Chlorine dioxide

from Wikipedia, the free encyclopedia
Structural formula
Structural formula of chlorine dioxide
General
Surname Chlorine dioxide
other names
  • E  926
  • Chlorine (IV) oxide
Molecular formula ClO 2
Brief description

yellowish-reddish gas with a sharp, suffocating odor

External identifiers / databases
CAS number 10049-04-4
EC number 233-162-8
ECHA InfoCard 100.030.135
PubChem 24870
Wikidata Q422080
properties
Molar mass 67.46 g mol −1
Physical state

gaseous

density

3.01 g l −1

Melting point

−59 ° C

boiling point

11 ° C (decomposition from 45 ° C)

Vapor pressure

140 k Pa at 20 ° C

solubility
  • soluble in water
  • soluble in alkaline solutions and sulfuric acid
safety instructions
GHS hazard labeling from  Regulation (EC) No. 1272/2008 (CLP) , expanded if necessary
04 - gas bottle 03 - Oxidising 06 - Toxic or very toxic
05 - Corrosive 09 - Dangerous for the environment

danger

H and P phrases H: 270-330-314-400
P: ?
MAK
  • DFG : 0.1 ml m −3 or 0.28 mg m −3
  • Switzerland: 0.1 ml m −3 or 0.3 mg m −3
Toxicological data
  • 94 mg kg −1 ( LD 50ratoral )
  • 292 mg · kg −1 ( LD 50ratoral , "stabilized chlorine dioxide", a mixture of chlorite and lactic acid)
  • 260 ppm 2h −1 ( LC Loratinh. )
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Chlorine dioxide is a chemical compound of chlorine and oxygen with the empirical formula ClO 2 . The notation OClO is used instead of ClO 2 when a distinction is to be made between the short-lived compound with the same empirical formula, chlor peroxide , ClOO. As a food additive , it was approved for bleaching flour in Germany until 1957 , later it had the E number E926; for health reasons, however, it is no longer permitted for the treatment of food these days.

At room temperature, chlorine dioxide is an amber-colored, toxic gas with a pungent, chlorine-like odor. Mixtures of air with more than 10 % by volume of chlorine dioxide can explode. It is therefore mostly used in an aqueous solution that is not explosive. Chlorine dioxide is a radical with oxidizing properties .

The uses of chlorine dioxide are based on its oxidative effect. It is often used in place of chlorine because it forms less toxic or harmful chlorinated organic compounds when it reacts with organic substances. As a bleaching agent of the ECF bleaching ( E lementar- C hlor- f ree bleaching) of wood pulp , such as paper , it has almost completely replaced elemental chlorine. It is also used for disinfection in drinking water treatment instead of chlorine.

The detection of chlorine dioxide in the earth's atmosphere over the Antarctic in 1986 contributed to the discovery of the causes of the ozone hole . It is one of the chlorine oxides that are there in the stratosphere from the previously often referred to as propellant gas or refrigerant used chlorofluorocarbon materials form (CFC) and in the destruction of the ozone layer are involved.

history

Humphry Davy

The discovery of chlorine dioxide is generally attributed to Humphry Davy , who was the first known halogen oxide to obtain it in 1811 by disproportionation (splitting) of chloric acid (HClO 3 ) . In previous experiments, chlorine dioxide was produced, but due to its solubility in water, it was not obtained as a gas and was therefore neither isolated nor recognized as a new compound. Davy noted that " when the gas is collected, it often explodes [...] with an immediate release of heat and light " ( translation ). Chlorine itself was still known as oxymuric acid at this time , as it was considered a compound of oxygen and hydrochloric acid (muric acid) before Davy recognized chlorine as an element. Today it is assumed that Davy did not isolate pure chlorine dioxide either, but rather a mixture of chlorine and chlorine dioxide, which he called "Euchlorine" ('very yellow').

In 1921, Erich Schmidt and Erich Graumann described chlorine dioxide as a selective bleaching agent that does not react with carbohydrates (polysaccharides) and can be used to break down lignin while preserving cellulose :

" Plant parts of incrustations, z. B. to rid wood of lignin in such a way that completely encrusted carbohydrates such as cellulose and hemi-cellulose are obtained, has so far only succeeded with simultaneous attack by the polysaccharides. However, if the encrusting substances are removed using chlorine dioxide, carbohydrates that are free of encrusts and completely unaffected can be obtained, since chlorine dioxide does not act on them "

Industrial use for pulp bleaching began over 20 years later, shortly after the end of World War II . Initially, chlorine dioxide was only used in the last phase of bleaching, later it increasingly replaced chlorine in the earlier phases to break down lignin as this improved the quality of the end product.

In the 1990s there was an increased awareness of the problem of chlorinated waste products, especially the very toxic dioxins , when bleaching with chlorine. In the course of this, chlorine dioxide displaced the chlorine more and more from the processes for pulp bleaching.

Increasing bans and restrictions on chlorine bleaching and its waste products made it the most important industrial bleach. In 2004, according to Greenpeace, 82% of the “chlorine-free bleached” paper was actually not bleached with elemental chlorine, but with chlorine dioxide and / or chlorine peroxide (Cl 2 O 2 ) (see illustration on pulp production in the bleaching agent section ).

presentation

Because chlorine dioxide is unstable and can explode, it is produced on site just before use. The representation is based on either chlorite (ClO 2 - ) or chlorate (ClO 3 - ); The application determines which starting substance is more suitable.

Applications that require large amounts of chlorine dioxide but have low purity requirements start from sodium chlorate (NaClO 3 ). All large-scale manufacturing processes for use as bleaching agents nowadays use sodium chlorate as the starting material. Today's pulp mills each produce over a million tonnes of pulp per year, with up to 40 tonnes of chlorine dioxide being bleached per day. With this ratio of chlorine dioxide to pulp, it took around one million tons of chlorine dioxide to produce 70 million tons of ECF-bleached pulp in 2005.

Sodium chlorite (NaClO 2 ) is mainly used for the production of small to medium quantities with high purity requirements, such as drinking water treatment . The reaction conditions in the production from chlorite are easier to control, on the other hand sodium chlorite is more expensive and less stable than sodium chlorate and therefore less suitable for large-scale industrial applications. It is inevitably more expensive if sodium chlorite itself is produced by reducing chlorine dioxide in an alkaline solution:

Sodium chlorite then has the function of an intermediate storage device in which chlorine dioxide is converted into a form that is easier to transport and store, which at the same time results in an end product with greater purity.

Large-scale application

Chlorine dioxide is generated by reducing chlorate with a suitable reducing agent in a strongly acidic solution. The reduction has a standard redox potential E 0 of 1.152 V:

In the 1950s, the Mathieson process was developed as the first large-scale process, in which sulfur dioxide (SO 2 ) is used to reduce :

In the reaction, sulfuric acid is used in excess, since the space required for the reaction strongly acidic pH should be maintained. Since the reaction only takes place under strongly acidic conditions, a lot of unused residual acid remains in this process. It also produces a lot of sodium sulphate as a by-product , which is used in the sulphate process to supplement the sodium and sulfur losses in the pulping process. In order to avoid the spontaneous decomposition or explosion of the chlorine dioxide, which occurs at room temperature from a partial vapor pressure of 10 kPa, air is blown through the reaction mixture. The mixture of chlorine dioxide and air is then passed in to dissolve the ClO 2 in chilled water (8-10 ° C). Usual concentrations of the cooled aqueous solution are 8 g / l to 10 g / l ClO 2 .

The variant of reduction with hydrochloric acid is inexpensive, but has the disadvantage that more chlorine is produced as a by-product. In the equation shown here, hydrochloric acid is also produced from sulfuric acid and sodium chloride (table salt):

Therefore, the hydrochloric acid in the Solvay variant was replaced by the use of methanol as a reducing agent:

However, the direct reaction of methanol with chlorate to form chlorine dioxide is very slow. Here, too, the actually active reducing agent is seen as the chloride ion , which is formed from a reaction of methanol with chlorine. Both chlorine and chloride anions are essential for the reaction. A complete consumption of all chloride ions leads to a standstill of the reaction called a “white-out” until chloride is reproduced again. For this reason, a small amount of chloride ion is constantly added to the reaction mixture in some of the processes.

The processes introduced after the Mathieson and Solvay processes are listed under the names R2 to R10. These are in each case further developments in which the weaknesses of the earlier processes are to be avoided by adapting the reaction conditions. The goals of the optimization are a high chlorine dioxide yield, low formation of chlorine as a by-product and a small amount of residual acid and salts. In most of these newer processes, the chlorine dioxide is continuously removed from the reaction solution under reduced pressure and dissolved in water, for example in the variants R8.SVP-MeOH, R9 and R10 based on reduction with methanol. The R8 process aims to reduce the amount of residual acid and salts formed in the reaction (52% less than the R3 process, 66% less than Solvay). In the R9 process, the remaining sodium sulphate-sodium hydrogen sulphate mixture is split electrolytically into sodium hydroxide and sulfuric acid, which are fed back into the process, while in the R10 process, neutral sodium sulphate is removed by precipitation while the acid is reused.

Drinking water treatment

In Germany, Section 11 of the Drinking Water Ordinance , as well as a list of substances from the Federal Ministry of Health named therein, regulates which substances may be added to drinking water and thus which methods are permitted for drinking water disinfection.

The list also includes worksheets W 224 and W 624 of the DVGW (German Association for Gas and Water) for chlorine dioxide . The manufacturing processes described in the worksheets are used to manufacture chlorine dioxide. The peroxodisulfate-chlorite, hydrochloric acid-chlorite and chlorine-chlorite processes for use in drinking water treatment are described there.

In the chlorine-chlorite process, chlorinated water with an acidic pH value (<2) is reacted with 10% sodium chlorite solution:

Chlorine is always used in excess to prevent unreacted sodium chlorite from remaining in the water.

In the hydrochloric acid-chlorite process, sodium chlorite is reacted with hydrochloric acid to form chlorine dioxide, common salt and water:

When reacting with sodium peroxodisulfate , sodium sulfate is formed as a by-product, which also occurs naturally in drinking water:

The disadvantage of the processes mentioned is the fact that hazardous substances are used in liquid (hydrochloric acid-chlorite process and peroxodisulfate-chlorite process) or gaseous form (chlorine-chlorite process) and the kinetics of the reactions mentioned are different. While the hydrochloric acid-chlorite process leads to the immediate formation of chlorine dioxide, the peroxodisulfate-chlorite process takes approx. 24 hours to set the target concentration. At the same time, the necessary, stabilizing equilibria are different, so that the chlorine dioxide preparation from the hydrochloric acid-chlorite process disintegrates within 1 to 4 days (room temperature, exclusion of light), while the peroxodisulfate-chlorite preparation is stable for up to 10 days under the same conditions having. Furthermore, it is common practice to cool the chlorine dioxide concentrate solutions that have already been introduced for stabilization, which also represents additional expenditure.
A manufacturing process that circumvents the disadvantages mentioned has been available since 2013. This process is mapped using a one-component solid mixture. The individual components required are pressed in an inert manner, so that there is no reaction during storage (storage stability over 3 years). The one-component solid process combines the three mentioned equilibria in one process. The reaction is primarily initiated by an acid-chlorite equilibrium, followed by the reaction with peroxodisulfate-chlorite. As an intermediate there is still a chlorine-chlorite equilibrium, which reduces the degradation kinetics. In this way, on the one hand, metering in proportion to the volume flow (drinking water, raw water) is simplified (since natural decay (disproportionation) is reduced), on the other hand, work safety is increased and the logistical effort is reduced. Handling is further simplified by pre-packaged dosage forms (e.g. tablets).
The process leads to an immediate formation of chlorine dioxide, at the same time the degradation kinetics at room temperature and with exclusion of light are approx. 10 to 15% per month.

Laboratory application

On a laboratory scale, chlorine dioxide is also obtained by oxidizing chlorite. The oxidizing agent for this is either sodium peroxodisulfate Na 2 S 2 O 8 or chlorine gas :

Alternatively, chlorine dioxide can also be obtained by disproportionating sodium chlorite in an acidic solution:

Another method is the conversion of potassium chlorate using concentrated sulfuric acid . To reduce the risk of explosion, oxalic acid is added, creating a chlorine dioxide-carbon dioxide mixture:

Another possibility for the production of moderate amounts of chlorine dioxide for technical use is the reduction of chlorate with hydrogen peroxide:

The redox reaction is started by acidification with sulfuric acid. A suitable mixture contains 40% sodium chlorate and 8% hydrogen peroxide and is commercially available. The resulting chlorine dioxide is either expelled with air and dissolved in cold water or the mixture is diluted and the strongly acidic solution is processed further directly. Although the reduction of chlorate with hydrogen peroxide is also suitable for large plants, methanol is usually preferred because of its lower cost.

properties

Chlorine dioxide: bond length and angle

Molecular Properties

The molecule is due to the free electron pairs constructed angled at the central chlorine atom, the bond angle is 117 °, the Cl-O bond length of 147 pm. Since chlorine occurs in the form of two different isotopes on earth (76% 35 Cl and 24% 37 Cl), individual chlorine dioxide molecules have a mass of 67  u or 69 u. However, the bond angle and bond length as a property of the identical electron configuration are the same in both cases. Due to its uneven number of 19  valence electrons , the molecule is a paramagnetic radical . Below -59 ° C crystallizes chlorine dioxide and form dimers (molecular pairs), which chlorine dioxide below -84 ° C diamagnetic is. In 1933, Lawrence Olin Brockway (1907–1979) proposed a three-electron bond. Linus Pauling later developed this idea further into a theory that assumes a weaker bond between the third electron. Later studies showed that the unpaired third electron occupies the highest occupied molecular orbital ( HOMO ).

Physical Properties

Crystal structure of chlorine dioxide with ClO 2 dimers (ClO 2 ) 2

Chlorine dioxide is 2.3 times as heavy as air. When dissolved in water, it has a broad absorption band at a wavelength of 350 nm. With gaseous chlorine dioxide, several maxima are visible due to vibration-coupled absorption (see illustration of the UV spectrum in the section on the ozone hole ). The vibrations responsible for the coupling were determined in 1933 and found in accordance with an extended Franck-Condon principle . The spectroscopic properties received renewed attention to elucidate the processes in the ozone hole.

On transition to the solid state below −59 ° C, it forms explosive orange-red crystals. It crystallizes in the orthorhombic crystal system in the space group Pbca (space group no. 61) with the lattice parameters a  = 1087  pm , b  = 671 pm and c  = 559 pm determined at 198  K , and eight formula units per unit cell . In the crystal structure , the chlorine dioxide molecules are present as dimers (ClO 2 ) 2 . The dimerization takes place via Cl · ·· O contacts of two neighboring ClO 2 molecules, the Cl · ·· O distance is 278 pm. Template: room group / 61

Percentage of chlorine dioxide at normal pressure in the gas mixture above an aqueous solution
Chlorine dioxide solution in water

Chemical properties

At temperatures between −59 ° C and 11 ° C, chlorine dioxide is an amber-colored, oily liquid that becomes unstable above −40 ° C and tends to explode. At standard temperature it is present as a gas which is explosive in mixtures with air of over 10% by volume or at a partial pressure of over 76  mmHg (10 kPa; 0.1 atm). The mechanism of the explosion of gaseous chlorine dioxide is considered to be an accumulation of radicals in the gas, which arise from intermediate products of the slow decomposition of chlorine dioxide. The radicals catalyze further decay until the decay takes place explosively. When it explodes, it breaks down into chlorine and oxygen:

Solutions in water are yellow to brown-yellow in color and are not explosive as long as they cannot generate a chlorine dioxide-air mixture with more than 10% by volume of chlorine dioxide. At normal pressure, this limit of 10% by volume is reached by solutions with 6 g (30 ° C) to 13 g (10 ° C) chlorine dioxide per liter of water (see figure). At low temperatures, clathrates crystallize from aqueous solutions , in which the gas crystallizes together with the water and is enclosed in voids in the crystal. The gas hydrates formed have the empirical formula ClO 2  ·  n  H 2 O ( n  ≈ 6–10).

Chlorine dioxide and its aqueous solutions have a strong oxidizing effect. Depending on the reduction partner, chlorides (Cl - ), chlorites (ClO 2 - ) or hypochlorites (OCl - ) are formed. An oxidation of ClO 2 is also possible with even more powerful oxidizing agents, among others, fluorine (F 2 ), ozone (O 3 ) and fuming sulfuric acid .

At neutral and acidic pH values, chlorine dioxide is relatively insensitive to hydrolysis (splitting by water). At pH values ​​above 10, however, considerable disproportionation (reaction with itself) sets in which, depending on the pH value, leads to the formation of chlorate / chloride or chlorate / chlorite.

When ClO 2 is introduced into strongly alkaline solutions, this reaction takes place in a stormy manner. Exposure to light also leads to decomposition.

use

Chlorine dioxide is used as a bleaching agent in the textile and pulp industry, where it has largely replaced chlorine. It was also used to bleach flour or starch, lubricants, ointments and wax. It can be used to bleach textile fibers, but hydrogen peroxide is mostly used there, as well as chlorine in countries without strict environmental regulations.

Used as a disinfectant , it is particularly important in the disinfection of drinking water , but is also used to disinfect waste water and to combat mold. During the anthrax attacks in 2001, it was used to disinfect buildings in one case.

In the laboratory, chlorine dioxide is used in the production of chlorous acid . In organic chemistry, it can be used as an oxidizing agent, for example to convert sulfides and thioethers into sulfoxides .

Pulp bleaching

Worldwide pulp production by bleaching method: chlorine (green, below), E lementar- C hlor- F rei (blue, middle), with chlorine dioxide or chlorite and T otal- C hlor- F rei (gray top,) with ozone or hydrogen peroxide bleached

In pulp bleaching, chlorine dioxide has now almost completely replaced bleaching with chlorine. Depending on the degree of development and environmental awareness, chlorine is no longer used for bleaching in some countries. In Scandinavia, for example, chlorine has not been used for pulp bleaching since 1994. This happened after chlorinated hydrocarbons , including the poisonous dioxins , were found in the wastewater from chlorine bleach. Chlorine bleaching has therefore been replaced by (elemental chlorine-free) ECF bleaching or (total chlorine-free) TCF bleaching. ECF bleaches use chlorine -containing compounds, mostly chlorine dioxide, as the oxidizing agent instead of elemental chlorine (Cl 2 ). TCF bleaches, on the other hand, are based on the bleaching effect of chlorine-free oxidizing agents based on oxygen such as oxygen (O 2 ), hydrogen peroxide (H 2 O 2 ) or ozone (O 3 ). Among these, however, ozone, for example, is only suitable for cellulose bleaching to a limited extent, as it is too reactive and also attacks the cellulose.

In the bleaching process degradation products of are lignin from the sulfate process or sulfite , residual lignin and dyes discolored pulp and oxidative degradation. The displacement of chlorine is due to the fact that chlorine dioxide, in contrast to elemental chlorine, only acts as an oxidizing agent, but not as a chlorinating agent. However, some hypochlorous acid and elemental chlorine are always formed in the reaction mixture . As with bleaching with chlorine itself, this leads to the formation of chlorinated hydrocarbons. The course and mechanism of the oxidation of aromatic hydrocarbons with chlorine dioxide depends on the pH of the reaction mixture. Normally the concentration of hypochlorous acid and elemental chlorine is very low and both substances degrade quickly so that they can no longer be detected in the filtrates of the bleach solution. The formation of organochlorine compounds is influenced by many factors and can be reduced by using lower amounts of chlorine dioxide and adding dimethyl sulfoxide or amidosulfonic acid . However, waste water from ECF-bleached wood still contains chloroform and other chlorinated hydrocarbons and therefore continues to pose an environmental risk. According to an estimate from 1999, however, according to data at the time, chloroform emissions should be reduced to 3% when elemental chlorine is replaced by ECF processes to let. The total amount of chlorinated hydrocarbons formed is stated to be around five times less for chlorine dioxide than for chlorine.

Food

Chlorine dioxide is used as an antimicrobial substance in the processing of food against pathogens such as Salmonella Typhimurium , Escherichia coli O157: H7 , Listeria monocytogenes and Campylobacter jejuni . Chlorine dioxide offers a broad spectrum of activity and breaks down into harmless chloride ions and oxygen when it comes into contact with food. It is used to treat poultry, red meat, fish and seafood, and fruit and vegetables, among other things.

Chlorine dioxide had the E number E926, was a bleaching agent for flour (until 1957) and nutshells and is used to disinfect drinking water. Today, for health reasons (e.g. because of severe kidney damage in animal experiments), chlorine dioxide is not an approved food additive ; its E number E926 is not included in current lists of food additives. Its use for disinfecting poultry in the European Union was discussed, but was rejected by the Council of Ministers of the European Union in 2008. In 1997 the EU banned the import of such poultry cuts. In January 2009, the US filed a lawsuit with the WTO's Dispute Settlement Body because neither the EU Commission nor the member states had provided a serious, scientific-based justification for the ban. In German-language media reports on relations between the EU and the USA, chicken parts treated in this way are referred to as "chlorine chicken", "chlorine chicken" or " chlorine chicken" .

In the opinion of EFSA , the disinfection of poultry meat with chlorine dioxide under the proposed conditions of use does not give rise to any safety concerns. In addition, despite a long history of use, there is no published data to suggest that the use of chlorine dioxide leads to increased bacterial tolerance to chlorine dioxide or increased resistance to therapeutic antibiotics and other antimicrobial agents. The Federal Institute for Risk Assessment is of the opinion that chicken meat treated with chlorine dioxide is not harmful to the health of the consumer and even has advantages in terms of sterility.

Misuse

In addition to MMS , a ready-to-drink chlorine dioxide solution (CDL, also CDS for chlorine dioxide solution ) with numerous supposedly health-promoting properties is marketed. Like MMS, CDL is intended to treat various diseases (e.g. cancer , diabetes , hepatitis , multiple sclerosis , Alzheimer's , AIDS , malaria , potency problems), alleged diseases ( autism ; there in the form of enemas in children) or the fight against pathogens ( Fungi , viruses , bacteria , parasites ). Reputable evidence does not exist for this. It is not an approved drug and it is harmful to health. So after taking a CDL u. a. reports of nausea , vomiting , diarrhea , kidney failure , intestinal damage and drop in blood pressure. An alleged effect against SARS-CoV-2 is also misinformation.

Disinfection of drinking and wastewater

Effectiveness of chlorine dioxide and chlorine (concentration determined iodometrically ) against the bacterium Staphylococcus aureus : Percentage of bacteria killed at different pH values after 20 minutes of disinfection with chlorine dioxide or chlorine

Chlorine dioxide is used in drinking water disinfection , in which it has largely replaced chlorine in individual countries. In Germany it is approved for drinking water treatment according to § 11 of the Drinking Water Ordinance.

It is just as or better effective against bacteria than chlorine and, in contrast to chlorine, also effective against viruses and many protozoa (single cells). Compared to chlorine, it has the advantage of producing significantly fewer chlorinated hydrocarbons from organic material. These cannot arise from chlorine-free ozone (O 3 ) during drinking water disinfection, but ozone can react with bromide naturally occurring in water and form carcinogenic bromate from it . This reaction does not take place with the weaker oxidizing agent chlorine dioxide.

In Germany around 9% of the waterworks used chlorine dioxide in 1998 ( sodium hypochlorite : 53%, chlorine: 27%); in the USA, chlorine dioxide was used as the primary disinfectant in around 10% of waterworks. The maximum value for the chlorite formed in the water after disinfection is 0.2 mg per liter in Germany and Switzerland. In Germany, the highest permitted concentration for chlorine dioxide after disinfection is also 0.2 mg / l, in Switzerland it is 0.05 mg / kg.

In the USA, chlorine dioxide is also used to improve the taste and smell of drinking water if it is unsatisfactory due to residual contamination from algae or rotting plants. Chlorine dioxide is also active against malodorous phenolic impurities, as it oxidatively breaks down phenols in a similar way to cellulose bleaching.

Chlorine dioxide is also used to deodorize malodorous waste and sewage. It is suitable for the latter because, in contrast to chlorine, it does not have a chlorinating effect and therefore does not release any persistent organochlorine compounds into the environment and maintains its effectiveness over a wider pH range. This is also a decisive factor when disinfecting drinking water , as chloroform, dichloroacetic acid or trichloroacetic acid has been detected in chlorinated drinking water.

Disinfectants

Hart Senate Office, the office building of the Capitol, which was disinfected with chlorine dioxide after receiving a letter with anthrax spores

Chlorine dioxide can be used to disinfect buildings because it has a broad spectrum of effectiveness against microorganisms and, as a gas, can also reach otherwise inaccessible places. It has been approved by the Environmental Protection Agency (EPA) in the USA for the disinfection of laboratory equipment, tools and room surfaces since 1988 . Thanks to its fungicidal effect, it is also ideal for combating mold . In 1991, for example, by spraying a two percent solution, the mold infestation in a library was stopped for years, which would otherwise noticeably occur when the ventilation system failed.

In 2002 an article in the New York Times made a company known that specialized in the disinfection of entire houses by flooding with the gas chlorine dioxide. After contamination with anthrax ( anthrax ), an office building (the Hart Senate Office) in the Capitol was disinfected with chlorine dioxide in 2007, and after Hurricane Katrina , a restaurant building in New Orleans was flooded with chlorine dioxide to kill mold and spores.

Chlorine dioxide is also increasingly being used for disinfection prior to filling PET bottles. Similar to drinking water, the limit values ​​for chlorite must not be exceeded due to residues of the disinfectant solution remaining in the bottle.

Since chlorite levels (over 1 mg / l) were found to be too high in the water when chlorine dioxide was used as a disinfectant in swimming pools, ClO 2 is no longer used in Germany . Nowadays, the water in public pools is disinfected with chlorine gas , sodium hypochlorite or calcium hypochlorite , which at the same time ensures a depot effect in the swimming pool.

proof

Evidence of chlorine dioxide is mostly based either on its spectroscopic properties or on its reactivity. A simple spectroscopic detection method is the measurement of light absorption at 350 nm, which was used in particular for detection in the atmosphere when exploring the ozone hole (see section Ozone hole and the absorption spectrum shown in it ).

Spectroscopic detection is most common in drinking water because it is the easiest to automate. For this purpose, direct methods detect chlorine dioxide either at 360 nm or in the range 320–400 nm. Much more frequently, however, have been published measuring methods that monitor the bleaching of a dye through reaction with chlorine dioxide. The colorants used are chlorophenol red , rhodamine , amaranth or neutral red . The measurements can also be differentiated according to the extraction technique. Either a sample with a certain volume is taken and analyzed, or the measurement is carried out as a flow injection analysis , in which the reaction is observed after a constant time interval in the flow behind an addition point. The disappearance of the fluorescence of dyes when reacting with chlorine dioxide, including fluorescein or rhodamine S , is also used less frequently than the analysis of the absorption . The advantage of detection using fluorescence is that the absorption of other substances can interfere with the measurement less.

The concentration of chlorine dioxide can also be determined iodometrically . For the determination in drinking water, the formation of iodine from the added iodide is then also measured spectrometrically. A detection of chlorine dioxide alone in addition to chlorine, hypochlorous acid, hypochlorites, chloramines, chlorites and chlorates is difficult. If chlorine dioxide is to be detected alongside other oxidizing (e.g. iron) salts, it can be converted into a solution without these interfering salts by blowing in air and introducing it into cold water.

Ozone hole

McMurdo Station , Antarctica. In 1986 the concentration of chlorine dioxide in the atmosphere was determined here. The absorption of moonlight or scattered sunlight by chlorine dioxide was measured.
The absorption spectrum of chlorine dioxide in aqueous solution (bottom, blue) and as a gas (top, red). The spectrum of the gas is temperature-dependent, the signals become wider at lower temperatures, and the signal height is therefore lower.

In the stratosphere , chlorine dioxide is formed from chlorine monoxide (ClO), usually with the help of bromine monoxide, which is then reduced by the reaction of the bromine radical formed with ozone:

The notation OClO instead of ClO 2 is used in connection with reactions in the atmosphere to distinguish chlorine dioxide from its unstable isomer, chloroperoxide, ClOO. In a peroxide , the two oxygen atoms are connected to one another, as indicated by the notation ClOO. Chlorine peroxide is also produced in the reactions of chlorine radicals in the atmosphere, but it breaks down again in less than a nanosecond .

The role of chlorine dioxide in the discovery of the ozone hole is based on its absorption at 350 nm. Gaseous chlorine dioxide does not show a single, broad absorption band, but many (around 20) absorption peaks can be seen individually (see picture). In 1986 Susan Solomon and her co-workers determined the chlorine dioxide concentration in the atmosphere in Antarctica by measuring the light absorption from the ground. They used moonlight at night and scattered sunlight during the day and during twilight. The spectrometer used could only measure wavelengths above 400 nm and therefore only use the less intense absorption peaks between 400 nm and 450 nm. From the measured values, they were able to estimate the concentration of chlorine monoxide, which was known to play a major role in the breakdown of ozone and the resulting ozone hole .

One of the ozone depletion cycles involves a chlorine dioxide intermediate stage through which chlorine radicals are recovered from chlorine monoxide. After initial uncertainties about the relative importance of the degradation cycles, this degradation cycle is now held responsible for around 75% of ozone depletion.

Overall balance:
With M = O 2 or M = O 2 + N 2

However, chlorine dioxide released on the earth's surface itself has little or no influence on the formation of the ozone hole, as sunlight breaks it down in the air into chlorite and chlorate in a very short time, which then return to the surface with rain, where it is reduced become:

However, volatile chlorinated hydrocarbons (e.g. chloroform), such as those produced during pulp bleaching, are also responsible for the ozone hole (see section on pulp bleaching ).

toxicology

The “Very Toxic” (T +) classification of the substance is due to its toxicity when inhaled. The inhalation of 19 ppm chlorine dioxide in the air is said to have led  to the death of a person concerned during an industrial accident in a tank for an indefinite period of time. The maximum workplace concentration in the breathing air is 0.1 ml / m 3 , which corresponds to 0.1 ppm. This is also the value most often given for the perceptibility limit of the pungent, chlorine-like odor of chlorine dioxide. The gas is thus self-warning. This value is similar to that of chlorine, the odor limit of which, depending on the individual sense of smell, is given as 0.2 to 3.5 ppm.

For the long-term toxicity of chlorine dioxide in drinking water, chlorite (ClO 2 - ) and chlorate (ClO 3 - ) must always be taken into account, since chlorine dioxide can be converted into them under suitable conditions. The action of UV light catalyzes the breakdown of chlorine dioxide into chlorite and chlorate:

Chlorite can also be formed as a product of the oxidation of other substances, and chlorine dioxide is also produced by acidifying chlorite solutions. Much higher doses are used in the United States for treating drinking water with chlorine dioxide. There, unlike in Europe, organic components are not first removed by activated carbon filters, which is why a higher concentration of chlorine dioxide is necessary for disinfection.

Higher organisms are relatively insensitive to the uptake of chlorine dioxide through ingestion. In a study on humans, for example, no negative changes were found after a single intake of 24 mg chlorine dioxide in one liter or 2.5 mg chlorite in 500 ml water in ten healthy men. That is a factor of twenty or a hundred higher than the maximum value for drinking water treatment in Germany of 0.2 mg per liter of drinking water. This insensitivity to oral ingestion of chlorine dioxide is probably due to rapid deactivation through reaction with substances in the stomach. In a study of Ethiopian green monkeys at a dose of 1.8 mg chlorine dioxide could (30 ml, 60 mg · l -1 ) and immediate recovery of the gastric fluid within 5 minutes only 8% of the original oxidising effect of the added chlorine dioxide by titration to determine , the rest had become ineffective by reaction with the gastric juices. Already diluted saliva of the monkeys deactivated chlorine dioxide in the test tube within one minute in the order of 0.15 mg (95% deactivated) to 1.5 mg (88% deactivated) chlorine dioxide per milliliter of saliva.

In organisms, chlorine dioxide reacts easily with the amino acids cysteine and tryptophan as well as free fatty acids . In one study, however, it hardly or only slowly reacted with (viral) RNA or the DNA of spores , much more slowly than they were killed.

In some cases, the antiviral activity could be traced back to a separation of the viral RNA from the capsid , the protein envelope of the virus. While chlorine dioxide is effective against viruses , bacteria , spores , molds and even prions , some types of the slower-growing mycobacteria show high resistance.

literature

Web links

Commons : Chlorine Dioxide  - Collection of Pictures, Videos and Audio Files

Individual evidence

  1. Entry on E 926: Chlorine dioxide in the European database on food additives, accessed on June 27, 2020.
  2. a b c d e f g h i Entry on chlorine dioxide in the GESTIS substance database of the IFA , accessed on May 13, 2020(JavaScript required) .
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This article was added to the list of excellent articles on May 5, 2010 in this version .