trader_4 wrote, on Wed, 25 Jun 2014 15:21:36 -0700:

I've never seen that table. What makes you think it's right?

J. O'Brien, J. Morris and J. Butler, €œEquilibria in Aqueous Solutions
of Chlorinated Isocyanurate€, Chapter 14 in A. Rubin, ed., Chemistry of
Water Supply, Treatment and Distribution, 1973 Symposium, (published 1974),
Ann Arbor Science, Ann Arbor, MI, pp. 333-358.


The following is an approximate formula you can use so long as your CYA ppm is at least 5 times your FC (the formula really falls apart terribly below a ratio of CYA/FC of 3).

(HOCl as ppm Cl2) = (FC as ppm Cl2) / ( 2.7*(ppm CYA) - 4.9*(FC as ppm Cl2) + 5 )

and if you are interested in the FC for a given HOCl (to construct the equivalent of Ben's table, for example), you can use the following which just solves for ppm FC from the above.

(FC as ppm Cl2) = ( 2.7*(ppm CYA) + 5 ) / ( 4.9 + 1/(ppm HOCl) )

The constants in the above formulas are for a pH of 7.5 (which is the only parameter that significantly affects these constants). With the spreadsheet I can easily calculate the constants for other pH, but remember that the above formulas are approximate. For example, with FC of 3 and CYA of 15 the formula gives HOCl as 0.098 when the correct answer is 0.095. That's not terrible (about an 3% error). However, with FC of 5 and CYA of 15 the formula gives HOCl as 0.239 while the correct answer is 0.199 (about an 20% error) which isn't as good.

A rough rule of thumb that applies at a pH of 7.5 is that the effective chlorine level is reduced by a factor about equal to the ppm of the CYA. So, a CYA of 30 ppm reduces the disinfecting chlorine (HOCl) level to about 1/30th of what it would be with no CYA.