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With a boiling point of 11°C and similar to chlorine in appearance and odour, chlorine dioxide (ClO2) exists as a dense yellowish-green gas at room temperature. In this form, it is extremely reactive, readily decomposing to chlorine and oxygen:


ClO2 (g) = ½ Cl2 (g) + O2 (g)


This reaction is highly exothermic; significant heat is evolved. Chlorine dioxide gas can however be stabilised to a marked extent by dissolving it in water; aqueous solutions of up to 10 g/L may be stored without significant loss if chilled and protected from sunlight. (1)


There are numerous methods used industrially to prepare chlorine dioxide, involving either sodium chlorite (NaClO2) or sodium chlorate (NaClO3) as starting materials. The former are more relevant to pool and spa applications; some of these reactions are listed below.



Although its standard reduction potential (E° = 0.954 V at 25°C1) is just over 0.4 V less than that for chlorine, chlorine dioxide remains a powerful oxidant, even under neutral conditions. As such, it can oxidise a wide range of organics, including carbohydrates, amino acids and phenols, and inorganics, such as ammonia and carbon monoxide (see also Precautions, below).


Routine testing can be accommodated using a similar iodometric titration method to that used in the Bio-Lab recommended procedure for determining chlorine demand. This involves the reduction of iodide to iodine:


2 ClO2 + 2 I- = 2 ClO2- + I2


then titration of the resultant yellow-brown solution with thiosulfate to a colourless endpoint:


I2 + 2 S2O32- = 2 I- + S4O62-


It is advisable that this test is carried out under neutral (pH 7) conditions.


Microbial Activity

Chlorine dioxide has been shown to have good bactericidal and algaecidal properties. Its most important use microbiologically, however, is its efficacy against the parasitic protozoa Giardia and Cryptosporidium (see TIB BG- 0382). Spread by ingestion of oocysts from the faeces of infected individuals, both classes of parasite were confirmed as being responsible for the 1998 Sydney water crisis, and numerous media reports have described outbreaks in municipal pools in Australia, NZ and overseas.


Some species of Giardia show minor resistance towards chlorine at temperatures lower than 10°C but, at 20°C,Giardia oocysts can be destroyed within 10 minutes at free chlorine levels of 1.5 ppm. (3) The chlorine dioxide level required to achieve the same degree of Giardia inactivation is reportedly less than 0.25 ppm. (4)


On the other hand, Cryptosporidium oocysts are extremely resistant to chlorine. Only when the chlorine level was raised to 80 ppm was a 90% kill-rate achieved, and then only after 90 minutes.4 The NZ Recreation Association and Water Safety NZ recommend superchlorination at 20 ppm for 20 hours in cases where less than 10 oocysts are found per 100 litres; complete pool closure and disinfection is mandatory if the oocyst level exceeds this.5 In contrast, only 1.3 ppm of chlorine dioxide is required to achieve 90% inactivation after 1 hour. (4)



The Chlorine Dioxide Website1 lists a number of applications for chlorine dioxide. These include:


  • Treatment of municipal, industrial and wastewaters
  • Bleaching of paper pulp.
  • Cleaning of electronic circuit boards.
  • Sulfide treatment in the oil industry.
  • Disinfection in hospitals, disposal of biotoxic waste and other medical applications.


Selinger (6) adds that the National Health and Medical Research Council (NHMRC) have approved chlorine dioxide for the bleaching of flour in the production of white bread.



Solutions of chlorine dioxide are severe eye and skin irritants, with absorption causing blood cell and tissue damage. Burns can result from the decomposition to chlorine gas. Inhalation or ingestion can cause severe headaches and, in prolonged or high-level exposure, may give rise to bronchiospasm and pulmonary oedema. No information is currently available as to whether the compound has any carcinogenic, teratogenic or mutagenic properties. The TLV (Threshold Limit Value, the maximum concentration of contaminated air at 25°C and 760 mm Hg pressure that humans may be exposed to repeatedly without adverse effect (7)) is set at 0.1 ppm. (1)


In gaseous form, chlorine dioxide is highly reactive; explosions have occurred when mixed with ammonia, carbon monoxide, hydrogen sulfide, phosphorus or metallic mercury, or placed in contact with solid or concentrated solutions of potassium hydroxide. Vapours are reactive with most organics, including carbohydrates such as sucrose and glucose. (1)(7)


Chlorine Dioxide Products for Pool Applications

The Biotek TDC-800®8 generates chlorine dioxide on-site by the reaction of two precursors, designated Biotek 800C® and Biotek 800H®. These would appear to be sodium chlorite and hydrochloric acid, respectively, so the reaction is most probably that in equation [1], above.


In contrast, AquaPlus+5® is a stabilised aqueous solution of chlorine dioxide, supplied as a liquid additive. (9) The exact nature of the stabiliser is undisclosed. The manufacturer advises that the product be diluted 1 part in 10 with water then acidified with a small quantity of hydrochloric acid prior to use. The resultant yellowish-green solution of free chlorine dioxide can then be dosed into the pool's balance tanks or skimmer boxes. AquaPlus recommends a chlorine dioxide level of 1.25 ppm (0.25 litres AquaPlus+5 per 10,000 litres) if the presence of Cryptosporidium is confirmed. Filters are then run for 6 hours. Routine maintenance is achieved using a level of 0.2 ppm (40 mL AquaPlus+5 per 10,000 litres).


Importantly, AquaPlus stresses that the chlorine dioxide treatment is NOT a replacement for the existing chlorination system on a pool. It is a complementary treatment designed primarily to remove Giardia, Cryptosporidium and other pathogens not destroyed by chlorine.


Critical Assessment of Claims

1. Effective against all types of bacteria, algae and fungi (Biotek TDC-8008). As stated above, it is true that chlorine dioxide is more effective against certain parasites than chlorine. It is also a good bactericide and algaecide, but it tends to be more selective than chlorine.

2. Effective at low concentrations (Biotek TDC-800 (8) and AquaPlus+5 (9)). The reports of Korich et al. (4) and others support the contention that low levels of chlorine dioxide are effective in combating Cryptosporidium and Giardia.

3. Chlorine dioxide is a powerful oxidising agent, many times stronger than chlorine or hydrogen peroxide (AquaPlus+5 9). This claim appears to confuse oxidising strength and reactivity. On the basis of standard reduction potentials - the usual indicator of the relative strength of oxidants and reductants - it is not as powerful as either chlorine (E° = 1.359 V at 25°C 10) or acidified hydrogen peroxide (E° = 1.776 V at 25°C (10)). It is also slightly weaker an oxidant than aqueous bromine (E° = 1.087 V at 25°C 10). What makes chlorine dioxide so reactive is that it exists as a free radical. To explain, the electrons in most species exist in pairs. A free radical is different in that one electron is missing from one of the pairs. This makes a species extremely reactive as it strives to obtain a second electron to establish the pair. Free radicals such as nitrogen oxide (NO) are responsible for the formation of "holes" in the ozone layer.

4. Softens the water (AquaPlus5+ (9)). "Hard" water is due to the presence of calcium and magnesium carbonates. Exactly why chlorine dioxide should reduce this insoluble carbonate formation is not clear. It may be due to the conversion of the carbonate ion to carbon dioxide, although this would then lower the pH of the pool water.

5. Reactivity and stability at elevated temperature (Biotek TDC-800 (8)). The claim is made that aqueous solutions of chlorine dioxide are more thermally stable than similar solutions of chlorine (i.e. as hypochlorite), although the manufacturer concedes that losses of chlorine dioxide will accelerate at elevated temperatures. The lack of stabilisation in this particular product is of obvious concern, particularly in terms of economics.



Further investigation into chlorine dioxide is certainly warranted. With new outbreaks of Cryptosporidium occurring at an alarmingly increasing rate, chlorine dioxide's effectiveness as a supplement to conventional chlorination regimes is attractive, particularly given the low maintenance concentrations (around 0.2 ppm) required.


For a number of reasons, including initial outlay, operating and maintenance costs and the potentially explosive reactivity of sodium chlorite, (11) AquaPlus+5 would appear the better alternative of the two products reviewed here. One reservation, however, is in the lack of information on the stabiliser. Given the reactivity of chlorine dioxide and the exposure of pool water to a range of climatic extremes and organic contaminants, the nature and effectiveness of this stabiliser would need to be ascertained to make any more conclusive recommendations.



1. Chlorine Dioxide Website at www.clo2.com. This is a highly useful resource which provides a concise summary of the chemical and biochemical properties of chlorine dioxide without corporate hype.

2. "Cryptosporidium", Bio-Lab Australia Technical Information Bulletin BG-038.

3. R.S. Kebabjian, "Disinfection of Public Pools and Management of Fecal Accidents", Journal of Environmental Health, Volume 58 (1995) 8.

4. D.G. Korich, J.R. Mead, M.S. Madore, N.A. Sinclair and C.R. Sterling, "Effects of Ozone, Chlorine Dioxide, Chlorine and Monochloramine on Cryptosporidium parvum Oocysts Viability", Applied Environmental Microbiology, Volume 56 (1990) 1423.

5. "Cryptosporidium Protocol", Memorandum to Pool Managers, Water Safety New Zealand and New Zealand Recreation Association, 18 June 1998.

6. B. Selinger, "Chemistry in the Market Place", 4th edition, Harcourt Brace Jovanovich, Sydney, 1988, p. 453.

7. G.D. Muir, "Hazards in the Chemical Laboratory", 2nd edition, Chemical Society, London, 1977, p. 185.

8. Biotek TDC-800 Chlorine Dioxide Generator Sales Communication, Western Specialty Chemicals.

9. AquaPlus+5 Sales Brochures, AquaPlus Marketing Pty. Ltd., 17 Dominion Road, Ashmore QLD 4217.

10. D.A. Skoog, D.M. West and F.J. Holler, "Fundamentals of Analytical Chemistry", 7th edition, Saunders College Publishing, Fort Worth TX, 1996, pp. A12-A15.

11. G.D. Muir, Reference 7, p. 385.


The above information is supplied by Bio-Lab and represents its best interpretation of available technical information at the time of preparation. The sole purpose is to supply factual information to Bio-Lab customers. It is not to be taken out of context nor used as support for any other claim not made herein.