Chlorine Defined

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© 2013 Special Education Issue / June 3, 2013 / Anthony Kozuh / STEM Research – Education 

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EPA Defines chlorine
History of chlorine
Scientific definition
Health Hazard Information
Acute Effects
Chronic Effects (Noncancer)
Reproductive/Developmental Effects
Cancer Risk
Science research material
Physical characteristics of chlorine and its compounds
Chemical characteristics

EPA Defines Chlorine

Chlorine is a strong oxidant commonly used in water treatment for oxidation and disinfection. As an oxidant, chlorine is applied to control biological growth and to remove color, taste and odor compounds, iron and manganese, and other dissolved inorganic contaminants such as arsenic. As a primary disinfectant, chlorine is applied to disinfect and to control microbial activity in the distribution system. It is also used as a secondary disinfectant after chlorine, ozone, UV irradiation, or chlorine dioxide. Chlorine is commonly applied at one or two points during treatment. Downstream residual chlorine concentrations make chlorination concurrent with other treatment processes. Figure 1 shows multiple possible chlorine (Cl2) application points during conventional treatment. Chlorine residuals are common during filtration to inhibit microbial (biofilm) growth on filter media that could increase filter head loss (pressure) build up.

Chlorine is available as compressed elemental gas, sodium hypochlorite solution (NaOCl) or solid calcium hypochlorite (Ca(OCl)2). All forms of chlorine, when applied to water, form hypochlorous acid (HOCl). Gaseous chlorine acidifies the water and reduces the alkalinity, whereas the liquid and solid forms of chlorine increase the pH and the alkalinity at the application point. The pH of the water will affect the dominating chlorine species such that HOCl dominates at lower pH, while the hypochlorite ion (OCl-) dominates at higher pH. Of the two species, HOCl is the stronger oxidant. Therefore, chlorine is more effective as an oxidant and a disinfectant at lower pH. Both forms, HOCl and OCl-, are referred to as free chlorine.

The concentration (C), contact time (T), pH and temperature affect the effectiveness of chlorine application.  CHLORINEThe product of concentration and time (CT) is the most important operational parameter in disinfection and inactivation. Although increasing the dose increases the ability of chlorine to oxidize and disinfect, it may also lead to taste and odor issues and to the formation of disinfection byproducts (DBPs) by chlorine’s reaction with natural organic matter (NOM). The dose is also affected by the application point, chlorine demand of the water, and desired residual concentration. Total organic carbon (TOC) and ultraviolet absorbance (UV) are two measures of DBP-reactive NOM and of chlorine demand. Information courtesy of EPADrinking Water Treatability Database

EPA Contaminants Treated by Chlorine

EPA Research references


Wikimedia Foundation, Inc. The most common compound of chlorine, sodium chloride, has been known since ancient times; archaeologists have found evidence that rock salt was used as early as 3000 BC and brine as early as 6000 BC.[21] Around 1630, chlorine was recognized as a gas by the Belgian chemist and physician Jan Baptist van Helmont.[22]

Elemental chlorine was first prepared and studied in 1774 by Swedish chemist Carl Wilhelm Scheele, and, therefore, he is credited for its discovery.[23] He called it “dephlogisticated muriatic acid air” since it is a gas (then called “airs”) and it came from hydrochloric acid (then known as “muriatic acid”).[23] However, he failed to establish chlorine as an element, mistakenly thinking that it was the oxide obtained from the hydrochloric acid (see phlogiston theory).[23] He named the new element within this oxide as muriaticum.[23] Regardless of what he thought, Scheele did isolate chlorine by reacting MnO2 (as the mineral pyrolusite) with HCl:[22]

4 HCl + MnO2 → MnCl2 + 2 H2O + Cl2

Scheele observed several of the properties of chlorine: the bleaching effect on litmus, the deadly effect on insects, the yellow green color, and the smell similar to aqua regia.[24]

At the time, common chemical theory was: any acid is a compound that contains oxygen (still sounding in the German and Dutch names of oxygensauerstoff or zuurstof, both translating into English as acid stuff), so a number of chemists, including Claude Berthollet, suggested that Scheele’s dephlogisticated muriatic acid air must be a combination of oxygen and the yet undiscovered element, muriaticum.[25][26][27]

In 1809, Joseph Louis Gay-Lussac and Louis-Jacques Thénard tried to decompose dephlogisticated muriatic acid airby reacting it with charcoal to release the free element muriaticum (and carbon dioxide).[23] They did not succeed and published a report in which they considered the possibility that dephlogisticated muriatic acid air is an element, but were not convinced.[28]

In 1810, Sir Humphry Davy tried the same experiment again, and concluded that it is an element, and not a compound.[23] He named this new element as chlorine, from the Greek word χλωρος (chlōros), meaning green-yellow.[29] The name halogen, meaning “salt producer,” was originally used for chlorine in 1811 by Johann Salomo Christoph Schweigger. However, this term was later used as a generic term to describe all the elements in the chlorine family (fluorine, bromine, iodine), after a suggestion by Jöns Jakob Berzelius in 1842.[30][31] In 1823, Michael Faraday liquefied chlorine for the first time,[32][33] and demonstrated that what was then known as “solid chlorine” had a structure of chlorine hydrate (Cl2•H2O).[22]

Chlorine gas was first used by French chemist Claude Berthollet to bleach textiles in 1785.[34][35] Modern bleaches resulted from further work by Berthollet, who first produced sodium hypochlorite in 1789 in his laboratory in the town ofJavel (now part of Paris, France), by passing chlorine gas through a solution of sodium carbonate. The resulting liquid, known as “Eau de Javel” (“Javel water“), was a weak solution of sodium hypochlorite. However, this process was not very efficient, and alternative production methods were sought. Scottish chemist and industrialist Charles Tennant first produced a solution of calcium hypochlorite (“chlorinated lime”), then solid calcium hypochlorite (bleaching powder).[34] These compounds produced low levels of elemental chlorine, and could be more efficiently transported than sodium hypochlorite, which remained as dilute solutions because when purified to eliminate water, it became a dangerously powerful and unstable oxidizer. Near the end of the nineteenth century, E. S. Smith patented a method of sodium hypochlorite production involving electrolysis of brine to produce sodium hydroxide and chlorine gas, which then mixed to form sodium hypochlorite.[36] This is known as the chloralkali process, first introduced on an industrial scale in 1892, and now the source of essentially all modern elemental chlorine and sodium hydroxide production (a related low-temperature electrolysis reaction, the Hooker process, is now responsible for bleach andsodium hypochlorite production).

Elemental chlorine solutions dissolved in chemically basic water (sodium and calcium hypochlorite) were first used as anti-putrification agents and disinfectants in the 1820s, in France, long before the establishment of the germ theory of disease. This work is mainly due to Antoine-Germain Labarraque, who adapted Berthollet’s “Javel water” bleach and other chlorine preparations for the purpose (see a more complete history, see below). Elemental chlorine has since served a continuous function in topical antisepsis (wound irrigation solutions and the like) as well as public sanitation (especially of swimming and drinking water). In 1826, silver chloride was used to produce photographic images for the first time.[37] Chloroform was first used as an anesthetic in 1847.[37]

Polyvinyl chloride (PVC) was invented in 1912, initially without a purpose.[37]Chlorine gas was first introduced as a weapon on April 22, 1915, at Ypres by the German Army,[38][39] and the results of this weapon were disastrous because gas masks had not been mass distributed and were tricky to get on quickly.

Chlorine Definition as Accepted Today

Wikimedia Foundation, Inc. Chlorine is a chemical element with symbol Cl and atomic number 17. Chlorine is in thehalogen group (17) and is the second lightest halogen after fluorine. The element is a yellow-green gas under standard conditions, where it forms diatomic molecules. It has the highest electron affinity and the third highestelectronegativity of all the elements; for this reason, chlorine is a strong oxidizing agent. Free chlorine is rare on Earth, and is usually a result of direct or indirect oxidation by oxygen.

The most common compound of chlorine, sodium chloride, has been known since ancient times. Around 1630 chlorine gas was first synthesized in a chemical reaction, but not recognized as a fundamentally important substance. Characterization of chlorine gas was made in 1774 by Carl Wilhelm Scheele, who supposed it an oxide of a new element. In 1809 chemists suggested that the gas might be a pure element, and this was confirmed by Sir Humphry Davy in 1810, who named it from Ancient Greek: χλωρóς khlôros “pale green”.

Nearly all chlorine in the Earth’s crust occurs as chloride in various ionic compounds, including table salt. It is thesecond most abundant halogen and 21st most abundant chemical element in Earth’s crust. Elemental chlorine is commercially produced from brine by electrolysis. The high oxidizing potential of elemental chlorine led commercially to free chlorine’s bleaching and disinfectant uses, as well as its many uses of an essential reagent in the chemical industry. Chlorine is used in the manufacture of a wide range of consumer products, about two-thirds of them organic chemicals such as polyvinyl chloride, as well as many intermediates for production of plastics and other end products which do not contain the element. As a common disinfectant, elemental chlorine and chlorine-generating compounds are used more directly in swimming pools to keep them clean and sanitary.

In the form of chloride ions, chlorine is necessary to all known species of life. Other types of chlorine compounds are rare in living organisms, and artificially produced chlorinated organics range from inert to toxic. In the upper atmosphere, chlorine-containing organic molecules such as chlorofluorocarbons have been implicated in ozone depletion. Small quantities of elemental chlorine are generated by oxidation of chloride to hypochlorite in neutrophils, as part of the immune response against bacteria. Elemental chlorine at high concentrations is extremely dangerous and poisonous for all living organisms, and was historically used in World War I as the first gaseous chemical warfare agent.

Wikipedia research references to forgoing history and current definition

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Health Hazard Information

Acute Effects:

  • Chlorine is a potent irritant in humans to the eyes, the upper respiratory tract, and the lungs. Several acute (short-term) studies have reported the following effects: tickling of the nose at 0.014 to 0.054 parts per million (ppm); tickling of the throat at 0.04 to 0.097 ppm; itching of the nose and cough, stinging, or dryness of the nose and throat at 0.06 to 0.3 ppm; burning of the conjunctiva and pain after 15 minutes at 0.35 to 0.72 ppm; and discomfort ranging from ocular and respiratory irritation to coughing, shortness of breath, and headaches above 1.0 ppm. (4)
  • Higher levels of chlorine have resulted in the following effects in humans: mild mucous membrane irritation at 1 to 3 ppm; chest pain, vomiting, dypsnea, and cough at 30 ppm; and toxic pneumonitis and pulmonary edema at 46 to 60 ppm. (3)
  • Chlorine is extremely irritating to the skin and can cause severe burns in humans. (3)
  • Acute animal tests in rats and mice have shown chlorine to have high acute toxicity via inhalation. (6)

Chronic Effects (Noncancer):

  • Workers chronically exposed to chlorine gas have exhibited respiratory effects, such as eye and throat irritation, and airflow obstruction. (8)
  • Animal studies have reported decreased body weight gain, eye and nose irritation, and nonneoplastic lesions and respiratory epithelial hyperplasia from chronic inhalation exposure to chlorine. (4,8)
  • The Reference Dose (RfD) for chlorine is 0.1 milligrams per kilogram body weight per day (mg/kg/d) based on no observed adverse effects in rats. The RfD is an estimate (with uncertainty spanning perhaps an order of magnitude) of a daily oral exposure to the human population (including sensitive subgroups) that is likely to be without appreciable risk of deleterious noncancer effects during a lifetime. It is not a direct estimator of risk but rather a reference point to gauge the potential effects. At exposures increasingly greater than the RfD, the potential for adverse health effects increases. Lifetime exposure above the RfDdoes not imply that an adverse health effect would necessarily occur. (2)
  • EPA has medium confidence in the RfD based on (1) medium to high confidence in the principal study because relevant endpoints in two animal species were examined after prolonged exposure by an appropriate route, but an effect level was not observed in this study and higher levels may not be practicable due to taste aversion, and (2) medium confidence in the database because information is available for rats and mice on the noncarcinogenic toxicity of oral exposure to chlorine for subchronic periods. Developmental and reproductive toxicity of chlorine have been examined in rats and mice, but with suboptimal studies; due to the chemical relationship between chlorine and monochloramine, reproductive and developmental studies for monochloramine may be used to satisfy data gaps for chlorine. (2)
  • EPA has not established a Reference Concentration (RfC) for chlorine. (2)
  • CalEPA has established a chronic reference exposure level of 0.00006 milligrams per cubic meter (mg/m3) based on respiratory epithelial lesions in rats. The CalEPA reference exposure level is a concentration at or below which adverse health effects are not likely to occur. (8)

Reproductive/Developmental Effects:

  • No information is available on the developmental or reproductive effects of chlorine in humans or animals via inhalation exposure.
  • Animal studies have demonstrated no evidence of reproductive or developmental effects from ingestion exposure to chlorine. (2)
  • Since chlorine is highly reactive, uptake at sites such as the ovaries and testes which are remote from the respiratory tract, is anticipated to be minimal. (2)

Cancer Risk:

  • No information is available on the carcinogenic effects of chlorine in humans from inhalation exposure.
  • Several human studies have investigated the relationship between exposure to chlorinated drinking water and cancer. These studies were not designed to assess whether chlorine itself causes cancer, but whether trihalomethanes or other organic compounds occurring in drinking water as a result of chlorination are associated with an increased risk of cancer. These studies show an association between bladder and rectal cancer and chlorination byproducts in drinking water. (5)
  • An NTP study reported no evidence of carcinogenic activity in male rats or male and female mice, and equivocal evidence, based on an increase in mononuclear cell leukemia, in female rats, from ingestion of chlorinated or chloraminated water. (9)
  • EPA has not classified chlorine for carcinogenicity. (2)

EPA research reference

Chlorine: Further science research material