOH → NaOCl + NaCl + H2O

The resulting product contains equal molecular amounts of sodium hypochlorite and common salt (NaCl). It also contains water and some free sodium hydroxide, which is necessary for stability during storage. Depending on the strength required, the product may be diluted further with additional water. Other ingredients such as natural and synthetic smectite clays may also be added to optimise stability and thicken the final product.

Information on the properties / activity of sodium hypochlorite and an explanation of its uses

The objective of using sodium hypochlorite is to remove soiling, stains, bacteria and odours. When it comes into contact with large, chain-linked soiling molecules, it breaks them down into smaller units. These units have a much harder time sticking to fabrics or surfaces. Therefore the combination of sodium hypochlorite and the physical or mechanical action being used, work together to remove the offending substances.

The cleaning process itself turns nearly all of the bleach back into salt water. Any remaining bleach passes either into ...

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Information on the properties / activity of sodium hypochlorite and an explanation of its uses

The cleaning process itself turns nearly all of the bleach back into salt water. Any remaining bleach passes either into the main drainage and is completely deactivated at a municipal treatment facility or in a septic system through biodegradation.

Sodium hypochlorite is widely used in homes, schools, hospitals, swimming pools, drinking water supplies, and for disinfecting hard surfaces and surgical instruments. Because of its low cost and availability sodium hypochlorite is an invaluable weapon for the maintenance of human health and proper preventative hygiene, throughout the world.

In the developing world this is particularly so, as it is a major contributor in the efforts to stem the debilitating consequences of cholera, dysentery, typhoid and other waterborne biological diseases.

In the developed world it also has an important public health role as there is increasing consumer awareness and concern that the home can be a haven for disease causing organisms such as salmonella and E. Coli. By following good hygiene practices, hypochlorite bleach has been shown to be the most effective means of minimising these dangers. It is also very effective in the removal of mold and mildew.

In hospitals and other health care facilities sodium hypochlorite is used to disinfect surfaces against HIV and Hepatitis B. In fact many authorities specifically promote such practices.

It has been used by NASA to assure destruction of any potentially harmful organisms introduced from space missions.

Industrial applications of sodium hypochlorite include:

The control of algae in open reservoirs
The treatment of sewerage to reduce odours and increase the digesting efficiency of bacteria
Chemical toilets and industrial wastes for odour control
Cyanide waste treatment in metal finishing
Treatment for cyanide effluent in gold mining
Air scrubbing
Food processing: dairy equipment sanitising, fruit and vegetable processing, mushroom production, meat production, fish processing
Precious metal recovery
Cooling water and boiler water treatment to prevent fouling

Details of any pollution / environmental problems associated with the use or production

During recent years there has been a concerted effort to investigate and assess the human and environmental safety aspects of sodium hypochlorite. The conclusions drawn are that it is safe for both humans and the environment. The way in which it is manufactured and the way in which consumers use it has no impact on the earth’s ozone layer.

Sodium hypochlorite bleach does not contain or generate chlorine gas when used by itself or when used as directed, with other products. The formation of chlorine gas can occur only if sodium hypochlorite bleach is mixed with acids or ammonia. Although chlorine is a component of CFC’s (chloroflourocarbons), it only contributes to ozone depletion when a chlorine atom is carried into the stratosphere by CFC’s. It is here that the chlorine atom can affect oxygen molecules and diminish the ozone layer.

Under normal usage, sodium hypochlorite is broken down in the environment into common salt (NaCl), oxygen and water. To a small extent, other substances may be formed and these by-products are most often referred to as AOX (adsorbable organic halides). Studies to provide a risk assessment of the amount of AOX in household bleach concludes it is very small both in absolute terms and relative to other human activities and natural sources. The majority of these AOX are biodegradable, the AOX formed are primarily water soluble and dioxins are not formed.

Depending on the concentration involved, sodium hypochlorite solutions can be classified as either an irritant or corrosive and appropriate precautions should be taken when using the product by carefully reading the label, following usage directions and adhering to cautionary warnings.

Through skin contact and ingestion, irritation can occur when the exposure concentration is greater than 5%, although there are no known delayed effects. There may be local tissue damage but these effects are generally reversible. The unpleasant taste of sodium hypochlorite bleach prevents unintentional ingestion of significant quantities and thereby limits the extent of injuries.

Inhalation of chlorine gas produced when sodium hypochlorite is mixed with acid or ammonia will induce coughing, chest discomfort, shortness of breath, wheezing, throat and nasal irritation. With treatment though, these symptoms normally subside within a short amount of time.

Eye exposure to household bleach is painful and may cause blurred vision or temporary blindness. These symptoms normally disappear if the eye is immediately flushed with large amounts of water. If this action is delayed the reaction is more severe but again normal vision should return within a couple of days.

The overall safety of sodium hypochlorite is further documented by reports from poison control centres in North America and Europe, which show no major health effects after unintentional ingestion or skin contact. (http://www.esemag.com/0596/bleach.html).

Conclusion

Studies by numerous independent research institutes have supported the high level of disinfection effectiveness of sodium hypochlorite. It is recognised as being the most effective disinfectant against virtually all known bacteria, fungi and viruses.

History is a good indicator of the problems we would still be incurring today without its discovery. Diseases would spread much more easily resulting in higher infant mortality, livestock loss, water contamination and crop contamination to name a few.

We can therefore conclude that sodium hypochlorite is an extremely versatile and important product in the modern world.