Cyanuric acid

Cyanuric acid or 1,3,5-triazine-2,4,6-triol is an chemical compound with the formula (CNOH)₃. Like many industrially useful chemicals, this triazine has many synonyms. This white solid is the cyclic trimer of cyanic acid, HOCN. It finds use as a precursor or a component of bleach, disinfectants, and herbicides.

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Chemical and properties

Cyanuric acid is odorless, hygroscopic, and commonly used as a crystalline powder. On heating above 320 °C, the solid decomposes to toxic nitrogen oxides and cyanic acid.^{[citation needed]}

The two chemical structures shown in the infobox readily interconvert: they are tautomers. The triol tautomer has aromatic character. Consequently, the hydroxyl (-OH) groups assume phenolic character, becoming more acidic than hydroxyl groups in typical alcohols. Deprotonation with base affords a cyanurate salt.

Synthesis

Cyanuric acid (CYA) was first synthesized by Wöhler in 1829 by the thermal decomposition of urea and uric acid.^[1] The current industrial route to CYA entails the thermal decomposition of urea (thermal decomposition). The production of CYA and ammelide commence simultaneously at approximately 175 °C, in the following two reactions involving both biuret and isocyanic acid as intermediates:

H₂N-CO-NH-CO-NH₂(m) + HNCO(g) → CYA(s) + NH₃(g)

H₂N-CO-NH-CO-NH₂(m) + HNCO(g) → ammelide(s) + H₂O(g)

As temperature exceeds 190 °C, other reactions begin to dominate the process.

The first appearance of ammeline occurs prior to 225 °C and is suspected also to occur from decomposition of biuret but is produced at a slower rate than that of CYA or ammelide, by the following reaction:

H₂N-CO-NH-CO-NH₂(m) + HNCO(g) → ammeline + 2 H₂O

The first appearance of melamine occurs between 325 and 350 °C and only in very small quantities.^[2]

Cyanuric acid can be produced by hydrolysis of crude or waste melamine and isolating the pure cyanuric acid product by crystallizing out and filtering off the cyanuric acid. Acid waste streams from plants producing these materials contain cyanuric acid and on occasion, dissolved amino-substituted triazines, namely, ammeline, ammelide and melamine. In one method, an ammonium sulfate solution is heated to the boil and treated with a stoichiometric amount of melamine, by which means the cyanuric acid present precipitates as melamine cyanurate. The various waste streams containing cyanuric acid and amino-substituted triazines may be combined for disposal and during upset conditions, undissolved cyanuric acid may be present in the waste streams. ^[3][4]

Applications

Cyanuric acid is used as a stabilizer in recreational water treatment to minimize the decomposition of hypochlorous acid by sunlight in outdoor swimming pools and hot tubs. It achieves this by absorbing UV radiation.

Chlorinated derivatives of cyanuric acid, such as trichloro-s-triazinetrione and sodium dichloro-s-triazinetrione, are used as algacides or microbiocides in swimming pool water and large scale water systems in industry.

Testing for cyanuric acid concentration is commonly done with a turbidometric test, which uses a reagent, melamine, to precipitate the cyanuric acid. The relative turbidity of the reacted sample quantifies the CYA concentration. Referenced in 1957.^[5] This test works because melamine combines with the cyanuric acid in the water to form a fine, insoluble, white precipitate that causes the water to cloud in proportion to the amount of cyanuric acid in it.

FDA permits a certain amount of cyanuric acid to be present in some non-protein nitrogen (NPN) additives used in animal feed and drinking water.^[6] Cyanuric acid has been used as NPN. For example, Archer Daniels Midland manufactures a NPN supplement for cattle, which contains biuret, triuret, cyanuric acid and urea.^[7]

Possible connection to pet food recall of spring, 2007

Further information: 2007 pet food recalls

Current research has focused on melamine and cyanuric acid in combination

Further information: melamine

Researchers have focused on the role of melamine and related compounds in causing renal failure. Beginning on April 19, it was reported that researchers had ruled out aminopterin contamination and had found a "spoke-like crystal" in contaminated wheat gluten and rice protein concentrate and the tissues and urine of affected animals. (It was previously known that melamine and cyanuric acid can form networks of hydrogen bonds, creating a tile-like planar structure through molecular self-assembly.)^[8] The crystal has been said to serve as a biomarker for contamination and is approximately 30% melamine. The remainder has been identified as cyanuric acid, ammelide and ammeline, with crystals recovered from urine reported to be approximately 70% cyanuric acid. While some researchers have theorized that the three latter chemicals might have been formed as the animals metabolized the melamine, or as by-products of bacterial metabolism (cyanuric acid is a known intermediate byproduct of bacterial metabolism of melamine), their presence in the crystals found in contaminated protein itself, combined with media reports of widespread adulteration with both melamine and cyanuric acid in China, has focused research efforts on their combined effects in animals. Neither melamine nor cyanuric acid, a chemical commonly used in pool chlorination, have been thought to be particularly toxic by themselves. The current hypothesis is that, although these contaminants are not very toxic individually, their potency appears to be increased when they are present together. ^{[9][10][11][12][13]}

On April 27 researchers from the University of Guelph, in Ontario announced that they had created crystals chemically similar to the ones found in contaminated animals by combining melamine and cyanuric acid in the laboratory under pH conditions similar to that in animal kidneys.^[14][15]

In light of these findings, on May 1, the American Veterinary Medical Association noted in a press release that the "extremely insoluble" crystals formed in animal kidneys are suspected of blocking kidney function.^[16] On May 7, however, Barbara Powers, president of the American Association of Veterinary Laboratory Diagnosticians and a professor of veterinary diagnostics at Colorado State University cautioned "There's something more going on than just the mechanical blockage. Because you wouldn't see so much necrosis (cell death) and inflammation.”^[17]

On May 2, in further inquiry into the source of the cyanuric acid in the contaminated ingredients and the toxic effects of the chemical combination, Richard Goldstein of the Cornell University College of Veterinary Medicine, in response to reports that the contaminant might be "melamine scrap" left over from processing coal into melamine, hypothesized: “It’s possible the other stuff they were left with was the bottom-of-the-barrel stuff, leftover melamine and possibly cyanuric acid. I think it’s this melamine with other compounds that is toxic.”^[18] The composition of the crystals analyzed in contaminated pet food ingredients is similar to the composition of a waste product produced in cyanuric acid production.^[19]

A toxicology study conducted after the recalls concluded that the combination of melamine and cyanuric acid in diet does lead to acute renal failure in cats.^[20][21] Wilson Rumbeiha, an associate professor in MSU’s Diagnostic Center for Population and Animal Health, commenting on results from a survey commissioned by the American Association of Veterinary Laboratory Diagnosticians and designed and implemented by MSU toxicologists which was also presented at the AAVLD's October 2007 meeting, said: "Unfortunately, these [melamine cyanurate] crystals don’t dissolve easily. They go away slowly, if at all, so there is the potential for chronic toxicity.”^[22][23][24]

Media reports of cyanuric acid adulteration in China

On May 8 2007, The International Herald Tribune reported that three Chinese chemical makers have said that animal feed producers often purchase, or seek to purchase, the chemical, cyanuric acid, from their factories to blend into animal feed to give the false appearance of a higher level of protein, suggesting another potentially dangerous way that melamine and cyanuric acid might combine in protein products.^[25]

References

• International Chemical Safety Card 1313