Interference in Melamine-based Determination of CYA

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chem geek

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I recently ran into the article Interference in Melamine-based Determination of Cyanuric Acid Concentration from the Journal of the Swimming Pool and Spa Industry published in 1995. The author found that some pools over-reported the CYA level in the standard CYA turbidity test. The pools with overly high readings tended to have high Combined Chlorine (CC) levels. Though several possible culprits for the interference were mentioned in the article, I suspect that it might be urea as this can be correlated with higher CC levels in higher bather-load pools. See the chemical structure of melamine-cyanurate and of urea and see how it could bind rings and therefore a lattice of melamine, though with only half of the hydrogen bonding (so a weaker precipitate).

Fortunately, most of our pools that we test do not have high CC levels, but if we suspect a falsely high reading when the CC is high, adding one drop of sodium hypochlorite to the precipitated sample (i.e. after melamine has been added and precipitate formed from the normal 30 second mixing) apparently resolved the problem. However, I would be concerned that the drop of chlorine might raise the pH too much since the melamine solutions for the CYA test intentionally contain an acidic pH buffer to precipitate more melamine-cyanurate since the precipitate at a pH of 7.0 has a solubility of around 20 ppm. I would also be concerned that the high chlorine level might chlorinate the cyanurate to the extent that it might not produce as much precipitate, even at equilibrium though this is may not be a problem if the bonds for the precipitate are much stronger than that of the chlorinated cyanurate (in equilibrium with chlorine).
 
chem geek said:
I recently ran into the article Interference in Melamine-based Determination of Cyanuric Acid Concentration from the Journal of the Swimming Pool and Spa Industry published in 1995. The author found that some pools over-reported the CYA level in the standard CYA turbidity test. The pools with overly high readings tended to have high Combined Chlorine (CC) levels. Though several possible culprits for the interference were mentioned in the article, I suspect that it might be urea as this can be correlated with higher CC levels in higher bather-load pools. See the chemical structure of melamine-cyanurate and of urea and see how it could bind rings and therefore a lattice of melamine, though with only half of the hydrogen bonding (so a weaker precipitate).

Fortunately, most of our pools that we test do not have high CC levels, but if we suspect a falsely high reading when the CC is high, adding one drop of sodium hypochlorite to the sample apparently resolved the problem. However, I would be concerned that the drop of chlorine might raise the pH too much since the melamine solutions for the CYA test intentionally contain an acidic pH buffer to precipitate more melamine-cyanurate since the precipitate at a pH of 7.0 has a solubility of around 20 ppm. I would also be concerned that the high chlorine level might chlorinate the cyanurate to the extent that it might not produce as much precipitate, even at equilibrium though this is may not be a problem if the bonds for the precipitate are much stronger than that of the chlorinated cyanurate (in equilibrium with chlorine).
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This is the reason why you are "THE CHEM GEEK" and I'm not worthy :bowdown:
I think I need to relax to keep my head from explodeing. ::epds::
 
Thanks. Do you know which volume or article for the rebuttal/comments from Wojtowicz? I can't find anything that looks like that on the JSPSI link. Perhaps it's in one of the prologues that hasn't yet been converted to PDF.

As for exposure to UV, yes that should also work to reduce CC but of course that's part of the reason why we don't see high CC in our outdoor residential pools (the other reason being the relatively low bather loads). Note, however, that reduction of urea may require UV that is more like sunlight that actually breaks down some chlorine to form free radicals such as hydroxyl radicals. The UV used in some UV systems is focused more on inorganic chloramine destruction with minimal effect on chlorine, but that's not necessarily a good thing with regard to more powerful oxidation needed to more rapidly oxidize urea.
 
chem geek said:
Thanks. Do you know which volume or article for the rebuttal/comments from Wojtowicz?
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It's volume 2 number 1, pages 14 to 17.

In that same issue, Wojtowicz also responds to an opinion paper by one Ben Powell.

In his comments on the CYA interferences, Wojtowicz says that ideally the available chlorine should be reduced with thiosulfate prior to precipitation as some of the CYA is in the form of chloroisocyanurate. He points out omissions in Latta's measurements and explains why ammelide and ammeline, ammonia and chloramine cannot cause such interferences. "All that has been established is that a lower reading is obtained if the test solution is treated with NaOCl solution. This does not prove that the procedure gives the correct CYA concentration" he notes.

In a previous discussion about this with Waterbear (either here on another forum), he pointed out that anyways, the CYA test is not all that precise and that all this has a very minor impact, if any, using the standard turbidity test.
 
Thanks. This link shows the letter to the editor and the relevant pages for this topic are on pages 16-17. Wojtowicz proposes shocking the sample to 10 ppm and then later dechlorinating before doing the CYA test or alternatively using Oxone (MPS). He has concerns, some similar to mine, that the high chlorine and pH levels could also produce incorrect results. He refers to chlorine oxidation of CYA as well which is known to occur though only more quickly at high pH (but adding concentrated bleach will raise both FC and pH -- using one drop of 10% chlorinating liquid would raise pH to around 12 and FC to around 600 ppm according to Wojtowicz).

Interestingly, neither Latta nor Wojtowicz referred to urea as being a possible interfering chemical. Since high CC is often associated with high bather loads, especially in pools with higher CYA that have slower oxidation rates, that would seem to correlate with potentially higher urea levels as well. Note how urea and cyanuric acid both have similar structure and hydrogen bonding potential (both have atoms lying in the same plane as well). As I noted earlier, urea would likely precipitate with melamine though more weakly with only half the bonding sites. To get rid of urea more rapidly, one can lower the pH and warm the water and add some chlorine. The melamine reagent is already buffered at low pH, so perhaps when there is chlorine in the water already then this is sufficient to remove any urea interference. It would be interesting to measure the FC level of the precipitate solution, but I don't know if the FAS-DPD test will still work in those conditions.

Code: 
  H           O
    \       //
      N---C
    /       \
O==C         N--H        CYANURIC ACID
    \       /
      N---C
    /       \\
  H           O


  H
    \
      N---H
    /
O==C                     UREA
    \
      N---H
    /
  H
Anyway, as I wrote earlier, this is probably not a concern for us with our relatively low bather-load residential pools, but it's something to consider when measuring CYA levels in high bather load pools. Interestingly, I haven't seen any problems in residential spas which are also high bather load, but the much higher temperatures may keep urea levels in check getting mostly oxidized daily since the chlorine demand based on nitrogenous compounds from sweat and urine is pretty much as predicted. Also, though the chlorine demand does slowly increase in such spas, it's only over months that it climbs from roughly 25% of the FC level daily to 50% of the FC level daily (for no bather load 24-hour chlorine demand). I suspect that this is from buildup of slower-to-oxidize organics introduced at a slower quantity rate than urea.
 
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