How Bleach Works: Disinfection, Stain Removal, and Dangers
A Versatile Household Product
Bleach is a multipurpose product that is very useful in homes, hospitals, science laboratories, and industry. It's a potent germ killer that can also whiten and brighten fabrics and remove stains. Some people even use it to create special effects in their art projects.
There are several chemicals that can act as bleaches. The most commonly used one is sodium hypochlorite, or NaOCl. (The formula is also written as NaClO.) Sodium hypochlorite dissolved in water is sometimes known as chlorine bleach. It destroys a wide range of bacteria, algae, fungi, and viruses.
Other hypochlorites can also act as bleaches, including calcium hypochlorite. This is sold as bleaching powder. Some chemicals that don't belong to the hypochlorite family are bleaches as well, such as hydrogen peroxide and sodium perborate. This article is concerned with sodium hypochlorite in water, however, which is easily obtainable and widely used. It's a very helpful product, but it must be used with caution since it's potentially dangerous.
The History of Bleaches
The discovery that sunlight can bleach fabrics is a very ancient one. The ultraviolet radiation in sunlight is responsible for the colour fading. Like chemical bleaches, UV light also kills germs if it's sufficiently intense.
The discovery of chemical bleaches was based on the work of three scientists in the eighteenth and nineteenth centuries.
- A Swedish scientist named Carl Wilhelm Scheele discovered chlorine. (This is significant because sodium hypochlorite contains chlorine.)
- A French scientist named Claude Berthollet discovered that chlorine could bleach fabrics. He was also the first person to make a solution of sodium hypochlorite, which he called "Eau de Javel" or Javel water. The name came from the part of Paris in which Berthollet worked.
- Antoine Germain Labarraque, another French scientist, discovered that hypochlorites could act as disinfectants.
Sodium hypochlorite is a white powder in its pure form. The bleach that is bought in stores contains sodium hypochlorite dissolved in water. It's a clear solution with a slightly yellow colour. Household bleach that is intended to be used for disinfection generally contains about 5.25% sodium hypochlorite by weight, although one brand in my local supermarket contains 7.4% sodium hypochlorite.
Sodium hypochlorite is a very unstable substance and reacts chemically with the water in the bleach container. A variety of reactions may occur, but the most common ones are described below.
Production of Hypochlorous Acid
The reaction between NaOCl and water produces produces HOCl, or hypochlorous acid, and sodium hydroxide, or caustic soda, as shown in the following chemical equation.
NaOCl + H2O → HOCl + NaOH
Hypochlorous acid is responsible for bleach's ability to remove colour from objects and for its ability to disinfect surfaces.
Production of the Hypochlorite Ion and Oxygen
The NaOCl also breaks down to produce the hypochlorite ion, or OCl-. This ion decomposes into a very reactive form of oxygen and a chloride ion. Like hypochlorous acid, the oxygen can remove colour from items, but to a lesser extent.
Removing Colour with Bleach: An Art Technique
How Does Bleach Whiten Fabrics?
Sodium hypochlorite is classified as an oxidizing agent. An oxidizing agent takes electrons from other chemicals when it reacts with them. Using its oxidizing ability, NaOCl (or the HOCl that it produces) breaks chemical bonds inside chromophores, which are the parts of molecules which give them colour. This causes the chromophores to either change their bonding structure or to break up. The ability of the chromophores to absorb and reflect light is altered and they are unable to produce colour. In this way the NaOCl removes stains from fabrics and also lightens their overall colour.
How Does Bleach Kill Germs?
Sodium hypochlorite reacts with proteins in microbes, denaturing them, or changing their shape. A protein is made of one or more chains of amino acids. Each chain is twisted and folded into a specific shape. If the shape changes, the protein can no longer do its job.
The hypochlorous acid that forms when sodium hypochlorite reacts with water causes microbe proteins to denature and then clump together, forming a non-functional mass. This kills the microbes.
Antibacterial Action of Bleach
The Importance of Dilution
Bleach that is bought in stores for cleaning and disinfecting needs to be diluted with water before use. The dilution factor depends on the starting concentration of the product. It's important to look at the container to see the manufacturer's recommendations. The company's website should also be a good resource for dilution recipes for different uses and for cleaning suggestions. The Centers for Disease Control and Prevention (CDC) webpage described at the end of this article should be helpful in this respect as well. The idea is to dilute the bleach so that it's safe and economical to use but not to dilute it so much that it's no longer effective.
Diluted bleach will only be effective for about a day (twenty-four hours) or sometimes for an even shorter time. Even the undiluted product has a shelf life and will eventually become ineffective. Once the NaOCl has finished reacting, salt (NaCl) and water are left. The container's "use by" date should be noted. This date is not a guarantee of the bleach's safety, though. Since we can't see the chemicals in a container of bleach, we don't know when they've finished reacting.
When diluting and using bleach it's important to work in a well-ventilated area, since the vapour may irritate the eyes and airways. It's also a good idea to wear protective gloves. Dilute solutions may be mildly irritating. Concentrated solutions can burn.
A Disinfectant and a Cleanser
Bleach is an excellent germ killer and can be very helpful in a home. For example, bleach can:
- remove mold and mildew from bathroom tiles and shower curtains
- clean shower mats
- clean porcelain toilets (but make sure that you read the "Potential Dangers" section below before you do this)
- remove plants from cracks in a driveway or path
- clean concrete
- clean surfaces on which raw foods have been placed, such as cutting boards
- clean kitchen counters, refrigerators, stoves, and floors
- disinfect door handles, toilet flushers, faucets, sinks, garbage cans, and non-porous toys
- disinfect pet areas, such as litter trays and bird cages
- disinfect garden tools to prevent spreading an infection from one plant to another
- act as a cut flower preservative
If cleaning instructions say to use a 1:10 bleach solution, they mean that one part of bleach should be mixed with nine parts of water. For example, mixing one cup of bleach with nine cups of water would create a 1:10 solution.
A Germ Killer
If bleach is being bought specifically to kill germs, it's important to read the bottle label carefully. The more concentrated solutions should have a word like "disinfectant" on the label. The less concentrated solutions are used as laundry bleaches and may not be able to kill germs.
Bleach that is being used to disinfect surfaces should be left in contact with the surface for at least five minutes (or ten minutes for some microbes) before being rinsed off. The surface should then be allowed to air dry whenever possible. It's important to think about the materials that are used to rinse or dry an item after it's been sanitized with bleach. If a contaminated cloth is used, it will re-introduce germs to the area.
Sodium Hypochlorite in Drinking Water and Swimming Pools
The safety of bleach is related to its concentration. Sodium hypochlorite is often used to disinfect drinking water and swimming pool water. When some people hear this, they think that they drink or swim in bleach. In fact, they do, since bleach is simply sodium hypochlorite dissolved in water! The concentration of NaOCl is kept to a safe level in water that's designed for human use, however.
Bleach can also be useful when doing the laundry. The product can be added to detergent to clean and brighten white fabrics or the fabrics can be soaked in bleach to remove a stain.
The washing instructions on a fabric and the instructions on the bottle of bleach should be followed carefully. The labels on some bleach containers say that the product is safe for certain types of colourfast fabrics, but it may be advisable to test the product on a small, hidden area of the fabric first.
Bleach may also weaken the material used to make an item of clothing if it's used over a long period of time. Some laundry bleaches contain an additive ("Fiber Guard") to protect fabrics and keep them strong.
Sodium hypochlorite is very reactive, so it's important to take safety precautions, even with diluted bleach. The product must be kept in a firmly closed container which is labelled carefully and kept out of reach of children and pets.
Some of the potential dangers of NaOCl are listed below.
- When NaOCl reacts with light it produces dangerous chlorine gas (Cl2). Chorine is also made when the solution is heated. Even at room temperature, some chlorine escapes from the solution.
- The caustic soda in bleach can irritate or burn skin, depending on its concentration.
- Bleach is corrosive, especially at higher concentrations.
- Bleach and cleansers containing ammonia must never be mixed. They react to produce a dangerous gas that contains toxic chloramine. The bleach must also be kept away from acids (including vinegar), rust remover, and toilet bowl cleaner.
If safety precautions are followed, bleach is a great substance to have in a home. It improves the appearance of fabrics, cleans surfaces, and can be an excellent germ killer. Even hospitals use the product to kill dangerous microbes. It's an impressive liquid.
- A chemistry professor at the University of Bristol describes sodium hypochlorite.
- A CDC webpage discusses disinfection in a hospital setting, but a lot of the information applies to homes, too. The "Chlorine and Chlorine Compounds" section describes the dilutions and exposure times needed to kill various microbes, using household bleach containing 5.25% to 6.15% sodium hypochlorite.
Questions & Answers
© 2013 Linda Crampton